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What is the model of the atom proposed by Ernest Rutherford?

What is the rutherford gold-foil experiment, what were the results of rutherford's experiment, what did ernest rutherford's atomic model get right and wrong, what was the impact of ernest rutherford's theory.

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Rutherford model

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UC Davis - The Rutherford Scattering Experiment
Chemistry LibreTexts - Rutherford's Experiment- The Nuclear Model of the Atom

The atom , as described by Ernest Rutherford , has a tiny, massive core called the nucleus . The nucleus has a positive charge. Electrons are particles with a negative charge. Electrons orbit the nucleus. The empty space between the nucleus and the electrons takes up most of the volume of the atom.

A piece of gold foil was hit with alpha particles , which have a positive charge. Most alpha particles went right through. This showed that the gold atoms were mostly empty space. Some particles had their paths bent at large angles. A few even bounced backward. The only way this would happen was if the atom had a small, heavy region of positive charge inside it.

The previous model of the atom, the Thomson atomic model , or the “plum pudding” model, in which negatively charged electrons were like the plums in the atom’s positively charged pudding, was disproved. The Rutherford atomic model relied on classical physics. The Bohr atomic model , relying on quantum mechanics, built upon the Rutherford model to explain the orbits of electrons.

The Rutherford atomic model was correct in that the atom is mostly empty space. Most of the mass is in the nucleus, and the nucleus is positively charged. Far from the nucleus are the negatively charged electrons. But the Rutherford atomic model used classical physics and not quantum mechanics. This meant that an electron circling the nucleus would give off electromagnetic radiation . The electron would lose energy and fall into the nucleus. In the Bohr model, which used quantum theory, the electrons exist only in specific orbits and can move between these orbits.

The gold-foil experiment showed that the atom consists of a small, massive, positively charged nucleus with the negatively charged electrons being at a great distance from the centre. Niels Bohr built upon Rutherford’s model to make his own. In Bohr’s model the orbits of the electrons were explained by quantum mechanics.

Rutherford model , description of the structure of atoms proposed (1911) by the New Zealand-born physicist Ernest Rutherford . The model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents , called electrons , circulate at some distance, much like planets revolving around the Sun .

what did the alpha scattering experiment show scientists

The nucleus was postulated as small and dense to account for the scattering of alpha particles from thin gold foil, as observed in a series of experiments performed by undergraduate Ernest Marsden under the direction of Rutherford and German physicist Hans Geiger in 1909. A radioactive source emitting alpha particles (i.e., positively charged particles, identical to the helium atom nucleus and 7,000 times more massive than electrons) was enclosed within a protective lead shield. The radiation was focused into a narrow beam after passing through a slit in a lead screen. A thin section of gold foil was placed in front of the slit, and a screen coated with zinc sulfide to render it fluorescent served as a counter to detect alpha particles. As each alpha particle struck the fluorescent screen , it produced a burst of light called a scintillation, which was visible through a viewing microscope attached to the back of the screen. The screen itself was movable, allowing Rutherford and his associates to determine whether or not any alpha particles were being deflected by the gold foil.

atom. Orange and green illustration of protons and neutrons creating the nucleus of an atom.

Most alpha particles passed straight through the gold foil, which implied that atoms are mostly composed of open space. Some alpha particles were deflected slightly, suggesting interactions with other positively charged particles within the atom. Still other alpha particles were scattered at large angles, while a very few even bounced back toward the source. (Rutherford famously said later, “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”) Only a positively charged and relatively heavy target particle, such as the proposed nucleus, could account for such strong repulsion. The negative electrons that balanced electrically the positive nuclear charge were regarded as traveling in circular orbits about the nucleus. The electrostatic force of attraction between electrons and nucleus was likened to the gravitational force of attraction between the revolving planets and the Sun. Most of this planetary atom was open space and offered no resistance to the passage of the alpha particles.

The Rutherford model supplanted the “plum-pudding” atomic model of English physicist Sir J.J. Thomson , in which the electrons were embedded in a positively charged atom like plums in a pudding. Based wholly on classical physics , the Rutherford model itself was superseded in a few years by the Bohr atomic model , which incorporated some early quantum theory . See also atomic model .

What is the 'Gold Foil Experiment'? The Geiger-Marsden experiments explained

Physicists got their first look at the structure of the atomic nucleus.

The gold foil experiments gave physicists their first view of the structure of the atomic nucleus and the physics underlying the everyday world.

J.J. Thomson model of the atom

Gold foil experiments, rutherford model of the atom.

The real atomic model

Additional Resources

Bibliography.

The Geiger-Marsden experiment, also called the gold foil experiment or the α-particle scattering experiments, refers to a series of early-20th-century experiments that gave physicists their first view of the structure of the atomic nucleus and the physics underlying the everyday world. It was first proposed by Nobel Prize -winning physicist Ernest Rutherford.

As familiar as terms like electron, proton and neutron are to us now, in the early 1900s, scientists had very little concept of the fundamental particles that made up atoms .

In fact, until 1897, scientists believed that atoms had no internal structure and believed that they were an indivisible unit of matter. Even the label "atom" gives this impression, given that it's derived from the Greek word "atomos," meaning "indivisible."

But that year, University of Cambridge physicist Joseph John Thomson discovered the electron and disproved the concept of the atom being unsplittable, according to Britannica . Thomson found that metals emitted negatively charged particles when illuminated with high-frequency light.

His discovery of electrons also suggested that there were more elements to atomic structure. That's because matter is usually electrically neutral; so if atoms contain negatively charged particles, they must also contain a source of equivalent positive charge to balance out the negative charge.

By 1904, Thomson had suggested a "plum pudding model" of the atom in which an atom comprises a number of negatively charged electrons in a sphere of uniform positive charge, distributed like blueberries in a muffin.

The model had serious shortcomings, however — primarily the mysterious nature of this positively charged sphere. One scientist who was skeptical of this model of atoms was Rutherford, who won the Nobel Prize in chemistry for his 1899 discovery of a form of radioactive decay via α-particles — two protons and two neutrons bound together and identical to a helium -4 nucleus, even if the researchers of the time didn't know this.

Rutherford's Nobel-winning discovery of α particles formed the basis of the gold foil experiment, which cast doubt on the plum pudding model. His experiment would probe atomic structure with high-velocity α-particles emitted by a radioactive source. He initially handed off his investigation to two of his protégés, Ernest Marsden and Hans Geiger, according to Britannica .

Rutherford reasoned that if Thomson's plum pudding model was correct, then when an α-particle hit a thin foil of gold, the particle should pass through with only the tiniest of deflections. This is because α-particles are 7,000 times more massive than the electrons that presumably made up the interior of the atom.

Here, an illustration of Rutherford's particle scattering device used in his gold foil experiment.

Marsden and Geiger conducted the experiments primarily at the Physical Laboratories of the University of Manchester in the U.K. between 1908 and 1913.

The duo used a radioactive source of α-particles facing a thin sheet of gold or platinum surrounded by fluorescent screens that glowed when struck by the deflected particles, thus allowing the scientists to measure the angle of deflection.

The research team calculated that if Thomson's model was correct, the maximum deflection should occur when the α-particle grazed an atom it encountered and thus experienced the maximum transverse electrostatic force. Even in this case, the plum pudding model predicted a maximum deflection angle of just 0.06 degrees.

Of course, an α-particle passing through an extremely thin gold foil would still encounter about 1,000 atoms, and thus its deflections would be essentially random. Even with this random scattering, the maximum angle of refraction if Thomson's model was correct would be just over half a degree. The chance of an α-particle being reflected back was just 1 in 10^1,000 (1 followed by a thousand zeroes).

Yet, when Geiger and Marsden conducted their eponymous experiment, they found that in about 2% of cases, the α-particle underwent large deflections. Even more shocking, around 1 in 10,000 α-particles were reflected directly back from the gold foil.

Rutherford explained just how extraordinary this result was, likening it to firing a 15-inch (38 centimeters) shell (projectile) at a sheet of tissue paper and having it bounce back at you, according to Britannica

Extraordinary though they were, the results of the Geiger-Marsden experiments did not immediately cause a sensation in the physics community. Initially, the data were unnoticed or even ignored, according to the book "Quantum Physics: An Introduction" by J. Manners.

The results did have a profound effect on Rutherford, however, who in 1910 set about determining a model of atomic structure that would supersede Thomson's plum pudding model, Manners wrote in his book.

The Rutherford model of the atom, put forward in 1911, proposed a nucleus, where the majority of the particle's mass was concentrated, according to Britannica . Surrounding this tiny central core were electrons, and the distance at which they orbited determined the size of the atom. The model suggested that most of the atom was empty space.

When the α-particle approaches within 10^-13 meters of the compact nucleus of Rutherford's atomic model, it experiences a repulsive force around a million times more powerful than it would experience in the plum pudding model. This explains the large-angle scatterings seen in the Geiger-Marsden experiments.

