discovery of electron discharge tube experiment

• Structure of Atom

Cathode Ray Experiment

What is Cathode Ray Tube?

A cathode-ray tube (CRT) is a vacuum tube in which an electron beam, deflected by applied electric or magnetic fields, produces a trace on a fluorescent screen.

The function of the cathode ray tube is to convert an electrical signal into a visual display. Cathode rays or streams of electron particles are quite easy to produce, electrons orbit every atom and move from atom to atom as an electric current.

Table of Contents

Cathode ray tube, recommended videos.

• J.J.Thomson Experiment

Apparatus Setup

Procedure of the experiment.

• Frequently Asked Questions – FAQs

In a cathode ray tube, electrons are accelerated from one end of the tube to the other using an electric field. When the electrons hit the far end of the tube they give up all the energy they carry due to their speed and this is changed to other forms such as heat. A small amount of energy is transformed into X-rays.

The cathode ray tube (CRT), invented in 1897 by the German physicist Karl Ferdinand Braun, is an evacuated glass envelope containing an electron gun a source of electrons and a fluorescent light, usually with internal or external means to accelerate and redirect the electrons. Light is produced when electrons hit a fluorescent tube.

The electron beam is deflected and modulated in a manner that allows an image to appear on the projector. The picture may reflect electrical wave forms (oscilloscope), photographs (television, computer monitor), echoes of radar-detected aircraft, and so on. The single electron beam can be processed to show movable images in natural colours.

J. J. Thomson Experiment – The Discovery of Electron

The Cathode ray experiment was a result of English physicists named J. J. Thomson experimenting with cathode ray tubes. During his experiment he discovered electrons and it is one of the most important discoveries in the history of physics. He was even awarded a Nobel Prize in physics for this discovery and his work on the conduction of electricity in gases.

However, talking about the experiment, J. J. Thomson took a tube made of glass containing two pieces of metal as an electrode. The air inside the chamber was subjected to high voltage and electricity flowing through the air from the negative electrode to the positive electrode.

J. J. Thomson designed a glass tube that was partly evacuated, i.e. all the air had been drained out of the building. He then applied a high electric voltage at either end of the tube between two electrodes. He observed a particle stream (ray) coming out of the negatively charged electrode (cathode) to the positively charged electrode (anode). This ray is called a cathode ray and is called a cathode ray tube for the entire construction.

The experiment Cathode Ray Tube (CRT) conducted by J. J. Thomson, is one of the most well-known physical experiments that led to electron discovery . In addition, the experiment could describe characteristic properties, in essence, its affinity to positive charge, and its charge to mass ratio. This paper describes how J is simulated. J. Thomson experimented with Cathode Ray Tube.

The major contribution of this work is the new approach to modelling this experiment, using the equations of physical laws to describe the electrons’ motion with a great deal of accuracy and precision. The user can manipulate and record the movement of the electrons by assigning various values to the experimental parameters.

A Diagram of JJ.Thomson Cathode Ray Tube Experiment showing Electron Beam – A cathode-ray tube (CRT) is a large, sealed glass tube.

The apparatus of the experiment incorporated a tube made of glass containing two pieces of metals at the opposite ends which acted as an electrode. The two metal pieces were connected with an external voltage. The pressure of the gas inside the tube was lowered by evacuating the air.

• Apparatus is set up by providing a high voltage source and evacuating the air to maintain the low pressure inside the tube.
• High voltage is passed to the two metal pieces to ionize the air and make it a conductor of electricity.
• The electricity starts flowing as the circuit was complete.
• To identify the constituents of the ray produced by applying a high voltage to the tube, the dipole was set up as an add-on in the experiment.
• The positive pole and negative pole were kept on either side of the discharge ray.
• When the dipoles were applied, the ray was repelled by the negative pole and it was deflected towards the positive pole.
• This was further confirmed by placing the phosphorescent substance at the end of the discharge ray. It glows when hit by a discharge ray. By carefully observing the places where fluorescence was observed, it was noted that the deflections were on the positive side. So the constituents of the discharge tube were negatively charged.

After completing the experiment J.J. Thomson concluded that rays were and are basically negatively charged particles present or moving around in a set of a positive charge. This theory further helped physicists in understanding the structure of an atom . And the significant observation that he made was that the characteristics of cathode rays or electrons did not depend on the material of electrodes or the nature of the gas present in the cathode ray tube. All in all, from all this we learn that the electrons are in fact the basic constituent of all the atoms.

Most of the mass of the atom and all of its positive charge are contained in a small nucleus, called a nucleus. The particle which is positively charged is called a proton. The greater part of an atom’s volume is empty space.

The number of electrons that are dispersed outside the nucleus is the same as the number of positively charged protons in the nucleus. This explains the electrical neutrality of an atom as a whole.

Uses of Cathode Ray Tube

• Used as a most popular television (TV) display.
• X-rays are produced when fast-moving cathode rays are stopped suddenly.
• The screen of a cathode ray oscilloscope, and the monitor of a computer, are coated with fluorescent substances. When the cathode rays fall off the screen pictures are visible on the screen.

