alpha particle in rutherford experiment

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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.

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 .

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.

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 .

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3.4: Rutherford's Experiment- The Nuclear Model of the Atom

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Learning Objectives

• Describe Thomson's "plum pudding" model of the atom and the evidence for it.
• Describe Rutherford's gold foil experiment and explain how this experiment altered the "plum pudding" model.

The electron was discovered by J.J. Thomson in 1897. The existence of protons was also known, as was the fact that atoms were neutral in charge. Since the intact atom had no net charge and the electron and proton had opposite charges, the next step after the discovery of subatomic particles was to figure out how these particles were arranged in the atom. This is a difficult task because of the incredibly small size of the atom. Therefore, scientists set out to design a model of what they believed the atom could look like. The goal of each atomic model was to accurately represent all of the experimental evidence about atoms in the simplest way possible.

Following the discovery of the electron, J.J. Thomson developed what became known as the " plum pudding " model (Figure \(\PageIndex{1}\) ) in 1904. Plum pudding is an English dessert similar to a blueberry muffin. In Thomson's plum pudding model of the atom, the electrons were embedded in a uniform sphere of positive charge like blueberries stuck into a muffin. The positive matter was thought to be jelly-like or a thick soup. The electrons were somewhat mobile. As they got closer to the outer portion of the atom, the positive charge in the region was greater than the neighboring negative charges and the electron would be pulled back more toward the center region of the atom.

However, this model of the atom soon gave way to a new model developed by New Zealander Ernest Rutherford (1871 - 1937) about five years later. Thomson did still receive many honors during his lifetime, including being awarded the Nobel Prize in Physics in 1906 and a knighthood in 1908.

Atoms and Gold

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.

According to the accepted atomic model, in which an atom's mass and charge are uniformly distributed throughout the atom, the scientists expected that all of the alpha particles would pass through the gold foil with only a slight deflection or none at all. Surprisingly, as shown in Figure \(\PageIndex{2}\) ( while most of the alpha particles were indeed undeflected, a very small percentage (about 1 in 8000 particles) bounced off the gold foil at very large angles. Some were even redirected back toward the source. No prior knowledge had prepared them for this discovery. In a famous quote, Rutherford exclaimed that it was "as if you had fired a 15-inch [artillery] shell at a piece of tissue and it came back and hit you."

Rutherford needed to come up with an entirely new model of the atom in order to explain his results. Because the vast majority of the alpha particles had passed through the gold, he reasoned that most of the atom was empty space. In contrast, the particles that were highly deflected must have experienced a tremendously powerful force within the atom. He concluded that all of the positive charge and the majority of the mass of the atom must be concentrated in a very small space in the atom's interior, which he called the nucleus. The nucleus is the tiny, dense, central core of the atom and is composed of protons and neutrons.

Rutherford's atomic model became known as the nuclear model . In the nuclear atom, the protons and neutrons, which comprise nearly all of the mass of the atom, are located in the nucleus at the center of the atom. The electrons are distributed around the nucleus and occupy most of the volume of the atom. It is worth emphasizing just how small the nucleus is compared to the rest of the atom. If we could blow up an atom to be the size of a large professional football stadium, the nucleus would be about the size of a marble.

Rutherford's model proved to be an important step towards a full understanding of the atom. However, it did not completely address the nature of the electrons and the way in which they occupied the vast space around the nucleus. For this and other insights, Rutherford was awarded the Nobel Prize in Chemistry in 1908. Unfortunately, Rutherford would have preferred to receive the Nobel Prize in Physics because he considered physics superior to chemistry. In his opinion, “All science is either physics or stamp collecting.”

• The plum pudding model is an early attempt to show what an atom looks like.
• Bombardment of gold foil with alpha particles showed that some particles were deflected.
• The nuclear model of the atom consists of a small and dense positively charged interior surrounded by a cloud of electrons.