Later Geiger-Marsden experiments were also instrumental; the 1913 tests helped determine the upper limits of the size of an atomic nucleus. These experiments revealed that the angle of scattering of the α-particle was proportional to the square of the charge of the atomic nucleus, or Z, according to the book "Quantum Physics of Matter," published in 2000 and edited by Alan Durrant.

In 1920, James Chadwick used a similar experimental setup to determine the Z value for a number of metals. The British physicist went on to discover the neutron in 1932, delineating it as a separate particle from the proton, the American Physical Society said .

What did the Rutherford model get right and wrong?

Yet the Rutherford model shared a critical problem with the earlier plum pudding model of the atom: The orbiting electrons in both models should be continuously emitting electromagnetic energy, which would cause them to lose energy and eventually spiral into the nucleus. In fact, the electrons in Rutherford's model should have lasted less than 10^-5 seconds.

Another problem presented by Rutherford's model is that it doesn't account for the sizes of atoms.

Despite these failings, the Rutherford model derived from the Geiger-Marsden experiments would become the inspiration for Niels Bohr 's atomic model of hydrogen , for which he won a Nobel Prize in Physics .

Bohr united Rutherford's atomic model with the quantum theories of Max Planck to determine that electrons in an atom can only take discrete energy values, thereby explaining why they remain stable around a nucleus unless emitting or absorbing a photon, or light particle.

Thus, the work of Rutherford, Geiger (who later became famous for his invention of a radiation detector) and Marsden helped to form the foundations of both quantum mechanics and particle physics.

Rutherford's idea of firing a beam at a target was adapted to particle accelerators during the 20th century. Perhaps the ultimate example of this type of experiment is the Large Hadron Collider near Geneva, which accelerates beams of particles to near light speed and slams them together.

See a modern reconstruction of the Geiger-Marsden gold foil experiment conducted by BackstageScience and explained by particle physicist Bruce Kennedy .
Find out more about the Bohr model of the atom which would eventually replace the Rutherford atomic model.
Rutherford's protege Hans Gieger would eventually become famous for the invention of a radioactive detector, the Gieger counter. SciShow explains how they work .

Thomson's Atomic Model , Lumens Chemistry for Non-Majors,.

Rutherford Model, Britannica, https://www.britannica.com/science/Rutherford-model

Alpha particle, U.S NRC, https://www.nrc.gov/reading-rm/basic-ref/glossary/alpha-particle.html

Manners. J., et al, 'Quantum Physics: An Introduction,' Open University, 2008.

Durrant, A., et al, 'Quantum Physics of Matter,' Open University, 2008

Ernest Rutherford, Britannica , https://www.britannica.com/biography/Ernest-Rutherford

Niels Bohr, The Nobel Prize, https://www.nobelprize.org/prizes/physics/1922/bohr/facts/

House. J. E., 'Origins of Quantum Theory,' Fundamentals of Quantum Mechanics (Third Edition) , 2018

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Robert Lea is a science journalist in the U.K. who specializes in science, space, physics, astronomy, astrophysics, cosmology, quantum mechanics and technology. Rob's articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University

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Rutherford Scattering

Michael Fowler, University of Virginia

Rutherford as Alpha-Male

[Rutherford was] a "tribal chief", as a student said.

(Richard Rhodes, The Making of the Atomic Bomb, page 46)

In 1908 Rutherford was awarded the Nobel Prize—for chemistry! The award citation read: "for his investigations into the disintegration of the elements, and the chemistry of radioactive substances." While at McGill University, he had discovered that the radioactive element thorium emitted a gas which was itself radioactive, but if the gas radioactivity was monitored separately from the thorium's, he found it decreased geometrically, losing approximately half its current strength for each minute that passed. The gas he had found was a short-lived isotope of radon, and this was the first determination of a "half-life" for a radioactive material. (Pais, Inward Bound , page 120).

The chemists were of course impressed that Rutherford was fulfilling their ancient alchemical dream of transmuting elements, or at least demonstrating that it happened. Rutherford himself remarked at the ceremony that he "had dealt with many different transformations with various time-periods, but the quickest he had met was his own transformation from a physicist to a chemist". Still, Nobel prizes of any kind are nice to get, so he played along, titling his official Nobel lecture: "The chemical nature of the alpha-particle from radioactive substances". (He established that his favorite particle was an ionized helium atom by collecting alphas in an evacuated container, where they picked up electrons. After compressing this very rarefied gas, he passed an electric discharge through it and observed the characteristic helium spectrum in the light emitted.)

Rutherford was the world leader in alpha-particle physics. In 1906, at McGill University, Montreal, he had been the first to detect slight deflections of alphas on passage through matter. In 1907, he became a professor at the University of Manchester, where he worked with Hans Geiger . This was just a year after Rutherford's old boss, J. J. Thomson , had written a paper on his plum pudding atomic model suggesting that the number of electrons in an atom was about the same as the atomic number. (Not long before, people had speculated that atoms might contain thousands of electrons. They were assuming that the electrons contributed a good fraction of the atom's mass.) The actual distribution of the electrons in the atom, though, was as mysterious as ever. Mayer's floating magnets (see previous lecture) were fascinating, but had not led to any quantitative conclusions on electronic distributions in atoms.

Rutherford's 1906 discovery that his pet particles were slightly deflected on passing through atoms came about when he was finding their charge to mass ratio, by measuring the deflection in a magnetic field. He detected the alphas by letting them impact photographic film. When he had them pass through a thin sheet of mica before hitting the film (so the film didn't have to be in the vacuum?) he found the image was blurred at the edges, evidently the mica was deflecting the alphas through a degree or two. He also knew that the alphas wouldn't be deflected a detectable amount by the electrons in the atom, since the alphas weighed 8,000 times as much as the electrons, atoms contained only a few dozen electrons, and the alphas were very fast. The mass of the atom must be tied up somehow with the positive charge . Therefore, he reasoned, analyzing these small deflections might give some clue as to the distribution of positive charge and mass in the atom, and therefore give some insight into his old boss J. J.'s plum pudding. The electric fields necessary in the atom for the observed scattering already seemed surprisingly high to Rutherford (Pais, page 189).

Scattering Alphas

Rutherford's alpha scattering experiments were the first experiments in which individual particles were systematically scattered and detected. This is now the standard operating procedure of particle physics. To minimize alpha loss by scattering from air molecules, the experiment was carried out in a fairly good vacuum, the metal box being evacuated through a tube T (see below). The alphas came from a few milligrams of radium (to be precise, its decay product radon 222) at R in the figure below, from the original paper, which goes on:

" By means of a diaphragm placed at D, a pencil of alpha particles was directed normally on to the scattering foil F. By rotating the microscope [M] the alpha particles scattered in different directions could be observed on the screen S."

Actually, this was more difficult than it sounds. A single alpha caused a slight fluorescence on the zinc sulphide screen S at the end of the microscope. This could only be reliably seen by dark-adapted eyes (after half an hour in complete darkness) and one person could only count the flashes accurately for one minute before needing a break, and counts above 90 per minute were too fast for reliability. The experiment accumulated data from hundreds of thousands of flashes.

Rutherford's partner in the initial phase of this work was Hans Geiger, who later developed the Geiger counter to detect and count fast particles. Many hours of staring at the tiny zinc sulphide screen in the dark must have focused his mind on finding a better way!

In 1909, an undergraduate, Ernest Marsden, was being trained by Geiger. To quote Rutherford (a lecture he gave much later):

"I had observed the scattering of alpha-particles, and Dr. Geiger in my laboratory had examined it in detail. He found, in thin pieces of heavy metal, that the scattering was usually small, of the order of one degree.

"One day Geiger came to me and said, "Don't you think that young Marsden , whom I am training in radioactive methods, ought to begin a small research?" Now I had thought that, too, so I said, " Why not let him see if any alpha-particles can be scattered through a large angle?"

"I may tell you in confidence that I did not believe that they would be, since we knew the alpha-particle was a very fast, massive particle with a great deal of energy, and you could show that if the scattering was due to the accumulated effect of a number of small scatterings, the chance of an alpha-particle's being scattered backward was very small. Then I remember two or three days later Geiger coming to me in great excitement and saying "We have been able to get some of the alpha-particles coming backward …" It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

Disproof of the Pudding

The back scattered alpha-particles proved fatal to the plum pudding model. A central assumption of that model was that both the positive charge and the mass of the atom were more or less uniformly distributed over its size, approximately 10 -10 meters across or a little more. It is not difficult to calculate the magnitude of electric field from this charge distribution. (Recall that this is the field that must scatter the alphas, the electrons are so light they will jump out of the way with negligible impact on an alpha.)

To be specific, let us consider the gold atom, since the foil used by Rutherford was of gold, beaten into leaf about 400 atoms thick. The gold atom has a positive charge of 79 e (balanced of course by that of the 79 electrons in its normal state). Neglect the electrons—they'll be scattered away with negligible impact on the heavy alpha.

See the animation here !