Frequently Asked Questions – FAQs

What are cathode ray tubes made of.

The cathode, or the emitter of electrons, is made of a caesium alloy. For many electronic vacuum tube systems, Cesium is used as a cathode, as it releases electrons readily when heated or hit by light.

Where can you find a cathode ray tube?

Cathode rays are streams of electrons observed in vacuum tubes (also called an electron beam or an e-beam). If an evacuated glass tube is fitted with two electrodes and a voltage is applied, it is observed that the glass opposite the negative electrode glows from the electrons emitted from the cathode.

How did JJ Thomson find the electron?

In the year 1897 J.J. Thomson invented the electron by playing with a tube that was Crookes, or cathode ray. He had shown that the cathode rays were charged negatively. Thomson realized that the accepted model of an atom did not account for the particles charged negatively or positively.

What are the properties of cathode rays?

They are formed in an evacuated tube via the negative electrode, or cathode, and move toward the anode. They journey straight and cast sharp shadows. They’ve got strength, and they can do the job. Electric and magnetic fields block them, and they have a negative charge.

What do you mean by cathode?

A device’s anode is the terminal on which current flows in from outside. A device’s cathode is the terminal from which current flows out. By present, we mean the traditional positive moment. Because electrons are charged negatively, positive current flowing in is the same as outflowing electrons.

Who discovered the cathode rays?

Studies of cathode-ray began in 1854 when the vacuum tube was improved by Heinrich Geissler, a glassblower and technical assistant to the German physicist Julius Plücker. In 1858, Plücker discovered cathode rays by sealing two electrodes inside the tube, evacuating the air and forcing it between the electrode’s electric current.

Which gas is used in the cathode ray experiment?

For better results in a cathode tube experiment, an evacuated (low pressure) tube is filled with hydrogen gas that is the lightest gas (maybe the lightest element) on ionization, giving the maximum charge value to the mass ratio (e / m ratio = 1.76 x 10 ^ 11 coulombs per kg).

What is the Colour of the cathode ray?

Cathode-ray tube (CRT), a vacuum tube which produces images when electron beams strike its phosphorescent surface. CRTs can be monochrome (using one electron gun) or coloured (using usually three electron guns to produce red, green, and blue images that render a multicoloured image when combined).

How cathode rays are formed?

Cathode rays come from the cathode because the cathode is charged negatively. So those rays strike and ionize the gas sample inside the container. The electrons that were ejected from gas ionization travel to the anode. These rays are electrons that are actually produced from the gas ionization inside the tube.

What are cathode rays made of?

Thomson showed that cathode rays were composed of a negatively charged particle, previously unknown, which was later named electron. To render an image on a screen, Cathode ray tubes (CRTs) use a focused beam of electrons deflected by electrical or magnetic fields.

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Discovery Of Electron | Discharge Tube Experiment

Discharge tube.

The discharge tube is a glass tube having two electrodes sealed into it connected to a vacuum pump to reduce the pressure of the gas taken into it. A slit is placed in the tube to get a sharp beam of radiations.

Experiment:

William Crooks a British scientist studied the passage of electricity through gases taken at different pressures in the gas discharge tube. He observed that air taken in the gas discharge tube at ordinary pressure did not allow the electricity to flow, even when a source of high potential of about 5000 Volt was used. However, when pressure of air was reduced by removing most of the air from the discharge tube then it allowed the current to flow and emitted light (as in neon sign). When pressure was reduced further to about 0.01 torr, then emission of light by air ceased. But the current still flows between the electrodes and produced fluorescent on striking the glass walls opposite to the cathode. This was the result of rays emitted by cathode . Rays emitted by cathode when electricity is passed through a gas taken in the discharge tube at very low pressure are called cathode rays.

Discovery of electron

Emission of cathode rays does not depend on the nature of the electrodes or the gas used in the discharge tube. This indicates that cathode rays (i.e., electrons) are the constituents of all types of matter.

Conclusion:

J.J. Thomson calculated the mass of cathode rays. He suggested that these rays are matter and not electromagnetic radiations. He proposed the name corpuscles for these particles. But as these particles were similar to the particles present in on the electricity, therefore, later on they were named electrons. J.J. Thomson won the 1906 Nobel Prize.

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JJ Thompson’s Discovery of Electron: Cathode Ray Tube Experiment Explained

JJ Thomson discovered the electron in 1897 and there are tons of videos about it.  However, most videos miss what JJ Thomson himself said was the motivating factor: a debate about how cathode rays move.  Want to know not only how but why electrons were discovered?

Table of Contents

The start of jj thomson, how thomson discovered electrons: trials and errors, thomson’s conclusion.

A short history of Thomson: Joseph John Thomson, JJ on papers, to friends, and even to his own son [1] , was born in Lancashire, England to a middle class bookseller.  When he was 14 years old, Thomson planned to get an apprenticeship to a locomotive engineer but it had a long waiting list, so, he applied to and was accepted at that very young age to Owen’s college. 

Thompson later recalled that, “the authorities at Owens College thought my admission was such a scandal – I expect they feared that students would soon be coming in perambulators  – that they passed regulations raising the minimum age for admission, so that such a catastrophe should not happen again.