Contributors and Attributions

Marisa Alviar-Agnew  ( Sacramento City College )

Henry Agnew (UC Davis)

• TextMap: Chemistry - The Central Science

• Structure of Atom
• Rutherford Atomic Model And Its Limitations

Rutherford Atomic Model and Limitations

Define rutherford atomic model.

Rutherford Atomic Model – The plum pudding model given by J. J. Thomson failed to explain certain experimental results associated with the atomic structure of elements. Ernest Rutherford, a British scientist conducted an experiment and based on the observations of this experiment, he explained the atomic structure of elements and proposed Rutherford’s Atomic Model.

Table of Contents

• Rutherfords Alpha Scattering Experiment

Observations of Rutherford’s Alpha Scattering Experiment

Rutherford atomic model, limitations of rutherford atomic model, recommended videos, frequently asked questions – faqs, rutherford’s alpha scattering experiment.

Rutherford conducted an experiment by bombarding a thin sheet of gold with α-particles and then studied the trajectory of these particles after their interaction with the gold foil.

Rutherford, in his experiment, directed high energy streams of α-particles from a radioactive source at a thin sheet (100 nm thickness) of gold. In order to study the deflection caused to the α-particles, he placed a fluorescent zinc sulphide screen around the thin gold foil. Rutherford made certain observations that contradicted Thomson’s atomic model .

The observations made by Rutherford led him to conclude that:

• A major fraction of the α-particles bombarded towards the gold sheet passed through the sheet without any deflection, and hence most of the space in an atom is empty .
• Some of the α-particles were deflected by the gold sheet by very small angles, and hence the positive charge in an atom is not uniformly distributed . The positive charge in an atom is concentrated in a very small volume .
• Very few of the α-particles were deflected back, that is only a few α-particles had nearly 180 o angle of deflection. So the volume occupied by the positively charged particles in an atom is very small as compared to the total volume of an atom .

Based on the above observations and conclusions, Rutherford proposed the atomic structure of elements. According to the Rutherford atomic model:

• The positive charge and most of the mass of an atom is concentrated in an extremely small volume. He called this region of the atom as a nucleus.
• Rutherford’s model proposed that the negatively charged electrons surround the nucleus of an atom. He also claimed that the electrons surrounding the nucleus revolve around it with very high speed in circular paths. He named these circular paths as orbits.
• Electrons being negatively charged and nucleus being a densely concentrated mass of positively charged particles are held together by a strong electrostatic force of attraction.

Although the Rutherford atomic model was based on experimental observations, it failed to explain certain things.

• Rutherford proposed that the electrons revolve around the nucleus in fixed paths called orbits. According to Maxwell, accelerated charged particles emit electromagnetic radiations and hence an electron revolving around the nucleus should emit electromagnetic radiation. This radiation would carry energy from the motion of the electron which would come at the cost of shrinking of orbits. Ultimately the electrons would collapse in the nucleus. Calculations have shown that as per the Rutherford model, an electron would collapse into the nucleus in less than 10 -8 seconds. So the Rutherford model was not in accordance with Maxwell’s theory and could not explain the stability of an atom .
• One of the drawbacks of the Rutherford model was also that he did not say anything about the arrangement of electrons in an atom which made his theory incomplete.
• Although the early atomic models were inaccurate and failed to explain certain experimental results, they formed the base  for future developments in the world of quantum mechanics .

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The Gold Foil Experiment

Structure of Atom Class 11 Chemistry

Drawbacks of Rutherford Atomic Model

What was the speciality of Rutherford’s atomic model?

Rutherford was the first to determine the presence of a nucleus in an atom. He bombarded α-particles on a gold sheet, which made him encounter the presence of positively charged specie inside the atom.

What is Rutherford’s atomic model?

Rutherford proposed the atomic structure of elements. He explained that a positively charged particle is present inside the atom, and most of the mass of an atom is concentrated over there. He also stated that negatively charged particles rotate around the nucleus, and there is an electrostatic force of attraction between them.

What are the limitations of Rutherford’s atomic model?