The maximum electric force the alpha will encounter is that at the surface of the sphere of positive charge,

E ⋅ 2 e = 1 4 π ε 0 ⋅ 79 e ⋅ 2 e r 0 2 = 9 ⋅ 10 9 158 ⋅ ( 1.6 ⋅ 10 − 19 ) 10 − 20 = 3.64 ⋅ 10 − 6 Newtons .

(In this model, once inside the sphere the electric force goes down, just as gravity goes down on going deep into the earth, to zero at the center. But the sideways component stays approximately constant if the path is nearly a straight line.)

If the alpha particle initially has momentum p , for small deflections the angle of deflection (in radians) is given by Δ p / p , where Δ p is the sideways momentum resulting from the electrically repulsive force of the positive sphere of charge.

A good estimate of the sideways deflection is given by taking the alpha to experience the surface force given above for a time interval equal to the time it takes the alpha to cross the atom—say, a distance 2 r 0 . (The force felt when outside the ball of charge is much smaller: it drops away as the inverse square, but at an angle that makes it effectively inverse cube. It can be shown to make only a small contribution.)

Note that since the alpha particle has mass 6.7x10 -27 kg, from F = m a , the electric force at the atomic surface above will give it a sideways acceleration of 5.4x10 20 meters per sec per sec (compare g = 10 !). But the force doesn't have long to act—the alpha is moving at 1.6x10 7 meters per second. So the time available for the force to act is the time interval a particle needs to cross an atom if the particle gets from New York to Australia in one second.

So the transit time for the alpha across the plum pudding atom is:

t 0 = 2 r 0 / v = 2 × 10 10 / 1.6 × 10 7 = 1.25 × 10 − 17 seconds .

Now, the magnitude of the total sideways velocity picked up on crossing the atom is the sideways acceleration multiplied by the time,

1.25 × 10 − 17 × 5.4 × 10 20 = 6750 m /sec .

This is a few ten-thousandths of the alpha's forward speed , so there is only a very tiny deflection . Even if the alpha hit 400 atoms in succession and they all deflected it the same way, an astronomically improbable event, the deflection would only be of order a degree. Therefore, the observed deflection through ninety degrees and more was completely inexplicable using Thomson's pudding model!

Emergence of the Nucleus

Rutherford pondered the problem for some months. He had been a believer in his former boss's pudding model, but he eventually decided there was simply no way it could generate the strength of electric field necessary to deflect the fast moving alphas. Yet it was difficult to credit there was much more positive charge around than that necessary to compensate for the electrons, and it was pretty well established that there were not more than a hundred or so electrons (we used 79, the correct value—that was not known exactly until a little later). The electric field from a sphere of charge reaches its maximum on the surface, as discussed above. Therefore, for a given charge, assumed spherically distributed, the only way to get a stronger field is to compress it into a smaller sphere . Rutherford concluded that he could only explain the large alpha deflections if the positive charge, and most of the mass of the atom, was in a sphere much smaller than the atom itself .

It is not difficult to estimate from the above discussion how small such a nucleus would have to be to give a substantial deflection. We found a sphere of radius 10 -10 meters gave a deflection of about 4x10 -4 radians. We need to increase this deflection by a factor of a few thousand. On decreasing the radius of the sphere of positive charge, the force at the surface increases as the inverse radius squared . On the other hand, the time over which the alpha experiences the sideways force decreases as the radius.

The total deflection , then, proportional to the product of force and time, increases as the inverse of the radius . This forces the conclusion that the positive charge is in a sphere of radius certainly less than 10 -13 meters, provided all the observed scattering is caused by one encounter with a nucleus.

Animation of scattering from a nuclear atom here !

Rutherford decided that the observed scattering was in fact from a single nucleus. He argued as follows: since the foil is only 400 atoms thick, it is difficult to see how ninety degree scatterings could arise unless the scattering by a single nucleus was at least one degree, say 100 times that predicted by the Thomson model. This would imply that the nucleus had a radius at most one-hundredth that of the atom, and therefore presented a target area for one-degree scattering (or more) to the incoming alphas only one ten-thousandth that of the atom. (In particle physics jargon, this target area is called the scattering cross section .) If an alpha goes through 400 layers of atoms, and in each layer it has a chance of one in ten thousand of getting close enough to the nucleus for a one-degree scatter, this is unlikely to happen twice. It follows that almost certainly only one scattering takes place. It then follows that all ninety or more degrees of scattering must be a single event, so the nucleus must be even smaller than one hundredth the radius of the atom -- it must be less than 10 -13 meters, as stated above.

Seeing the Nucleus

Having decided that the observed scattering of the alphas came from single encounters with nuclei, and assuming that the scattering force was just the electrostatic repulsion, Rutherford realized maybe just scaling down the radius in the plum pudding analysis given above wasn't quite right. Maybe the nucleus was so small that the alpha particle didn't even touch it. If that were the case, the alpha particle's entire trajectory was determined by a force law of inverse square repulsion, and could be analyzed precisely mathematically by the techniques already well-known to astronomers for finding paths of planets under inverse square attraction.

It turns out that the alpha will follow a hyperbolic path (see the animation). Imagine an alpha coming in along an almost straight line path, the perpendicular distance of the nucleus from this line is called the impact parameter (how close to the center the alpha particle would pass if the repulsion were switched off). The standard planetary math is enough to find the angle at which the alpha comes out (the scattering angle), given the impact parameter and speed. Although not exactly a hot shot theorist, Rutherford managed to figure this out after a few weeks.

The incoming stream of alphas all have the same velocity (including direction) , but random impact parameters: we assume the beam intensity doesn't vary much in the perpendicular direction, certainly on an atomic scale, so we average over impact parameters (with a factor 2 π p d p for the annular region p , p + d p ).

The bottom line is that for a nucleus of charge Z , and incident alpha particles of mass m and speed v , the rate of scattering to a point on the screen corresponding to a scattering angle of θ (angle between incident velocity and final velocity of alpha) is proportional to:

scattering into small area at θ ∝ ( 1 4 π ε 0 ⋅ Z e 2 m v 2 ) 2 ⋅ 1 sin 4 ( θ / 2 ) .

Analysis of the hundred thousand or more scattering events recorded for the alphas on gold fully confirmed the angular dependence predicted by the above analysis.

Modeling the Scattering

To visualize the path of the alpha in such a scattering, Rutherford "had a model made, a heavy electromagnet suspended as a pendulum on thirty feet of wire that grazed the face of another electromagnet set on a table. With the two grazing faces matched in polarity and therefore repelling each other, the pendulum was deflected" into a hyperbolic path.(Rhodes, page 50)

But it didn't work for Aluminum...

On replacing the gold foil by aluminum foil (some years later), it turned out that small angle scattering obeyed the above law, but large angle scattering didn't. Rutherford correctly deduced that in the large angle scattering, which corresponded to closer approach to the nucleus, the alpha was actually hitting the nucleus. This meant that the size of the nucleus could be worked out by finding the maximum angle for which the inverse square scattering formula worked, and finding how close to the center of the nucleus such an alpha came. Rutherford estimated the radius of the aluminum nucleus to be about 10 -14 meters.

The Beginnings of Nuclear Physics

The First World War lasted from 1914 to 1918. Geiger and Marsden were both at the Western front, on opposite sides. Rutherford had a large water tank installed on the ground floor of the building in Manchester, to carry out research on defense against submarine attack. Nevertheless, occasional research on alpha scattering continued. Scattering from heavy nuclei was fully accounted for by the electrostatic repulsion, so Rutherford concentrated on light nuclei, including hydrogen and nitrogen. In 1919, Rutherford established that an alpha impinging on a nitrogen nucleus can cause a hydrogen atom to appear! Newspaper headlines blared that Rutherford had "split the atom". (Rhodes, page 137)

Shortly after that experiment, Rutherford moved back to Cambridge to succeed J. J. Thomson as head of the Cavendish laboratory, working with one of his former students, James Chadwick , who had spent the war years interned in Germany. They discovered many unusual effects with alpha scattering from light nuclei. In 1921, Chadwick and co-author Bieler wrote: "The present experiments do not seem to throw any light on the nature of the law of variation of the forces at the seat of an electric charge, but merely show that the forces are of great intensity … It is our task to find some field of force which will reproduce these effects." I took this quote from Pais, page 240, who goes on to say that he considers this 1921 statement as marking the birth of the strong interactions.

In fact, Rutherford was beginning to focus his attention on the actual construction of the nucleus and the alpha particle. He coined the word "proton" to describe the hydrogen nucleus, it first appeared in print in 1920 (Pais). At first, he thought the alpha must be made up of four of these protons somehow bound together by having two electrons in the middle—this would get the mass and charge right, but of course nobody could construct a plausible electrostatic configuration. Then he had the idea that maybe there was a special very tightly bound state of a proton and an electron, much smaller than an atom. By 1924, he and Chadwick were discussing how to detect this neutron. It wasn't going to be easy—it probably wouldn't leave much of a track in a cloud chamber. In fact, Chadwick did discover the neutron, but not until 1932, and it wasn't much like their imagined proton-electron bound state. But it did usher in the modern era in nuclear physics.