[2] ”  While in school, his father died, and his family didn’t have enough money for the apprenticeship.  Instead, he relied on scholarships at universities – ironically leading him to much greater fame in academia. In 1884, at the tender age of 28, Thomson applied to be the head of the Cavendish Research Institute. 

He mostly applied as a lark and was as surprised as anyone to actually get the position!  “I felt like a fisherman who…had casually cast a line in an unlikely spot and hooked a fish much too heavy for him to land. [3] ”  Suddenly, he had incredible resources, stability and ability to research whatever he wished. 

He ended up having an unerring ability to pinpoint interesting phenomena for himself and for others. In fact, a full eight of his research assistants and his son eventually earned Nobel Prizes, but, of course, like Thomson’s own Nobel Prize, that was in the future.

Why did J. J. Thomson discover the electron in 1897?  Well, according to Thomson: “the discovery of the electron began with an attempt to explain the discrepancy between the behavior of cathode rays under magnetic and electric forces [4] .”  What did he mean by that? 

Well, a cathode ray, or a ray in a vacuum tube that emanates from the negative electrode, can be easily moved with a magnet.  This gave a charismatic English chemist named William Crookes the crazy idea that the cathode ray was made of charged particles in 1879! 

However, 5 years later, a young German scientist named Heinrich Hertz found that he could not get the beam to move with parallel plates, or with an electric field.  Hertz decided that Crookes was wrong, if the cathode ray was made of charged particles then it should be attracted to a positive plate and repulsed from a negative plate. 

Ergo, it couldn’t be particles, and Hertz decided it was probably some new kind of electromagnetic wave, like a new kind of ultraviolet light.  Further, in 1892, Hertz accidentally discovered that cathode rays could tunnel through thin pieces of metal, which seemed like further proof that Crookes was so very wrong.

Then, in December of 1895, a French physicist named Jean Perrin used a magnet to direct a cathode ray into and out of an electroscope (called a Faraday cylinder) and measured its charge.  Perrin wrote, “the Faraday cylinder became negatively charged when the cathode rays entered it, and only when they entered it; the cathode rays are thus charged with negative electricity .

[5] ”  This is why JJ Thomson was so confused, he felt that Perrin had, “conclusive evidence that the rays carried a charge of negative electricity” except that, “Hertz found that when they were exposed to an electric force they were not deflected at all.”  What was going on?

In 1896, Thomson wondered if there might have been something wrong with Hertz’s experiment with the two plates.  Thomson knew that the cathode ray tubes that they had only work if there is a little air in the tube and the amount of air needed depended on the shape of the terminals.

Thomson wondered if the air affected the results.  Through trial and error, Thomson found he could get a “stronger” beam by shooting it through a positive anode with a hole in it.  With this system he could evacuate the tube to a much higher degree and, if the vacuum was good enough, the cathode ray was moved by electrically charged plates, “just as negatively electrified particles would be.

[6] ” (If you are wondering why the air affected it, the air became ionized in the high electric field and became conductive.  The conductive air then acted like a Faraday cage shielding the beam from the electric field.)

As stated before, Heinrich Hertz also found that cathode rays could travel through thin solids.  How could a particle do that?  Thomson thought that maybe particles could go through a solid if they were moving really, really fast.  But how to determine how fast a ray was moving? 

Thomson made an electromagnetic gauntlet.  First, Thomson put a magnet near the ray to deflect the ray one-way and plates with electric charge to deflect the ray the other way.  He then added or reduced the charge on the plates so that the forces were balanced and the ray went in a straight line. 

He knew that the force from the magnet depended on the charge of the particle, its speed and the magnetic field (given the letter B).  He also knew that the electric force from the plates only depended on the charge of the particle and the Electric field.  Since these forces were balanced, Thomson could determine the speed of the particles from the ratio of the two fields. 

Thomson found speeds as big as 60,000 miles per second or almost one third of the speed of light.  Thomson recalled, “In all cases when the cathode rays are produced their velocity is much greater than the velocity of any other moving body with which we are acquainted. [7] ”  

Thomson then did something even more ingenious; he removed the magnetic field.  Now, he had a beam of particles moving at a known speed with a single force on them.  They would fall, as Thomson said, “like a bullet projected horizontally with a velocity v and falling under gravity [8] ”.  

Note that these “bullets” are falling because of the force between their charge and the charges on the electric plates as gravity is too small on such light objects to be influential.  By measuring the distance the bullets went he could determine the time they were in the tube and by the distance they “fell” Thomson could determine their acceleration. 

Using F=ma Thomson determine the ratio of the charge on the particle to the mass (or e/m).  He found some very interesting results.  First, no matter what variables he changed in the experiment, the value of e/m was constant.  “We may… use any kind of substance we please for the electrodes and fill the tube with gas of any kind and yet the value of e/m will remain the same.

[9] ”  This was a revolutionary result.  Thomson concluded that everything contained these tiny little things that he called corpuscles (and we call electrons).  He also deduced that the “corpuscles” in one item are exactly the same as the “corpuscles” in another.  So, for example, an oxygen molecule contains the same kind of electrons as a piece of gold!  Atoms are the building blocks of matter but inside the atoms (called subatomic) are these tiny electrons that are the same for everything .