Rutherford failed to explain the arrangement of electrons in an atom. Like Maxwell, he was unable to explain the stability of the atom.

What kind of experiment did Rutherford’s perform?

Rutherford performed an alpha scattering experiment. He bombarded α-particles on a gold sheet and then studied the trajectory of these α-particles.

What was the primary observation of Rutherford’s atomic model?

Rutherford observed that a microscopic positively charged particle is present inside the atom, and most of the mass of an atom is concentrated over there.

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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

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 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

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

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Alpha Particles and the Atom

Rutherford at manchester, 1907–1919.

Ernest Rutherford discovered the nucleus of the atom in 1911. We read this in textbooks and in popular writings. But what does that statement mean? Geographical discovery usually means that one sees a place for the first time. But can discovery be the same for a realm hidden from sight? One cannot see an atom in that sense. So this hints that perhaps the story of the discovery of the nucleus was more complicated. The story as it unfolded in Rutherford's lab at the University in Manchester revolved around real people. It involved frustrations and triumphs. It involved hard work and perplexity and inspiration.

Rutherford arrived in Manchester in the summer of 1907, months before the university's term began. He had been named Langworthy Professor of Physics, successor to Arthur Schuster (1851–1934), who retired at age 56 to recruit Rutherford. Schuster had built a modern physics building, hired Hans Geiger, Ph.D. (1882–1945) because of his experimental skill, and endowed a new position in mathematical physics to round out a full physics program. Rutherford entered the center of the physics world. Researchers came to him by the dozen.

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I found Rutherford's place very busy, hard working. But a very dirty place. Namely, Manchester is very foggy, foggy and smoky. And of course everywhere you see smoke there, everywhere the smoke. Now the technique used in Rutherford’s lab was to fit up an electroscope. You have to build it yourself of cocoa boxes, gold leaf and sulfur isolation. And you charge the electroscope by sealing wax which you rubbed on your trousers. So it was a very primitive technique. But of course also a microscope to read the electroscope. Now the microscope was fixed and then you were not supposed to touch it. And of course you were not supposed to clean it. So years went on without apparatus being cleaned. But apart from the shortcomings it was a very fine lab, nice rooms, and lots of people working there—able people.... I remember Moseley very well, with whom I was on very friendly terms. I will tell you later about his work. And Charles Darwin was there. He was lecturing in theoretical physics. And Russell, who later came to Oxford. An Italian, Rossi, did spectroscopic work. He showed that ionium and sodium have the same spectrum. And then Geiger was there. He was an assistant. And also an assistant named Makower, who died since. Geiger and Makower published a book together. And also a chap Robinson, who worked on beta rays. Gray, a New Zealand man. Marsden who came from Australia. Fajans who came from Germany. And Boltwood was there for a while. He came from Yale. Rutherford invited him in hope that Boltwood, a great chemist, would purify ionium, but he failed as many others.

Rutherford arrived with many research questions in mind. He was not done with the puzzles of the decay families of thorium, radium, etc., but he was passing much of this work to Boltwood, Hahn, and Soddy. Boltwood and Hahn both worked with Rutherford in Manchester, Boltwood in 1909–1910 and Hahn in 1907–1908. Rutherford was gradually turning his attention much more to the α (alpha), β (beta), and γ (gamma) rays themselves and to what they might reveal about the atom. That is, he was leaving radio-chemistry to others and turning to physics.

Rutherford's early team at Manchester included Geiger and William Kay (1879–1961), junior laboratory assistant since 1894. Rutherford promoted Kay to laboratory steward in 1908, to manage lab equipment and to aid him in his research. In 1957, Kay thought back to his youth with Rutherford in an interview. The language is quaint, but the description is as close to Rutherford's approach as we get. The questioner was Samuel Devons (1914–2006), who was one of Rutherford's last students in the 1930s.