If you wanted to know more about a dark room that you were unable to go into, you might be able to get an idea of its size and contents by throwing balls into the room and considering their behaviour (the sort of task you might get in an Adventure game made for the computer). you would be able to tell where obstacles were and roughly the size of the room.

Physicists do the same kind of thing when 'lookling at' tiny objects such as atoms. In 1911 Rutherford wanted to find out more about the structure of the atom so he set two of his research students (Geiger and Marsden) the task of bombarding gold atoms with alpha particles, gathering data as to what happened to the 'missiles' and making deductions about the atom's structure from that data. In those days they did not have particle accelerators providing a ready supply of protons or neutrons so they had to use something that could be used as a natural probe - alpha particles were ideal.

for an animation of this experiment

).To achieve this we must use a absorbs all of the alpha particles except those travelling in one direction - unless they are travelling parallel to the sides of the outlet of the collimator they will impact on the side and be absorbed - so only those travelling parallel to each other and in one direction get through!

' - that made angles of greater than 90 with their original trajectory. Rutherford was amazed, he said, 'It was quite the most incredible event that has happened to me in my life! It was almost as incredible as if you fired a 15" shell at a piece of tissue paper and it came back and hit you!'

. This gives an indication of the tedious work involved in taking a checking the results! They would have to be repeated many times to be sure they were not due to anomalies! Would you have been tempted to discount totally inexplicable results and give your professor a set of results he expected? Or would you, like Geiger and Marsden, repeat and repeat and report the strange findings to Rutherford so that a whole new model of the atom could be developed? It is the careful (and honest!) investigation into anomalies that often lead to new theories.

When Rutherford mathematically investigated the results he proposed a model that explained the results that Geiger and Marsden obtained.

The fact that the vast majority of the alpha particles got straight through led Rutherford to propose that the atom was composed primarily of empty space.

The fact that occurred in 1 in 8000 alpha particles indicated that there was a:

(that was why so few were affected)

(meaning containing lots of mass - he knew the electrons had very little mass and the fact that all of the positive charges were concentrated into a small area meant that the mass was concentrated there too)

(because it repelled the alpha particles) nucleus in the centre of the atom (neutrons had not been discovered at that time - so he made no mention of them!).

So his picture was one of the atom being like the solar system - the sun being the nucleus (taking a very small proportion of the volume of the solar system but being the vast bulk of the mass in it!) and the electrons being like the planets orbiting the 'sun'.

This model was later amended by Bohr (to take into account a couple of points that Rutherford's atom did not fully explain - like the motion of the electrons and the orbital paths that could explain what the Chemists understood of electron behaviour in bonding) to make the model of the atom that you are taught at GCSE but it was still a magnificent advance to our understanding of atomic structure.

The observations made in 1911 by Geiger and Masden carrying out the experiment for Rutherford were a fatal blow to the . J.J. Thomson had put forward this picture of what an atom was like in 1906 and it was accepted scientific theory in 1911.

It took Rutherford a number of months before he finally decided that the only way his results made sense was if the charge was concentrated in a nucleus - he was the first to put forward such an idea... and let's face it, the idea seemed counter-intuitive - who would think the atom was mainly empty space? He had to be sure his calculations were correct before he put his idea forward to the scientific community.

Once he presented his ideas and explained how he got the results that led to them scientists all over the world repeated his experiment to verify the results and checked his mathematics to see whether his theories were sound.

We now know that Rutherford was right, but this was a revolutionary suggestion at the time and is a good example of how precise, repeatable experimentation can lead to us changing our thinking about how the Universe works - and lead to better understanding.

If Geiger and Marsden had 'tinkered with' their results to make them do what 'made sense' and was expected we would have not made such a rapid advance in nuclear knowledge! Think of that the next time you are tempted to fabricate a repeat set of readings in class - or to get rid of an anomaly. The 1 in 8000 backscatter could have been seen as an anomaly, but Geiger and Marsden checked the 'anomalies' out - several times to verify they were nothing of the kind. If they had ignored them Rutherford would have been presented with a set of result that showed just what he expected to see... they would have been in accordance with current scientific thinking.

for a mathematical treatment of scattering - used to find the size of the nucleus

to see how .

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Experimental Evidence for the Structure of the Atom

George sivulka march 23, 2017, submitted as coursework for ph241 , stanford university, winter 2017, introduction.

A three-dimensional view of an apparatus similar to Geiger and Marsden's final cylindrical iteration, clearly showing the scattering of alpha particles by gold foil. (Source: )

The Rutherford Gold Foil Experiment offered the first experimental evidence that led to the discovery of the nucleus of the atom as a small, dense, and positively charged atomic core. Also known as the Geiger-Marsden Experiments, the discovery actually involved a series of experiments performed by Hans Geiger and Ernest Marsden under Ernest Rutherford. With Geiger and Marsden's experimental evidence, Rutherford deduced a model of the atom, discovering the atomic nucleus. His "Rutherford Model", outlining a tiny positively charged atomic center surrounded by orbiting electrons, was a pivotal scientific discovery revealing the structure of the atoms that comprise all the matter in the universe.

The experimental evidence behind the discovery involved the scattering of a particle beam after passing through a thin gold foil obstruction. The particles used for the experiment - alpha particles - are positive, dense, and can be emitted by a radioactive source. Ernest Rutherford discovered the alpha particle as a positive radioactive emission in 1899, and deduced its charge and mass properties in 1913 by analyzing the charge it induced in the air around it. [1] As these alpha particles have a significant positive charge, any significant potential interference would have to be caused by a large concentration of electrostatic force somewhere in the structure of the atom. [2]

Previous Model of the Atom

A comparison between J.J. Thompson's "plum pudding" atomic model and the Rutherford model and its nucleus. Alpha particles and their scattering or lack thereof are depicted by the paths of the black arrows. (Source: )

The scattering of an alpha particle beam should have been impossible according to the accepted model of the atom at the time. This model, outlined by Lord Kelvin and expanded upon by J. J. Thompson following his discovery of the electron, held that atoms were comprised of a sphere of positive electric charge dotted by the presence of negatively charged electrons. [3] Describing an atomic model similar to "plum pudding," it was assumed that electrons were distributed throughout this positive charge field, like plums distributed in the dessert. However, this plum pudding model lacked the presence of any significant concentration of electromagnetic force that could tangibly affect any alpha particles passing through atoms. As such, alpha particles should show no signs of scattering when passing through thin matter. [4] (see Fig. 2)

The Geiger Marsden Experiments

Testing this accepted theory, Hans Geiger and Ernest Marsden discovered that atoms indeed scattered alpha particles, a experimental result completely contrary to Thompson's model of the atom. In 1908, the first paper of the series of experiments was published, outlining the apparatus used to determine this scattering and the scattering results at small angles. Geiger constructed a two meter long glass tube, capped off on one end by radium source of alpha particles and on the other end by a phosphorescent screen that emitted light when hit by a particle. (see Fig. 3) Alpha particles traveled down the length of the tube, through a slit in the middle and hit the screen detector, producing scintillations of light that marked their point of incidence. Geiger noted that "in a good vacuum, hardly and scintillations were observed outside of the geometric image of the slit, "while when the slit was covered by gold leaf, the area of the observed scintillations was much broader and "the difference in distribution could be noted with the naked eye." [5]

The schematics for the original two meter long tube that Geiger constructed and used to first detect the scattering of alpha particles by the atomic nucleus. At the point labeled R is the radon particle emission source, and Z the detector screen. (Source: )

On Rutherford's request, Geiger and Marsden continued to test for scattering at larger angles and under different experimental parameters, collecting the data that enabled Rutherford to further his own conclusions about the nature of the nucleus. By 1909, Geiger and Marsden showed the reflection of alpha particles at angles greater than 90 degrees by angling the alpha particle source towards a foil sheet reflector that then would theoretically reflect incident particles at the detection screen. Separating the particle source and the detector screen by a lead barrier to reduce stray emission, they noted that 1 in every 8000 alpha particles indeed reflected at the obtuse angles required by the reflection of metal sheet and onto the screen on the other side. [6] Moreover, in 1910, Geiger improved the design of his first vacuum tube experiment, making it easier to measure deflection distance, vary foil types and thicknesses, and adjust the alpha particle stream' velocity with mica and aluminum obstructions. Here he discovered that both thicker foil and foils made of elements of increased atomic weight resulted in an increased most probable scattering angle. Additionally, he confirmed that the probability for an angle of reflection greater than 90 degrees was "vanishingly small" and noted that increased particle velocity decreased the most probably scattering angle. [7]

Rutherford's Atom

Backed by this experimental evidence, Rutherford outlined his model of the atom's structure, reasoning that as atoms clearly scattered incident alpha particles, the structure contained a much larger electrostatic force than earlier anticipated; as large angle scattering was a rare occurrence, the electrostatic charge source was only contained within a fraction of the total volume of the atom. As he concludes this reasoning with the "simplest explanation" in his 1911 paper, the "atom contains a central charge distributed through a very small volume" and "the large single deflexions are due to the central charge as a whole." In fact, he mathematically modeled the scattering patterns predicted by this model with this small central "nucleus" to be a point charge. Geiger and Marsden later experimentally verified each of the relationships predicted in Rutherford's mathematical model with techniques and scattering apparatuses that improved upon their prior work, confirming Rutherford's atomic structure. [4, 8, 9] (see Fig. 1)

With the experimentally analyzed nature of deflection of alpha rays by thin gold foil, the truth outlining the structure of the atom falls into place. Though later slightly corrected by Quantum Mechanics effects, the understanding of the structure of the the atom today almost entirely follows form Rutherford's conclusions on the Geiger and Marsden experiments. This landmark discovery fundamentally furthered all fields of science, forever changing mankind's understanding of the world around us.