The other result he found was that the value of e/m was gigantic, 1,700 times bigger than the value for a charged Hydrogen atom, the object with the largest value of e/m before this experiment.   So, either the “corpuscle” had a ridiculously large charge or it was, well, ridiculously small.   

A student of Thomson’s named C. T. R. Wilson had experimented with slowly falling water droplets that found that the charge on the corpuscles were, to the accuracy of the experiment, the same as the charge on a charged Hydrogen atom!   Thomson concluded that his corpuscles were just very, very, tiny, about 1,700 times smaller then the Hydrogen atom [1] .  These experiments lead Thomson to come to some interesting conclusions:

• Electrons are in everything and are well over a thousand times smaller then even the smallest atom. 
• Benjamin Franklin thought positive objects had too much “electrical fire” and negative had too little.  Really, positive objects have too few electrons and negative have too many.  Oops.
• Although since Franklin, people thought current flowed from the positive side to the negative, really, the electrons are flowing the other way.  When a person talks about “current” that flows from positive to negative they are talking about something that is not real!   True “electric current” flows from negative to positive and is the real way the electrons move. [although by the time that people believed J.J. Thomson, it was too late to change our electronics, so people just decided to stick with “current” going the wrong way!]
• Since electrons are tiny and in everything but most things have a neutral charge, and because solid objects are solid, the electrons must be swimming in a sea or soup of positive charges.  Like raisons in a raison cookie.

The first three are still considered correct over one hundred years later.  The forth theory, the “plum pudding model” named after a truly English “desert” with raisins in sweet bread that the English torture people with during Christmas, was proposed by Thomson in 1904. 

In 1908, a former student of Thomson’snamed Ernest Rutherford was experimenting with radiation, and inadvertently demolished the “plum pudding model” in the process.  However, before I can get into Rutherford’s gold foil experiment, I first want to talk about what was going on in France concurrent to Thomson’s experiments. 

This is a story of how a new mother working mostly in a converted shed discovered and named the radium that Rutherford was experimenting with.  That woman’s name was Marie Sklodowska Curie, and that story is next time on the Lightning Tamers.

[1] the current number is 1,836 but Thomson got pretty close

[1] p 14 “Flash of the Cathode Rays: A History of JJ Thomson’s Electron” Dahl

[2] Thompson, J.J. Recollections and Reflections p. 2 Referred to in Davis & Falconer JJ. Thompson and the Discovery of the Electron 2002 p. 3

[3] Thomson, Joseph John Recollections and Reflections p. 98 quoted in Davis, E.A & Falconer, Isabel JJ Thomson and the Discovery of the Electron 2002 p. 35

[4]   Thomson, JJ Recollections and Reflections p. 332-3

[5] “New Experiments on the Kathode Rays” Jean Perrin, December 30, 1985 translation appeared in Nature, Volume 53, p 298-9, January 30, 1896

[6] Nobel Prize speech?

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Chemistry library

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
• Bohr's model of hydrogen

• J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons .
• Thomson's plum pudding model of the atom had negatively-charged electrons embedded within a positively-charged "soup."
• Rutherford's gold foil experiment showed that the atom is mostly empty space with a tiny, dense, positively-charged nucleus .
• Based on these results, Rutherford proposed the nuclear model of the atom.

Introduction: Building on Dalton's atomic theory

• All matter is made of indivisible particles called atoms , which cannot be created or destroyed.
• Atoms of the same element have identical mass and physical properties.
• Compounds are combinations of atoms of 2 ‍   or more elements.
• All chemical reactions involve the rearrangement of atoms.

J.J. Thomson and the discovery of the electron

• The cathode ray is composed of negatively-charged particles.
• The particles must exist as part of the atom, since the mass of each particle is only ∼ ‍   1 2000 ‍   the mass of a hydrogen atom.
• These subatomic particles can be found within atoms of all elements.

The plum pudding model

Ernest rutherford and the gold foil experiment, the nuclear model of the atom.

• The positive charge must be localized over a very tiny volume of the atom, which also contains most of the atom's mass. This explained how a very small fraction of the α ‍   particles were deflected drastically, presumably due to the rare collision with a gold nucleus.
• Since most of the α ‍   particles passed straight through the gold foil, the atom must be made up of mostly empty space!
• Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup."

Attributions

• “ Evolution of Atomic Theory ” from Openstax, CC BY 4.0 .
• " Atomic Theory " from UC Davis ChemWiki, CC BY-NC-SA 3.0 US .

Additional References

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Discovering the electron: JJ Thomson and the Cathode Ray Tube

Concept Introduction: JJ Thomson and the Discovery of the Electron

The discovery of the electron was an important step for physics, chemistry, and all fields of science. JJ Thomson made the discovery using the cathode ray tube. Learn all about the discovery, the importance of the discovery, and JJ Thomson in this tutorial article.