[Devons] “When you were here [in Manchester], during this period... did Rutherford actually make any apparatus himself?” [Kay] “No, no, no, no. We used to, I used to set up nearly all his apparatus. You know, when he did his work, you know, oftener than not, he used to tell me and we did a rough experiment, re...” [D.] “Did he sketch out what he wanted?” [K.] “Well, he'd tell you what he wanted, roughly, you see, but he'd let you make what you wanted, you see, he'd tell you what he was going to do, which was very good, you see. It gives you......... it learnt you a lot and you knew what to do and what not to do. And then we would do a rough experiment, and get one or two curves you see, and then straight away button it on to somebody else to do the real work, and that's how he did his........ attacked these little things, you see.” [D.] “He tried them out himself first?” [K.] “He'd try a rough experiment himself on the little things, d'you see, and then he'd turn it over on to somebody...” (Quoted in Hughes, p. 104)

Rutherford and Hans Geiger worked closely in 1907 and 1908 on the detection and measurement of α particles. If they were to use α particles to probe the atom, they had first to know more about these particles and their behavior. Rutherford had tried and failed back at McGill to count α particles.

A year later in Manchester, he and Geiger succeeded with two methods of observing α particles. The first method involved scintillations excited by α particles on a thin layer of zinc sulfide. They observed these through a microscope and counted the scintillations at different angles of dispersion. They also developed an "electrometer" that could demonstrate the passage of an individual α particle to a large audience. The instrument, which evolved into the "Geiger counter," had a partially evacuated metal cylinder with a wire down its center. They applied a voltage between the cylinder and the wire high enough almost to spark. They admitted α particles through a thin mica window, where these particles collided with gasses, producing gas ions. These then collided with other molecules and produced more ions, and so on. Each α particle produced a cascade of ions, which partially discharged the cylinder and indicated the passage of an α particle. Geiger and Rutherford published several articles in 1908 and 1909 on these methods and their use.

Rutherford wrote to Henry Bumstead (1870–1920), an American physicist, on 11 July 1908:

Geiger is a good man and worked like a slave. I could never have found time for the drudgery before we got things going in good style. Finally all went well, but the scattering is the devil. Our tube worked like a charm and we could easily get a throw of 50 mm. for each particle. ... Geiger is a demon at the work of counting scintillations and could count at intervals for a whole night without disturbing his equanimity. I damned vigorously and retired after two minutes. (Quoted in Eve, p. 180.)

Although Rutherford suspected as early as 1906 that α particles were helium atoms stripped of their electrons, he demanded a high standard of proof. One kind of experiment was not enough. One kind of detector was not enough. He wanted more proof. For this, Rutherford desired "big voltages" and big electromagnets to divert α particles, but this method was not yet ripe. Lab steward William Kay recalled in the cited oral history interview that Rutherford in 1908 insisted that strong electric and magnetic fields were needed to measure more directly the charge and mass of the α and β particles:

And that's what he was after all the time. That's what he got at Cambridge [after 1919], which we never got here, you see, because we'd got no money. (Hughes, “William Kay,” 2008, pp. 109–110.)

Kay said Rutherford wanted a big, water-cooled magnet, but that he “dropped it like a hot cake” when he learned its cost. So he needed a new line of attack. The new line was very simple, a chemical procedure mixed with physics. For this work Rutherford recruited Thomas Royds (1884–1955), who had earned his Physics Honours degree in 1906. They collected α particles in a sealed glass tube, compressed them, and passed an electric spark through. They studied the emitted light in a spectroscope and found it to be identical to the spectrum of helium. Within a few months, Rutherford was awarded the Nobel Prize for Chemistry, "for his investigations into the disintegration of the elements, and the chemistry of radioactive substances." (Nobel citation) Rutherford and Royds had established the identity and primary properties of α particles. Rutherford next turned his attention to using them to probe the atom.