© George Sivulka. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

[1] E. Rutherford, "Uranium Radiation and the Electrical Conduction Produced By It," Philos. Mag. 47 , 109 (1899).

[2] E. Rutherford, "The Structure of the Atom," Philos. Mag. 27 , 488 (1914).

[3] J. J. Thomson, "On the Structure of the Atom: an Investigation of the Stability and Periods of Oscillation of a Number of Corpuscles Arranged at Equal Intervals Around the Circumference of a Circle; with Application of the Results to the Theory of Atomic Structure," Philos. Mag. 7 , 237 (1904).

[4] E. Rutherford, "The Scattering of α and β Particles by Matter and the Structure of the Atom," Philos. Mag. 21 , 669 (1911).

[5] H. Geiger, "On the Scattering of the α Particles by Matter," Proc. R. Soc. A 81 , 174 (1908).

[6] H. Geiger and E. Marsden, "On a Diffuse Reflection of the α-Particles," Proc. R. Soc. A 82 , 495 (1909).

[7] H. Geiger, "The Scattering of the α Particles by Matter," Proc. R. Soc. A 83 , 492 (1910).

[8] E. Rutherford, "The Origin of α and β Rays From Radioactive Substances," Philos. Mag. 24 , 453 (1912).

[9] H. Geiger and E. Marsden, "The Laws of Deflexion of α Particles Through Large Angles," Philos. Mag. 25 , 604 (1913).

PhysicsOpenLab Modern DIY Physics Laboratory for Science Enthusiasts

The rutherford-geiger-marsden experiment.

April 11, 2017 Alpha Spectroscopy , English Posts 84,364 Views

What made by Rutherford and his assistants Geiger and Marsden is perhaps one of the most important experiments of nuclear physics.

The experiments were performed between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester.

In the experiment, Rutherford sent a beam of alpha particles (helium nuclei) emitted from a radioactive source against a thin gold foil (the thickness of about 0.0004 mm, corresponding to about 1000 atoms).

Surrounding the gold foil it was placed a zinc sulfide screen that would show a small flash of light when hit by a scattered alpha particle. The idea was to determine the structure of the atom and understand if it were what supposed by Thomson (atom without a nucleus, also known as pudding model ) or if there was something different.

In particular, if the atom had an internal nucleus separated from external electrons, then they would have been able to observe events, or particles, with large angle of deviation . Obtained, actually, these results, the New Zealand physicist concluded that the atom was formed by a small and compact nucleus , but with high charge density, surrounded by an electron cloud. In the image below it is depicted the interaction of the alpha particles beam with the nuclei of the thin gold foil; one can see how the majority of the particles passes undisturbed, or with small angles of deflection, through the “empty” atom, some particles, however, passing close to the nucleus are diverted with a high angle or even bounced backwards.

The interaction between an alpha particle and the nucleus (elastic collision) is also known as Coulomb scattering , because the interaction in the collision is due to the Coulomb force. In the diagram below it is shown the detail of the interaction between an alpha particle and the nucleus of an atom.

Experimental Setup

In the PhysicsOpenLab “laboratory” we tried to replicate the famous Rutherford experiment. With the equipment already used in alpha spectroscopy we built a setup based on an alpha solid-state detector , a 0.9 μCi Am 241 source and a gold foil as a scatterer. In these post we describe the equipment used : Alpha Spectrometer , Gold Leaf Thickness . The main purpose is not to make precision measurements but to make a qualitative assessment of the scattering as a function of deflection. The images below show the experimental setup:

The alpha source is actually 0.9 μCi of Am 241 (from smoke detector) which emits alpha particles with energy of 5.4 MeV. The alpha particle beam is collimated by a simple hole in a wooden screen. Source and collimator are fixed on a arm free to rotate around a pivot, which hosts the gold foil that acts as a scatterer. The whole is placed inside a sealed box that acts as a vacuum chamber with the help of an ordinary oil rotary vacuum pump. The images below show the “vacuum chamber” and the electronic part for amplification and acquisition connected to the PC for counting events.

Linear Scale :

Semilog Scale

The results obtained in our experiment approach, albeit with obvious limitations, to the expected theoretical results, represented in the following graph:

For completeness, we report also at the side the formula that describes the distribution of the number of the counted particles in function of the scattering angle. Interestingly, this depends on the power of two the atomic number of the target and is inversely proportional to the fourth power of the sin (θ/2).

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KC761B: the new Gamma Spectrometer from DEEPACE

Abstract: in this article, we present an interesting new apparatus dedicated to gamma spectrometry and dosimetry measurements. It is a device based on a CsI(Tl) scintillator coupled to solid-state photomultipliers: SiPM. In addition to the scintillation sensor, the instrument has a PIN diode sensitive to beta radiation.

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Chapter 1: Matter in our Surroundings

Matter is Made of Tiny Particles
Why Solids, Liquids and Gases Have Different Properties
Classification of Matter
Brownian Movement
States of Matter: Solid, Liquid, Gas and Plasma
Evaporation
Effects of Relative Humidity and Wind Speed
How Does Evaporation Cause Cooling?
Effect of Change of Temperature
Melting Point
What is Vaporization?
Condensation
Effects of Change of Pressure
Difference between Rigidity and Fluidity of Matter
Prove That Liquids have No fixed Shape but have a Fixed Volume
Diffusion in Solids, Liquids, and Gases
What is the Unit of Temperature?
What is the Relationship Between Celsius and Kelvin Scale of Temperature?
Liquification of Gases
How to demonstrate the Presence of Water Vapour in Air?
What is Plasma and Bose-Einstein Condensate?

Chapter 2: Is Matter Around Us Pure?

Solution: Properties of Solution
Saturated and Unsaturated Solutions
Concentration of a Solution
Suspensions
How will you distinguish a Colloid from a Solution?
Classification of Colloids
Tyndall Effect
Separation of Mixtures
How to separate a Mixture of Two Solids?
Separation by a suitable solvent
Separation of Mixtures using Sublimation and Magnets
How to Separate a Mixture of a Solid and a Liquid?
Filtration: Definition, Process, Diagram and Examples
Water Purification
Centrifugation
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Difference Between Homogeneous and Heterogeneous Mixture
Difference Between Compound and Mixture
Factors affecting Solubility
Separation by Evaporation
Crystallization
Chromatography
Distillation
Separation of Mixtures of Two or More Liquids
Fractional Distillation
Pure and Impure Substances
What is an Element?
Metals, Non-Metals and Metalloids
Properties of Metals and Non-Metals

Chapter 3: Atoms and Molecules

Laws of Chemical Combination
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Verification of the Law of Conservation of Mass in a Chemical Reaction
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What is Atom?
Atomic Mass
How Do Atoms Exist?
Cations vs Anions
What are Ionic Compounds?
What are Monovalent Ions?
What are Divalent Ions?
Trivalent Ions - Cations and Anions
Polyatomic Ions
Formulas of Ionic Compounds
Chemical Formula of Common Compounds
Molecular Mass
Mole Concept
Problems Based on Mole Concepts
Dalton's Atomic Theory
Drawbacks of Dalton's Atomic Theory
Significance of the Symbol of Elements
Difference Between Molecules and Compounds
How to Calculate Valency of Radicals?
What is the Significance of the Formula of a Substance?
Gram Atomic and Gram Molecular Mass

Chapter 4: Structure of the Atom

Charged Particles in Matter
Thomson's Atomic Model

Rutherford Atomic Model

Drawbacks of Rutherford's Atomic Model
Bohr's Model of an Atom
Valence Electrons
Mass Number
Relation Between Mass Number and Atomic Number
Why do all the Isotopes of an Element have similar Chemical Properties?
Why Isotopes have different Physical Properties?
What is Fractional Atomic Mass?
Radioactive Isotopes
Discovery of Electrons
What is a Proton?
Rutherford's Alpha Scattering Experiment
Atomic Nucleus
How did Neil Bohr explained the Stability of Atom?
Electron Configuration
Potassium and Calcium - Atomic Structure, Chemical Properties, Uses
What is meant by Chemical Combination?
Difference between Electrovalency and Covalency

Rutherford’s Alpha Scattering Experiment

Rutherford’s Alpha Scattering Experiment is the fundamental experiment done by Earnest Rutherford’s Alpha Scattering Experiment that gives the fundamental about the structure of the atom. Rutherford in his experiment directed high-energy streams of α-particles from a radioactive source at a thin sheet (100 nm thickness) of gold. Then the deflection of these alpha particles tells us about the structure of atoms.