Further Reading on the Electron

Electron Orbital and Electron Shapes Writing Electron Configurations Electron Shells What are valence electrons? Electron Affinity Aufbau Principle

Who was JJ Thomson?

JJ Thomson was an English physicist who is credited with discovery of the electron in 1897. Thompson was born in December 1856 in Manchester, England and was educated at the University of Manchester and then the University of Cambridge, graduating with a degree in mathematics. Thompson made the switch to physics a few years later and began studying the properties of cathode rays. In addition to this work, Thomson also performed the first-ever mass spectrometr y experiments, discovered the first isotope and made important contributions both to the understanding of positively charged particles and electrical conductivity in gases.

Thomson did most of this work while leading the famed Cavendish Laboratory at the University of Cambridge. Although he received the Nobel Prize in physics and not chemistry, Thomson’s contributions to the field of chemistry are numerous. For instance, the discovery of the electron was vital to the development of chemistry today, and it was the first subatomic particle to be discovered. The proton and the neutron would soon follow as the full structure of the atom was discovered.

What is a cathode ray tube and why was it important?

Prior to the discovery of the electron, several scientists suggested that atoms consisted of smaller pieces. Yet until Thomson, no one had determined what these might be. Cathode rays played a critical role in unlocking this mystery. Thomson determined that charged particles much lighter than atoms , particles that we now call electrons made up cathode rays. Cathode rays form when electrons emit from one electrode and travel to another. The transfer occurs due to the application of a voltage in vacuum. Thomson also determined the mass to charge ratio of the electron using a cathode ray tube, another significant discovery.

How did Thomson make these discoveries?

Thomson was able to deflect the cathode ray towards a positively charged plate deduce that the particles in the beam were negatively charged. Then Thomson measured how much various strengths of magnetic fields bent the particles. Using this information Thomson determined the mass to charge ratio of an electron. These were the two critical pieces of information that lead to the discovery of the electron. Thomson was now able to determine that the particles in question were much smaller than atoms, but still highly charged. He finally proved atoms consisted of smaller components, something scientists puzzled over for a long time. Thomson called the particle “corpuscles” , not an electron. George Francis Fitzgerald suggested the name electron.

Why was the discovery of the electron important?

The discovery of the electron was the first step in a long journey towards a better understanding of the atom and chemical bonding. Although Thomson didn’t know it, the electron would turn out to be one of the most important particles in chemistry. We now know the electron forms the basis of all chemical bonds. In turn chemical bonds are essential to the reactions taking place around us every day. Thomson’s work provided the foundation for the work done by many other important scientists such as Einstein, Schrodinger, and Feynman.

Interesting Facts about JJ Thomson

Not only did Thomson receive the Nobel Prize in physics in 1906 , but his son Sir George Paget Thomson won the prize in 1937. A year earlier, in 1936, Thomson wrote an autobiography called “Recollections and Reflections”. He died in 1940, buried near Isaac Newton and Charles Darwin. JJ stands for “Joseph John”. Strangely, another author with the name JJ Thomson wrote a book with the same name in 1975. Thomson had many famous students, including Ernest Rutherford.

Discovery of the Electron: Further Reading

Protons, Neutrons & Electrons Discovering the nucleus with gold foil Millikan oil drop experiment Phase Diagrams

Subatomic science: JJ Thomson's discovery of the electron

Read about how JJ Thomson announced his discovery of the electron at the Royal Institution in this blog by our Head of Heritage and Collections. 

JJ Thomson, while familiar to scientists, is not necessarily a name most people would recognise; however, anyone who has undertaken any science at school will have heard of an electron.

It is Thomson we have to thank for discovering this fundamental breakthrough in science and announcing his discovery to the world during a lecture here at the Royal Institution in 1897.

What is an electron?

The technical definition is:

"An electron is a stable subatomic particle with a negative electrical charge. Unlike protons and neutrons, electrons are not constructed from even smaller components."

As a non-scientist this definition is something I have heard before but must confess is not something that means a great deal to me. It is an explanation in its basic form but doesn’t convey really what an electron is or what the impact of its discovery made

John Dalton's atomic theory

Prior to 1897, scientists had hypothesised about the makeup of the universe at the atomic and subatomic level but had not been able to prove any theories. The atom had been known about for many years.

In 1808, chemist John Dalton developed an argument that led to a realisation: that perhaps all matter, the things or objects that make up the universe are made of tiny, little bits.

These are fundamental and indivisible bits and named after the ancient Greek words ‘a’ meaning not and ‘tomos’ meaning cut therefore ‘atomos’ or uncuttable. Atoms.

JJ Thomson's cathode ray tube experiments

Thomson, a highly respected theoretical physics professor at Cambridge University, undertook a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube – he was attempting to solve a long-standing controversy regarding the nature of cathode rays, which occur when an electric current is driven through a vessel from which most of the air or other gas has been pumped out.

This was something that many scientists were investigating at the time. It was Thomson that made the breakthrough however, concluding through his experimentation that particles making up the rays were 1,000 times lighter than the lightest atom, proving that something smaller than atoms existed.

Thomson likened the composition of atoms to plum pudding, with negatively-charged ‘corpuscles’ dotted throughout a positively charged field.