The autumn of 1908 began an important series of researches. Geiger had been passing beams of α particles through gold and other metallic foils, using the new detection techniques to measure how much these beams were dispersed by the atoms in the foils. Geiger thought Ernest Marsden (1889–1970), a 19-year-old student in Honours Physics, was ready to help on these experiments and suggested it to Rutherford. Since Rutherford often pushed third-year students into research, saying this was the best way to learn about physics, he readily agreed.

Geiger and Marsden began with small-angle dispersion and tried various thicknesses of foils, seeking mathematical relationships between dispersion and thickness of foil or number of atoms traversed. Marsden later recalled that Rutherford said to him amidst these experiments: "See if you can get some effect of alpha-particles directly reflected from a metal surface." (Reported by Marsden in Birks, 1962, p. 8). Marsden doubted that Rutherford expected back scatter of α particles, but as Marsden wrote

...it was one of those 'hunches' that perhaps some effect might be observed, and that in any case that neighbouring territory of this Tom Tiddler's ground might be explored by reconnaissance. Rutherford was ever ready to meet the unexpected and exploit it, where favourable, but he also knew when to stop on such excursions. (Birks, 1962, p. 8)

This was Rutherford's playful approach in action. His students and others tried out his ideas, many of which were dead-ends. This idea to look for backscattering of α particles, however, paid off. Rutherford wrote:

Experiment, directed by the disciplined imagination either of an individual or, still better, of a group of individuals of varied mental outlook, is able to achieve results which far transcend the imagination alone of the greatest philosopher. (Quoted in Eve, 1939, Frontmatter)

Sometime later in 1908 or 1909, Marsden said, he reported his results to Rutherford. Rutherford recalled this a little differently:

I remember ...later Geiger coming to me in great excitement and saying, 'We have been able to get some of the α -particles coming backwards...' It was quite the most incredible event that has ever happened to me in my life. It was almost incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. ( Rutherford , 1938, p. 68)

Human memory is fallible. Whether Marsden or Geiger told Rutherford, the effect was the same. Rutherford said they should prepare a publication from this research, which they submitted in May 1909. Moreover, this started Rutherford thinking toward what ultimately, almost two years later, he published as a theory of the atom.

What was Rutherford doing for the rest of 1909 and all of 1910? For one thing, his close friend Boltwood was in Manchester for the academic year working with Rutherford on radioactive decay products of radium. He was also reviewing and speaking on earlier ideas about atomic structure. Most importantly, he was taking the phenomenon of the scattering of α particles apart systematically and testing each piece. Rutherford did not have his bold idea — the nuclear atom — instantly, but he came to it gradually by considering the problem from many sides.

In the autumn of 1910 he brought Marsden back to Manchester to complete rigorous experimental testing of his ideas with Geiger. They re-established rates of emission and the ranges of α particles by radioactive sources and they re-examined their statistical analyses. Rutherford tried to reconcile scattering results with different atomic models, especially that of J.J. Thomson, in which the positive electricity was considered as dispersed evenly throughout the whole sphere of the atom.

At some point in the winter of 1910–1911, Rutherford worked out the basic idea of an atom with a "charged center." As Geiger and Marsden pointed out in their 1909 article:

If the high velocity and mass of the α -particle be taken into account, it seems surprising that some of the α -particles, as the experiment shows, can be turned within a layer of 6 x 10 -5 cm. of gold through an angle of 90°, and even more. To produce a similar effect by a magnetic field, the enormous field of 109 absolute units would be required. (Birks, p. 179)

Rutherford concluded in his May 1911 paper that such a remarkable deviation in the path of a massive charged particle could only be achieved if most of the mass of, say, an atom of gold and most of its charge were concentrated in a very small central body. Note: at this point in 1911, Rutherford did not call this a "nucleus."