In this article, we will study about constituents of atoms, Rutherford’s Alpha Scattering Experiment,

What are Constituents of an Atom?

An atom consists of Electrons, Protons, and Neutrons are the fundamental particles or sub-atomic particles that build the structure of an atom. Let us understand each term.

Electron: In 1897, J. J. Thomson discovered negatively charged particles towards the anode, these rays are emitted by the cathode in a cathode ray experiment. Then these negatively charged particles are proposed as Electrons .
Protons: In 1886, Ernest Goldstein discovered that an anode emitted positively charged particles with a different condition in the same tube, known as Canal rays or as Protons .
Neutrons: A subatomic particle with no charge and a mass equivalent to protons in the nucleus of all atoms was discovered by J. Chadwick. These neutrally charged particles are termed Neutrons .

The image added below shows the structure of an atom.

Learn more about, Atomic Structure

Structure of Atom

Isotopes are the elements that have the same atomic number but different mass. e.g. Isotopes of the Hydrogen atoms are Protium ( 1 H 1 ), Deuterium ( 2 H 1 ) and Tritium( 3 H 1 ). Isotopes of the Carbon atoms are 12 C 6 , 13 C 6 , 14 C 6 .

Isobars are the elements that have different atomic number but have same mass number. e.g. 19 K 40 , 18 Ar 40 , 20 Ca 40 , here all the elements having same mass number hence they are isobars.

He conduct an experiment by bombarding alpha particles into a thin sheet of gold and then notices their interaction with the gold foil and trajectory or path followed by these particles.

In the experiment, Rutherford passes very high streams of alpha-particles from a radioactive source i.e. alpha-particle emitter, at a thin sheet of100 nm thickness of gold. In order to examine the deflection produced by the alpha particles, he placed a screen of fluorescent zinc sulphide around the thin gold foil. Rutherford made certain observations that oppose Thomson’s atomic model.

Observations of Rutherford’s Alpha Scattering Experiment

The observations of Rutherford’s Alpha Scattering Experiment are:

First, he observe that most of the α-particles that are bombarded towards the gold sheet pass away the foil without any deflection, and hence it shows most of the space is empty.
Out of all, some of the α-particles were deflected through the gold sheet by very small angles, and hence it shows the positive charge in an atom is non-uniformly distributed. The positive charge is concentrated in a very small volume in an atom.
Very few of the alpha-particles(1-2%) were deflected back, i.e. only a very less amount of α-particles had nearly 180° angle of deflection. this shows that the volume occupied by the positively charged particles is very small as compared to the total volume of an atom.

Rutherford proposed the atomic structure of elements, on the basis of his experiment. According to Rutherford’s atomic model:

Positively charged particle was concentrated in an extremely small volume and most of the mass of an atom was also in that volume. He called this a nucleus of an atom.
Rutherford proposed that there is negatively charged electrons around the nucleus of an atom. the electron surrounding the nucleus revolves around it in a circular path with very high speed. He named orbits to these circular paths.
Nucleus being a densely concentrated mass of positively charged particles and electrons being negatively charged are held together by a strong force of attraction called electrostatic forces of attraction.

Learn about, Rutherford Atomic Model

Limitations of Rutherford Atomic Model

The Rutherford atomic model is failed to explain certain things.

According to Maxwell, an electron revolving around the nucleus should emit electromagnetic radiation due to accelerated charged particles emit electromagnetic radiation. but Rutherford model says that the electrons revolve around the nucleus in fixed paths called orbits. The radiation would carry energy from the motion which led to the shrinking of orbit. Ultimately electrons would collapse inside the nucleus.
As per the Rutherford model, calculations have shown that an electron would collapse in the nucleus in less than 10 -8 seconds. So Rutherford model has created a high contradiction with Maxwell’s theory and Rutherford later could not explain the stability of an atom.
Rutherford also did not describe the arrangement of electrons in the orbit as one of the other drawbacks of his model.

Regardless of seeing the early atomic models were inaccurate and failed to explain certain experimental results, they were the base for future developments in the world of quantum mechanics.

Sample Questions on Rutherford’s Alpha Scattering Experiment

Some sample questions on Rutherford’s Alpha Scattering Experiment is,

Q1: Represent Element ‘X’ which contains 15 electrons and 16 neutrons.

Atomic number of element = No. of electron = 15 Mass number of element = no. of electrons + no. of neutrons = 15 + 16 = 31 Correct representation of element X is 31 X 15 .

Q2: Name particle and give its location in the atom which has no charge and has a mass nearly equal to that of a proton.

The particle which has no charge and has a mass nearly equal to that of a proton is a neutron and it is present in the nucleus of the atom.

Q3: An atom has both electron attribute negative charge and protons attribute positive charge but why there is no charge?

Positive and negative charges of protons and electrons are equal in magnitude, they cancel the effect of each other. So, the atom as a whole is electrically neutral.

Q4: What is Valency of Sodium Atom (Na)?

The atomic number of sodium = 11. Electronic configuration (2, 8, 1). By losing one electron it gains stability hence its valency is 1.

Q5: Which property do the following pairs show? 209 X 84 and 210 X 84

Atomic number of X is the same hence the pair shows an isotopic property. So, 209 X 84 and 210 X 84 are isotopes.

Dalton’s Atomic Theory Thomson’s Atomic Model Quantum Numbers

Rutherford’s Alpha Scattering Experiment FAQs

What is name of atom which has one electron, one proton and no neutron.

Atom with one electron, one proton and no neutron is Hydrogen, ( 1 H 1 ).

What is Ground State of an Atom?

It is the state of an atom where all the electrons in the atom are in their lowest energy state or levels is called the ground state.

What was Rutherford’s Alpha Particle Scattering Experiment?

Rutherford’s Alpha Particle Scattering Experiment is the fundamental experiment that gives the basic structure of an atom.

What was Conclusion of Rutherford’s Alpha Scattering Experiment?

Conclusion of Rutherford’s Alpha Scattering Experiment is, Atom is largely empty and has a heavy positive-charged body at the center called the nucleus. The central nucleus is positively charged and the negatively-charged electrons revolve around the nucleus.

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Alpha-Particle Scattering and Rutherford’s Nuclear Model of Atom

In 1911, Rutherford, along with his assistants, H. Geiger and E. Marsden, performed the Alpha Particle scattering experiment , which led to the birth of the ‘nuclear model of an atom ’ – a major step towards how we see the atom today.

Browse more Topics under Atoms

Atomic Spectra
Bohr Model of the Hydrogen Atom

The Alpha Particle Scattering Experiment

They took a thin gold foil having a thickness of 2.1×10 -7 m and placed it in the centre of a rotatable detector made of zinc sulfide and a microscope. Then, they directed a beam of 5.5MeV alpha particles emitted from a radioactive source at the foil. Lead bricks collimated these alpha particles as they passed through them.

After hitting the foil, the scattering of these alpha particles could be studied by the brief flashes on the screen. Rutherford and his team expected to learn more about the structure of the atom from the results of this experiment.

Source: Wikipedia

Observations

Here is what they found:

Most of the alpha particles passed through the foil without suffering any collisions
Around 0.14% of the incident alpha particles scattered by more than 1 o
Around 1 in 8000 alpha particles deflected by more than 90 o

These observations led to many arguments and conclusions which laid down the structure of the nuclear model on an atom.

Conclusions and arguments

The results of this experiment were not in sync with the plum-pudding model of the atom as suggested by Thomson. Rutherford concluded that since alpha particles are positively charged, for them to be deflected back, they needed a large repelling force. He further argued that for this to happen, the positive charge of the atom needs to be concentrated in the centre, unlike scattered in the earlier accepted model.

Hence, when the incident alpha particle came very close to the positive mass in the centre of the atom, it would repel leading to a deflection. On the other hand, if it passes through at a fair distance from this mass, then there would be no deflection and it would simply pass through.

He then suggested the ‘nuclear model of an atom’ wherein the entire positive charge and most of the mass of the atom is concentrated in the nucleus. Also, the electrons are moving in orbits around the nucleus akin to the planets and the sun. Further, Rutherford also concluded from his experiments that the size of the nucleus is between 10 -15 and 10 -14 m.

According to Kinetic theory, the size of an atom is around 10 -10 m or around 10,000 to 100,000 times the size of the nucleus proposed by Rutherford. Hence, the distance of the electrons from the nucleus should be around 10,000 to 100,000 times the size of the nucleus.