G Johnstone Stoney coins the term 'electron'

Thomson explained within his lecture all of his experiments and the results, never mentioning the word electron but instead sticking to corpuscles to explain these tiny particles in the same terms as biological cells (corpuscles are a minute body or cell in an organism).

Such would they have remained if not for the term 'electron' coined by G Johnstone Stoney who in 1891 denoted the unit of charge found in experiments that passed electrical current through chemicals.

It was then in 1897 after Thomson’s publication of his research that Irish physicist George Francis Fitzgerald suggested that the term be applied to Thomson's research instead of corpuscles to better describe these newly discovered subatomic particles.

JJ Thomson and the Royal Institution

Thomson had a long-standing relationship with the Royal Institution during his long academic career in Cambridge, lecturing many times on the development of physics through Discourses and educational lectures to all ages.

Thomson was a great friend of Sir William Henry Bragg and Sir William Lawrence Bragg, who jointly won the Nobel Prize in 1915 for the development of x-ray crystallography, and who were both former Director’s of the Royal Institution.

JJ Thomson's Nobel Prize

Thomson received the Nobel Prize for his work in Physics in 1906 and was knighted in 1908. The studies of nuclear organisation that continue even to this day and the further identification of elementary particles have all followed the accomplishments of Thomson and his discovery in 1897.

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Letters to gwendoline – wwi bragg family correspondence.

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The birth of electric motion.

As we celebrate the bicentenary of Faraday's invention of the electric motor in 1821, our Head of Heritage and Collections

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John Tyndall set the foundation for our modern understanding of the greenhouse effect, climate change, meteorology, and weather

J.J. Thomson

by: Ann Johnson

• 1.1 Biography
• 2 Electron Discovery
• 3 Cathode Ray Experiments
• 4 Isotopes and Mass Spectrometry
• 5.1 Further reading
• 5.2 External links
• 6 References

The Main Idea

J. J. Thomson was a Nobel Prize winning English physicist who used cathode rays to discover electrons. He also developed the mass spectrometer.

J. J. Thomson was born on December 18th, 1856 in England. His father wished he would become an engineer, however he could not find an apprenticeship. He attended Trinity College at Cambridge, and eventually headed the Cavendish Laboratory. Thomson married one of his students, Rose Paget, in 1892. They had two children, Joan and George Thomson. George eventually became a physicist and earned a Nobel Prize of his own. J. J. Thomson published over 200 papers and 13 books. He died on August 30th, 1940 in Cambridge and is buried in Westminster Abbey.

Electron Discovery

J. J. Thomson discovered the electron in 1897 while performing experiments on electric discharge in a high-vacuum cathode ray tube. He interpreted the deflection of the rays by electrically charged plates and magnets as "evidence of bodies much smaller than atoms." He later suggested that the atom is best represented as a sphere of positive matter, through which electrons are positioned by electrostatic forces.

Cathode Ray Experiments

A cathode ray tube is a glass tube with wiring inserted on both ends, and as much air as possible pumped out of it. Cathode rays were discovered to travel in straight lines, just like waves do. Physicists knew that the ray had an electric charge, and they were trying to figure out if that electric charge could be separated from the ray.

Thomson had the hypothesis that the ray and charge were inseparable, and designed experiments using a magnetic field to prove this was true. He first built a cathode ray tube with a metal cylinder at the end. The cylinder had slits in it that were attached to electrometers, that could measure electric charges. When he applied a magnetic field across the tube, no activity was recorded by the electrometers. This meant the charge had been bent away by the magnet. This proved his theory that the charge and the ray were inseparable.

Isotopes and Mass Spectrometry

After discovering the electron, Thomson started studying positive rays. Positive rays behaved very differently from cathode rays, and he found that each ray followed its own parabolic path based on its detection on the photographic plate. He reasoned that no two particles would follow the same path unless they possessed the same mass-to-charge ratio. He correctly suggested that the positively charged particles were formed by the loss of an electron (isotopes). This created the field of mass spectrometry, which is still used very heavily today.

Properties of matter, including mass and charge, are related to Thomson's work with electrons and the mass spectrometer.

Further reading

Thomson, J. J. (June 1906). "On the Number of Corpuscles in an Atom". Philosophical Magazine 11: 769–781. doi:10.1080/14786440609463496. Archived from the original on 19 December 2007. Retrieved 4 October 2008. Leadership and creativity : a history of the Cavendish Laboratory, 1871 - 1919

External links

http://www.cambridgenetwork.co.uk/news/cambridge-physicist-is-streets-ahead/

http://thomson.iqm.unicamp.br/thomson.phphttp://www.chemheritage.org/discover/online-resources/chemistry-in-history/themes/atomic-and-nuclear-structure/thomson.aspx http://www.biography.com/people/jj-thomson-40039 http://study.com/academy/lesson/jj-thomsons-cathode-ray-tube-crt-definition-experiment-diagram.htmlhttps://explorable.com/cathode-ray-experiment

[[Category:Notable Scientists]

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30.2 Discovery of the Parts of the Atom: Electrons and Nuclei

Learning objectives.