The first public announcement of the nuclear theory by Rutherford was made at a meeting of the Manchester Literary and Philosophical Society, and he invited us young boys to go to the meeting. He said he’d got some interesting things to say and he thought we’d like to hear them. We didn’t know what it was about at that time. The older people in the laboratory did, of course Geiger and Marsden knew because they were already doing the experiments. In fact, unless they had done some which were sufficient to be decisive, Rutherford never mentioned it publicly. And, of course, Darwin knew about it much earlier. But that must have been early in 1911, and we went to the meeting and he told us. And he mentioned then that there was some experimental evidence which had been obtained by Geiger and Marsden. He did not, as far as I remember, say more about the results than that they were quite decisive. And, as I said before, he would never have made a public announcement of that kind if he hadn’t had good evidence. And that is one of the characteristics that runs through all Rutherford’s work, particularly all his work up to the end of the Manchester period. If you look at some of his papers in the early days — I call McGill the early days — he was quite convinced that the alpha particles were atoms of helium, but he never said that in those words. He always said they were either atoms of helium or molecules of hydrogen or perhaps he may have said something else of that weight. It was quite characteristic of him that he would never say a thing was so unless he had experimental evidence for it that really satisfied him.

In fact, Rutherford was exceedingly cautious in drawing conclusions about this central charge: “A simple calculation shows that the atom must be a seat of an intense electric field in order to produce such a large deflexion at a single encounter.” (Birks, p. 183). He worked out quickly and roughly that several quantitative relationships should be true if this basic theory were correct. First, the number of α particles scattered through a given angle should be proportional to the thickness of the foil. Second, that number should be proportional to the square of the nuclear charge. Lastly, it should be inversely proportional to the fourth power of the velocity of the α particle. These three ideas laid out the experimental program of Geiger and Marsden for the next year.

Rutherford’s interest was then almost entirely in the research. He had done very little teaching in McGill. He was research professor. I suppose he gave some lectures but it would have been very few. And his interest was quite naturally on the research side. He did give some lectures, but elementary lectures, the kind of thing you would expect a man to know before he came to the University. They were the lectures to the engineers. They were a rowdy lot and Rutherford could keep them under control. There was perhaps only one other man in the department who could have done it, and he (Rutherford?) enjoyed them because he was able to show them the very interesting experiments one can perform in elementary courses.

It's often been said to me that Rutherford was a bad lecturer. I never heard such nonsense. It is quite true that on occasion he would be a bit dull, a bit mixed up, but that was only on very rare occasions. There were other occasions when he was really most stimulating. There was a tremendous enthusiasm about him.

Rutherford entertained the possibility that the charged center is negative. That sounds odd today, so what made it reasonable? First, it wasn't very different from Thomson's model. Second, since Rutherford knew that α particles carry a double + charge, he thought this might act the same way the Sun does on a comet sweeping near it. It would slingshot the α particle around and back towards its source. He also considered a nearly forgotten model suggested by Japanese physicist Hantaro Nagaoka (1865–1950) — the Saturnian model. Nagaoka and Rutherford were in contact in 1910 and 1911 and Rutherford mentioned Nagaoka's model of "a central attracting mass surround by rings of rotating electrons" (Birks, p. 203). The end result in this critical Rutherford paper, however, was Rutherford's announcement that whether the atom were a disk or a sphere, and indeed whether the central charge were positive or negative, would not affect the calculations. Rutherford was always careful not to claim more than his results could support.

Rutherford did see possible tests of the nature of the central charge. The absorption of β particles, he said, should be different with a negative center versus a positive one. A positive center would explain the great velocity that α particles achieve during emission from radioactive elements. But these were only hints.

Geiger and Marsden did indeed work systematically through the testable implications of Rutherford's central charge hypothesis. The first major publication of their results was in German in the Proceedings of the Vienna Academy of Sciences ( Sitzungberichte der Wiener Akademie der Wissenschaften) in 1912. This 30-page version was followed by one in English in 1913 in the Philosophical Magazine: "The Laws of Deflexion of α Particles through Large Angles" The English version is the better known. Slight differences between the two led one historian to suggest that Rutherford decided in favor of a positively charged center by August 1912 (Trenn, 1974). Rutherford's other team members, especially Charles Galton Darwin (1887–1962), H.G.J. Moseley (1887–1915), and Niels Bohr (1885–1962) figured prominently in the ultimate establishment of Rutherford's nuclear atom.