This eventually implies that most of the atom is empty space and explains why most alpha particles went right through the foil. And, these particles are deflected or scattered through a large angle on coming close to the nucleus. Also, the electrons having negligible mass, do not affect the trajectory of these incident alpha particles.

Alpha Particle Trajectory

The trajectory traced by an alpha particle depends on the impact parameter of the collision. The impact parameter is simply the perpendicular distance of each alpha particle from the centre of the nucleus. Since in a beam all alpha particles have the same kinetic energy, the scattering of these particles depends solely on the impact parameter.

Hence, the particles with a small impact parameter or the particles closer to the nucleus, experience large angle of scattering. On the other hand, those with a large impact parameter suffer no deflection or scattering at all. Finally, those particles having ~zero impact parameter or a head-on collision with the nucleus rebound back.

Coming to the experiment, Rutherford and his team observed that a really small fraction of the incident alpha particles was rebounding back. Hence, only a small number of particles were colliding head-on with the nucleus. This, subsequently, led them to believe that the mass of the atom is concentrated in a very small volume.

Electron Orbits

In a nutshell, Rutherford’s nuclear model of the atom describes it as:

A small and positively charged nucleus at the centre
Surrounded by revolving electrons in their dynamically stable orbits

The centripetal force that keeps the electrons in their orbits is an outcome of:

The positively charged nucleus and
The negatively charged revolving electrons.

Solved Example for You

Question: Rutherford, Geiger and Marsden, directed a beam of alpha particles on a foil of which metal

Solution: Gold

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Rutherford Scattering ( AQA GCSE Physics )

Revision note.

Physics Project Lead

Rutherford Scattering

Alpha scattering.

Physicist, Ernest Rutherford was instructing two of his students, Hans Geiger and Ernest Marsden to carry out the experiment
They were directing a beam of alpha particles (He 2+ ions) at a thin gold foil
They expected the alpha particles to travel through the gold foil, and maybe change direction a small amount
Most of the alpha particles passed straight through the foil
Some of the alpha particles changed direction but continued through the foil
A few of the alpha particles bounced back off the gold foil
The bouncing back could not be explained by the Plum Pudding model, so a new model had to be created

rutherford-scattering, IGCSE & GCSE Physics revision notes

When alpha particles are fired at thin gold foil, most of them go straight through, some are deflected and a very small number bounce straight back

The Nuclear Model

Ernest Rutherford made different conclusions from the findings of the experiment
The table below describes the findings and conclusions of A, B and C from the image above:

Alpha Scattering Findings and Conclusions Table

Rutherford conclusions, downloadable IGCSE & GCSE Physics revision notes

Rutherford proposed the nuclear model of the atom
Nearly all of the mass of the atom is concentrated in the centre of the atom (in the nucleus)
The nucleus is positively charged
Negatively charged electrons orbit the nucleus at a distance
The nuclear model could explain experimental observations better than the Plum Pudding model

Nuclear model, downloadable IGCSE & GCSE Physics revision notes

The Nuclear model replaced the Plum Pudding model as it could better explain the observations of Rutherford’s Scattering Experiment

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What did Rutherford expect in his alpha scattering experiments? [closed]

At the time when Rutherford's gold foil experiment was performed, Thomson's plum pudding model was believed to be true (at least by Rutherford himself and his students).

With this model in mind Rutherford predicted that most of the alpha particles will be deflected by at most a fraction of a degree (sourced by this Wikipedia page ), but why?

In my opinion, since according to the plum pudding model the mass of an atom was assumed to be uniformly distributed and the atomic mass of gold is nearly 50 times larger than the mass of an alpha particle, and gold is solid, therefore much less intermolecular space will be present, so, most of the alpha particles should rebound or get deflected by a large angle.

experimental-chemistry

3 $\begingroup$ Perhaps see this answer to get started: chemistry.stackexchange.com/a/10392/79678 . Search for “Rutherford” here for more information. $\endgroup$ – Ed V Commented Jul 21, 2020 at 12:36
3 $\begingroup$ Strongly related (almost duplicate): chemistry.stackexchange.com/questions/106819/… $\endgroup$ – Nilay Ghosh Commented Jul 21, 2020 at 12:48
$\begingroup$ en.wikipedia.org/wiki/Rutherford_scattering $\endgroup$ – Buck Thorn ♦ Commented Jul 21, 2020 at 13:21
1 $\begingroup$ What is the alpha particle bouncing off of? What are the kinematics of that situation? Without using backspin, you don't get backscattering in pool/snooker because the balls are all the same mass. $\endgroup$ – Jon Custer Commented Jul 21, 2020 at 13:51
3 $\begingroup$ Further, given that Thompson's model was proposed after $\alpha$ backscattering was observed, it is clear that the assumption in the first sentence is not valid. $\endgroup$ – Jon Custer Commented Jul 21, 2020 at 14:35

The nucleus has a radius roughly 10⁴ times smaller than the size of the atom itself (imagine a sports ball in a stadium). That would mean that its volume were 10¹² smaller than the volume of an atom. Sure the gold nucleus is ~30 times as charged and is ~50 times heavier. But dilute that charge and mass by a factor of a trillion, and suddenly those don't seem so significant anymore.

That's the worst thing about those schematic diagrams that you see for the Rutherford experiment. They blow up the size of the nucleus to prove a point, but those diagrams are definitely "Not to Scale™".

$\begingroup$ I know that but I asked what led Rutherford think that Alpha particle should pass through the gold foil keeping in mind plum pudding model $\endgroup$ – Tushar Commented Jul 21, 2020 at 13:14
$\begingroup$ he expectesdthis i.e. before conducting the experiment $\endgroup$ – Tushar Commented Jul 21, 2020 at 13:15
$\begingroup$ That's what I'm saying to you. Rutherford is able to divide, so he can already estimate the density of matter and charge within the gold foil, and it's not that dense. $\endgroup$ – Zhe Commented Jul 21, 2020 at 13:59
2 $\begingroup$ Why is a question with 3 upvotes closed? Surely there was a known difference between an alpha particle and gold and this had to be in his mind. Ostensibly one conducts experiments to answer a question, What was the question? The 16in projectile analogy is foolish; shooting a pingpong ball at a pillow filled with pingpong balls and having it bounce back is more appropriate and more exciting. $\endgroup$ – jimchmst Commented Aug 29, 2023 at 21:00
$\begingroup$ @jimchmst I would expect that most ping pong ball collisions result in its bouncing back. That seems less than exciting. $\endgroup$ – Zhe Commented Aug 31, 2023 at 17:52

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You are here: Home > GCSE > Key ideas > Atomic structure > Development of the atomic model

Development of the atomic model

Aims of this page

After studying this page, you should be able to describe how the atomic model has changed over time, including:

Dalton’s simple model
Thomson’s plum pudding model
the Geiger-Marsden alpha scattering experiment
Rutherford’s nuclear model
Bohr’s shell model
the discovery of subatomic particles .

Dalton's model

John Dalton (1766 – 1844) was an English scientist who developed an atomic theory early in the 19th century. The technology did not exist then to investigate atoms in detail, so he imagined them as very tiny balls. Dalton also devised some ideas that are similar to scientific ideas today:

elements are made of atoms
the atoms of an element are all the same, but different from the atoms of other elements
atoms cannot be made, broken down or destroyed.

He also devised chemical ideas about compounds and reactions:

atoms of different elements join together in whole-number ratios to make compounds
atoms in substances are separated, joined or rearranged in chemical reactions.

The plum pudding model

J.J. Thomson (1856 – 1940) was a British physicist who discovered the electron in 1897. He knew that:

atoms contain electrons
electrons are negatively charged
atoms have no overall electric charge.

Joseph John Thomson was a talented scientist who began his university studies when he was just 14. Later in his career, Ernest Rutherford became one of his students.

Thomson devised a model of the atom in which electrons are embedded in a sphere of positive charge. This model became known as the plum pudding model because it resembled a type of dessert containing dried fruit.

Remember: at this time, no-one knew that an atom has a nucleus .

The alpha scattering experiment

Ernest Rutherford (1871 – 1937) was a New Zealand physicist who discovered alpha radiation. By 1907 he knew that alpha particles are positively charged helium ions . Rutherford wanted to find out more about their properties, so he asked his colleagues Hans Geiger and Ernest Marsden to carry out an experiment.

The scientists aimed a beam of alpha particles at very thin gold foil. The foil was surrounded by a fluorescent screen that flashed when an alpha particle hit it. Working in the dark, Geiger and Marsden recorded the numbers and positions of these flashes.

Atoms are neutral overall.

Positively charged ions form when atoms, or groups of atoms, lose electrons.

Negatively charged ions form when atoms, or groups of atoms, gain electrons.

Ernest Rutherford (1871 – 1937) was a New Zealand physicist who discovered alpha radiation. By 1907 he knew that alpha particles are positively charged helium ions. Rutherford wanted to find out more about their properties, so he asked his colleagues Hans Geiger and Ernest Marsden to carry out an experiment.