By the end of this section, you will be able to:

• Describe how electrons were discovered.
• Explain the Millikan oil drop experiment.
• Describe Rutherford’s gold foil experiment.
• Describe Rutherford’s planetary model of the atom.

Just as atoms are a substructure of matter, electrons and nuclei are substructures of the atom. The experiments that were used to discover electrons and nuclei reveal some of the basic properties of atoms and can be readily understood using ideas such as electrostatic and magnetic force, already covered in previous chapters.

Charges and Electromagnetic Forces

In previous discussions, we have noted that positive charge is associated with nuclei and negative charge with electrons. We have also covered many aspects of the electric and magnetic forces that affect charges. We will now explore the discovery of the electron and nucleus as substructures of the atom and examine their contributions to the properties of atoms.

The Electron

Gas discharge tubes, such as that shown in Figure 30.4 , consist of an evacuated glass tube containing two metal electrodes and a rarefied gas. When a high voltage is applied to the electrodes, the gas glows. These tubes were the precursors to today’s neon lights. They were first studied seriously by Heinrich Geissler, a German inventor and glassblower, starting in the 1860s. The English scientist William Crookes, among others, continued to study what for some time were called Crookes tubes, wherein electrons are freed from atoms and molecules in the rarefied gas inside the tube and are accelerated from the cathode (negative) to the anode (positive) by the high potential. These “ cathode rays ” collide with the gas atoms and molecules and excite them, resulting in the emission of electromagnetic (EM) radiation that makes the electrons’ path visible as a ray that spreads and fades as it moves away from the cathode.

Gas discharge tubes today are most commonly called cathode-ray tubes , because the rays originate at the cathode. Crookes showed that the electrons carry momentum (they can make a small paddle wheel rotate). He also found that their normally straight path is bent by a magnet in the direction expected for a negative charge moving away from the cathode. These were the first direct indications of electrons and their charge.

The English physicist J. J. Thomson (1856–1940) improved and expanded the scope of experiments with gas discharge tubes. (See Figure 30.5 and Figure 30.6 .) He verified the negative charge of the cathode rays with both magnetic and electric fields. Additionally, he collected the rays in a metal cup and found an excess of negative charge. Thomson was also able to measure the ratio of the charge of the electron to its mass, q e q e / m e / m e —an important step to finding the actual values of both q e q e and m e m e . Figure 30.7 shows a cathode-ray tube, which produces a narrow beam of electrons that passes through charging plates connected to a high-voltage power supply. An electric field E E is produced between the charging plates, and the cathode-ray tube is placed between the poles of a magnet so that the electric field E E is perpendicular to the magnetic field B B of the magnet. These fields, being perpendicular to each other, produce opposing forces on the electrons. As discussed for mass spectrometers in More Applications of Magnetism , if the net force due to the fields vanishes, then the velocity of the charged particle is v = E / B v = E / B . In this manner, Thomson determined the velocity of the electrons and then moved the beam up and down by adjusting the electric field.

To see how the amount of deflection is used to calculate q e / m e q e / m e , note that the deflection is proportional to the electric force on the electron:

But the vertical deflection is also related to the electron’s mass, since the electron’s acceleration is

The value of F F is not known, since q e q e was not yet known. Substituting the expression for electric force into the expression for acceleration yields

Gathering terms, we have

The deflection is analyzed to get a a , and E E is determined from the applied voltage and distance between the plates; thus, q e m e q e m e can be determined. With the velocity known, another measurement of q e m e q e m e can be obtained by bending the beam of electrons with the magnetic field. Since F mag = q e vB = m e a F mag = q e vB = m e a , we have q e / m e = a / vB q e / m e = a / vB . Consistent results are obtained using magnetic deflection.

What is so important about q e / m e q e / m e , the ratio of the electron’s charge to its mass? The value obtained is

This is a huge number, as Thomson realized, and it implies that the electron has a very small mass. It was known from electroplating that about 10 8 C/kg 10 8 C/kg is needed to plate a material, a factor of about 1000 less than the charge per kilogram of electrons. Thomson went on to do the same experiment for positively charged hydrogen ions (now known to be bare protons) and found a charge per kilogram about 1000 times smaller than that for the electron, implying that the proton is about 1000 times more massive than the electron. Today, we know more precisely that

where q p q p is the charge of the proton and m p m p is its mass. This ratio (to four significant figures) is 1836 times less charge per kilogram than for the electron. Since the charges of electrons and protons are equal in magnitude, this implies m p = 1836 m e m p = 1836 m e .

Thomson performed a variety of experiments using differing gases in discharge tubes and employing other methods, such as the photoelectric effect, for freeing electrons from atoms. He always found the same properties for the electron, proving it to be an independent particle. For his work, the important pieces of which he began to publish in 1897, Thomson was awarded the 1906 Nobel Prize in Physics. In retrospect, it is difficult to appreciate how astonishing it was to find that the atom has a substructure. Thomson himself said, “It was only when I was convinced that the experiment left no escape from it that I published my belief in the existence of bodies smaller than atoms.”

Thomson attempted to measure the charge of individual electrons, but his method could determine its charge only to the order of magnitude expected.