The ‘Great War’ totally disrupted work in Rutherford's Manchester department. Bohr returned to Denmark. Marsden accepted a professorship in New Zealand. Moseley died in the Battle of Gallipoli. James Chadwick (1891–1974), who was working with Geiger at the Technical University of Berlin when war broke out, spent several years interned in the Ruhleben camp for prisoners of war. Other students went off to war, too, and Rutherford devoted considerable energy to mobilizing science for the war effort and specifically to anti-submarine techniques.

Against this distracted background, Rutherford and his lab steward, William Kay, began in 1917 to explore the passage of α particles through hydrogen, nitrogen, and other gases. When the Great War ended, Ernest Marsden briefly helped with the tedious scintillation observations that provided clues to the nature of the nucleus. Rutherford reported the tentative results of these extensive experiments in 1919. Rutherford placed a source of radium C (bismuth-214) in a sealable brass container, fitted so that the position of the source could be changed and so that different gases could be introduced or a vacuum produced, as desired. The α particles traversed the interior of the container and passed through a slit, covered by a silver plate or other material, and hit a zinc sulfide screen, where a scintillation was observed in a darkened room. When hydrogen gas was introduced into the container and care was taken to absorb the α particles before they hit the screen, scintillations were still observed. Rutherford posited that as the α particles traversed the hydrogen gas, they occasionally collided with hydrogen nuclei. As Rutherford wrote, this produced “swift hydrogen atoms” which were mostly projected forward in the direction of the α particles’ original motion.

Rutherford had several subtle questions in mind during these experiments, mostly concerned with the nature of the nucleus. He asked his colleague Darwin to analyze these collisions based on a ‘simple theory’ of elastic collisions between point nuclei repelled according to an inverse square law, the α particles carrying a charge of 2 times that of an electron (and of opposite sign) and the hydrogen nuclei 1 times. Darwin found that all α particles approaching within 2.4x10 -13 cm would produce a ‘swift hydrogen atom.’ This simple theory, however, predicted far fewer accelerated hydrogen atoms than were observed in the experiments.

Rutherford rejected explanations of this variance based on different charges on the particles or other laws than inverse square laws. Rather, he concluded that for distances on the order of the diameter of the electron, ‘the structure of the helium nucleus can no longer be regarded as a point…’. He posited that the helium nucleus ( α particle) has a complex structure of four hydrogen nuclei plus two negatively charged electrons. (We would say it is composed of two protons.) Rutherford concluded that deformation of complex nuclei during collisions was a more likely explanation, the variation of the forces between the nuclei varying in a complex way on close approach.

Taking into account the intense forces brought into play in such collisions, it would not be surprising if the helium nucleus were to break up. No evidence of such a disintegration…has been observed, indicating that the helium nucleus must be a very stable structure.

We must remember that Rutherford could not directly observe the structure of the nucleus, so his conclusions were tentative. Nevertheless, he was openly considering the possibilities of a complex nucleus, capable of deformation and even of possible disintegration. These thoughts shaped this intense period of experimental researches.

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Course: chemistry library   >   unit 5.

• The history of atomic chemistry
• Dalton's atomic theory
• Discovery of the electron and nucleus

Rutherford’s gold foil experiment

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Video transcript

• 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.

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J.j thomson’s plum-pudding model.

In 1897-98, the first model of an atom was proposed by J.J. Thomson. Famously known as the Plum-pudding model or the watermelon model, he proposed that an atom is made up of a positively charged ball with electrons embedded in it. Further, the negative and positive charges were equal in number , making the atom electrically neutral.

Figure 1 shows what Thomson’s plum-pudding model of an atom looked like. Ernest Rutherford, a former research student working with J.J. Thomson, proposed an experiment of scattering of alpha particles by atoms to understand the structure of an atom.

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

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’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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