The table summarises the observations they made, and the explanations for these observations.

Observation	Explanation
Most particles passed straight through the foil	Atoms are mostly empty space
Some particles are deflected	Positively charged particles in atoms repelled alpha particles
A few particles are deflected by large angles	Atoms contain a relatively small object, with a relatively high mass and charge

All the alpha particles should have passed straight through the gold foil if the plum pudding model were correct. As this did not always happen, the alpha scattering experiment disproved the plum pudding model. Rutherford developed his nuclear model to explain the unexpected results.

Rutherford described the results as “almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

Rutherford's model

Rutherford developed his nuclear model in 1911. This showed:

a central, positively charged nucleus
electrons arranged in a cloud around the nucleus.

This was quickly modified to show electrons travelling around the nucleus, rather like planets around the Sun.

Bohr's model

Rutherford’s model had a problem. Orbiting electrons would lose energy and spiral down into the nucleus. Niels Bohr (1885 – 1962) was a Danish physicist whose model overcame this problem. His 1913 model was based on theoretical calculations, but the results of experiments confirmed them later. In the Bohr model:

electrons orbit the nucleus at different distances
each orbit has a particular energy
electrons can only move from one orbit to another by gaining or losing the difference in energy between the orbits.

The orbits are called shells or energy levels .

Subatomic particles

Remember that the existence of electrons was already known when the plum pudding model, and Rutherford’s and Bohr’s models were devised. The atomic model was developed further when the proton and neutron were discovered.

The table shows the locations of subatomic particles in atoms, their relative masses and relative charges.

	Proton	Neutron	Electron
Location	nucleus	nucleus	shells (energy levels)
Relative charge	+1	0	–1
Relative mass	1	1	⁄

The existence of these particles was suspected before they were discovered:

electron, 1897, Thomson
proton, 1917, Rutherford
neutron, 1932, James Chadwick

Protons, neutrons and electrons are called subatomic particles because they are smaller than atoms.

The relative mass of an electron may be described as zero or negligible in some GCSE specifications – check yours.

The GCSE model

Scientists have continued to develop the atomic model, but the one you need to know about is based on the Bohr model and subatomic particles. In this model:

there is a central nucleus containing protons and neutrons
electrons surround the nucleus in shells or energy levels
the nucleus contains most of the mass of an atom
the nucleus is very small compared to an atom.

Atoms are neutral overall because the number of protons is equal to the number of electrons. The number of neutrons affects the mass of an atom but not its charge.

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Published: 21 June 2024

Scattering theory

Information gets into the flow with wave scattering

Arthur Goetschy ORCID: orcid.org/0000-0002-2307-5422 1

Nature Physics ( 2024 ) Cite this article

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140 years ago, John Henry Poynting demonstrated “by what paths and according to what law” electromagnetic energy travels. Theory and experiment now also reveal how waves acquire information as they scatter through their environment.

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Poynting, J. H. Phil. Trans. R. Soc. Lond. 175 , 343–361 (1884).

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Hüpfl, J. et al. Nat. Phys. https://doi.org/10.1038/s41567-024-02519-8 (2024).

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Bouchet, D. et al. Nat. Phys. 17 , 564–568 (2021).

Giovannetti, V. et al. Nat. Photon. 5 , 222–229 (2011).

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COMMENTS

Rutherford scattering experiments
A replica of an apparatus used by Geiger and Marsden to measure alpha particle scattering in a 1913 experiment. The Rutherford scattering experiments were a landmark series of experiments by which scientists learned that every atom has a nucleus where all of its positive charge and most of its mass is concentrated. They deduced this after measuring how an alpha particle beam is scattered when ...
Atomic structure
In the experiment, positively charged alpha particles close alpha particle Subatomic particle comprising two protons and two neutrons (the same as a helium nucleus). were fired at thin gold foil ...
6.7.1 Alpha Particle Scattering Experiment
The Rutherford scattering experiment directed parallel beams of α-particles at gold foil. The observations were: Most of the α-particles went straight through the foil. The largest value of n will therefore be at small angles. Some of the α-particles were deflected through small angles. n drops quickly with increasing angle of deflection θ.
Rutherford model
The nucleus was postulated as small and dense to account for the scattering of alpha particles from thin gold foil, as observed in a series of experiments performed by undergraduate Ernest Marsden under the direction of Rutherford and German physicist Hans Geiger in 1909. A radioactive source emitting alpha particles (i.e., positively charged particles, identical to the helium atom nucleus and ...
What is the 'Gold Foil Experiment'? The Geiger-Marsden experiments
Bibliography. The Geiger-Marsden experiment, also called the gold foil experiment or the α-particle scattering experiments, refers to a series of early-20th-century experiments that gave ...
Rutherford's alpha scattering experiment
Evidence of back-scattering. It is sometimes asked why, if alpha radiation was expected to pass right through the gold foil, the alpha-scattering experiment was designed to allow the detector to be moved round through an angle of greater than 90°. Of course, if the plum pudding model was correct, back-scattering would not be expected.
Models of the atom
The scientists were very surprised when other things happened: most of the alpha particles did pass straight through the foil a small number of alpha particles were deflected by large angles (> 4 ...
Rutherford Scattering
Rutherford's alpha scattering experiments were the first experiments in which individual particles were systematically scattered and detected. This is now the standard operating procedure of particle physics. To minimize alpha loss by scattering from air molecules, the experiment was carried out in a fairly good vacuum, the metal box being ...
Rutherford scattering
Figure 1. In a cloud chamber, a 5.3 MeV alpha particle track from a lead-210 pin source near point 1 undergoes Rutherford scattering near point 2, deflecting by an angle of about 30°. It scatters once again near point 3, and finally comes to rest in the gas. The target nucleus in the chamber gas could have been a nitrogen, oxygen, carbon, or hydrogen nucleus.
PDF The Rutherford Scattering Experiment
The scattering foil is an annulus located coaxially with the -source and detector with inner and outer diameters, 46.0 and 56.7 mm respectively. The angle is determined by a ﬁxed distance from source to scattering foil. The scattering angle is varied by changing the distance from the scattering plane to the plane of the detector.
A Level Physics: Rutherford's gold foil alpha-scattering experiment
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In 1911, Rutherford and coworkers Hans Geiger and Ernest Marsden initiated a series of groundbreaking experiments that would completely change the accepted model of the atom. They bombarded very thin sheets of gold foil with fast moving alpha particles. Figure 3.4.2 3.4. 2 (a) The experimental setup for Rutherford's gold foil experiment: A ...
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Rutherford's Alpha Scattering Experiment. The results of this experiment were so astounding that they made Rutherford say, "It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you."
Experimental Evidence for the Structure of the Atom
As such, alpha particles should show no signs of scattering when passing through thin matter. [4] (see Fig. 2) The Geiger Marsden Experiments. Testing this accepted theory, Hans Geiger and Ernest Marsden discovered that atoms indeed scattered alpha particles, a experimental result completely contrary to Thompson's model of the atom.
The Rutherford-Geiger-Marsden Experiment
The experiments were performed between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester. In the experiment, Rutherford sent a beam of alpha particles (helium nuclei) emitted from a radioactive source against a thin gold foil (the thickness of ...
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Rutherford, along with his assistants - H. Geiger and E. Marsden - started performing experiments to study the structure of an atom. In 1911, they performed the Alpha particle scattering experiment, which led to the birth of the 'nuclear model of an atom' - a major step towards how we see the atom today. Figure 1. Source: Wikipedia.
4.1.7 Rutherford Scattering
Alpha Scattering. In 1909 a group of scientists were investigating the Plum Pudding model. Physicist, Ernest Rutherford was instructing two of his students, Hans Geiger and Ernest Marsden to carry out the experiment. They were directing a beam of alpha particles (He 2+ ions) at a thin gold foil. They expected the alpha particles to travel ...
What did Rutherford expect in his alpha scattering experiments?
In my opinion, since according to the plum pudding model the mass of an atom was assumed to be uniformly distributed and the atomic mass of gold is nearly 50 times larger than the mass of an alpha particle, and gold is solid, therefore much less intermolecular space will be present, so, most of the alpha particles should rebound or get ...
Development of the atomic model
As this did not always happen, the alpha scattering experiment disproved the plum pudding model. Rutherford developed his nuclear model to explain the unexpected results. Rutherford described the results as "almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."
Information gets into the flow with wave scattering
140 years ago, John Henry Poynting demonstrated "by what paths and according to what law" electromagnetic energy travels. Theory and experiment now also reveal how waves acquire information as ...
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Bacteria have developed a highly diverse battery of defense mechanisms to combat infections by phages, which collectively have been coined the "immune system" of bacteria (1, 2).Restriction modification and CRISPR-CAS systems, both of which target and cleave foreign nucleic acids, are most common, but studies over the past 5 years have identified multiple new phage-restricting systems with ...

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