Since Faraday’s experiments with electroplating in the 1830s, it had been known that about 100,000 C per mole was needed to plate singly ionized ions. Dividing this by the number of ions per mole (that is, by Avogadro’s number), which was approximately known, the charge per ion was calculated to be about 1 . 6 × 10 − 19 C 1 . 6 × 10 − 19 C , close to the actual value.

An American physicist, Robert Millikan (1868–1953) (see Figure 30.8 ), decided to improve upon Thomson’s experiment for measuring q e q e and was eventually forced to try another approach, which is now a classic experiment performed by students. The Millikan oil drop experiment is shown in Figure 30.9 .

In the Millikan oil drop experiment, fine drops of oil are sprayed from an atomizer. Some of these are charged by the process and can then be suspended between metal plates by a voltage between the plates. In this situation, the weight of the drop is balanced by the electric force:

The electric field is produced by the applied voltage, hence, E = V / d E = V / d , and V V is adjusted to just balance the drop’s weight. The drops can be seen as points of reflected light using a microscope, but they are too small to directly measure their size and mass. The mass of the drop is determined by observing how fast it falls when the voltage is turned off. Since air resistance is very significant for these submicroscopic drops, the more massive drops fall faster than the less massive, and sophisticated sedimentation calculations can reveal their mass. Oil is used rather than water, because it does not readily evaporate, and so mass is nearly constant. Once the mass of the drop is known, the charge of the electron is given by rearranging the previous equation:

where d d is the separation of the plates and V V is the voltage that holds the drop motionless. (The same drop can be observed for several hours to see that it really is motionless.) By 1913 Millikan had measured the charge of the electron q e q e to an accuracy of 1%, and he improved this by a factor of 10 within a few years to a value of − 1 . 60 × 10 − 19 C − 1 . 60 × 10 − 19 C . He also observed that all charges were multiples of the basic electron charge and that sudden changes could occur in which electrons were added or removed from the drops. For this very fundamental direct measurement of q e q e and for his studies of the photoelectric effect, Millikan was awarded the 1923 Nobel Prize in Physics.

With the charge of the electron known and the charge-to-mass ratio known, the electron’s mass can be calculated. It is

Substituting known values yields

where the round-off errors have been corrected. The mass of the electron has been verified in many subsequent experiments and is now known to an accuracy of better than one part in one million. It is an incredibly small mass and remains the smallest known mass of any particle that has mass. (Some particles, such as photons, are massless and cannot be brought to rest, but travel at the speed of light.) A similar calculation gives the masses of other particles, including the proton. To three digits, the mass of the proton is now known to be

which is nearly identical to the mass of a hydrogen atom. What Thomson and Millikan had done was to prove the existence of one substructure of atoms, the electron, and further to show that it had only a tiny fraction of the mass of an atom. The nucleus of an atom contains most of its mass, and the nature of the nucleus was completely unanticipated.

Another important characteristic of quantum mechanics was also beginning to emerge. All electrons are identical to one another. The charge and mass of electrons are not average values; rather, they are unique values that all electrons have. This is true of other fundamental entities at the submicroscopic level. All protons are identical to one another, and so on.

The Nucleus

Here, we examine the first direct evidence of the size and mass of the nucleus. In later chapters, we will examine many other aspects of nuclear physics, but the basic information on nuclear size and mass is so important to understanding the atom that we consider it here.

Nuclear radioactivity was discovered in 1896, and it was soon the subject of intense study by a number of the best scientists in the world. Among them was New Zealander Lord Ernest Rutherford, who made numerous fundamental discoveries and earned the title of “father of nuclear physics.” Born in Nelson, Rutherford did his postgraduate studies at the Cavendish Laboratories in England before taking up a position at McGill University in Canada where he did the work that earned him a Nobel Prize in Chemistry in 1908. In the area of atomic and nuclear physics, there is much overlap between chemistry and physics, with physics providing the fundamental enabling theories. He returned to England in later years and had six future Nobel Prize winners as students. Rutherford used nuclear radiation to directly examine the size and mass of the atomic nucleus. The experiment he devised is shown in Figure 30.10 . A radioactive source that emits alpha radiation was placed in a lead container with a hole in one side to produce a beam of alpha particles, which are a type of ionizing radiation ejected by the nuclei of a radioactive source. A thin gold foil was placed in the beam, and the scattering of the alpha particles was observed by the glow they caused when they struck a phosphor screen.

Alpha particles were known to be the doubly charged positive nuclei of helium atoms that had kinetic energies on the order of 5 MeV 5 MeV when emitted in nuclear decay, which is the disintegration of the nucleus of an unstable nuclide by the spontaneous emission of charged particles. These particles interact with matter mostly via the Coulomb force, and the manner in which they scatter from nuclei can reveal nuclear size and mass. This is analogous to observing how a bowling ball is scattered by an object you cannot see directly. Because the alpha particle’s energy is so large compared with the typical energies associated with atoms ( MeV MeV versus eV eV ), you would expect the alpha particles to simply crash through a thin foil much like a supersonic bowling ball would crash through a few dozen rows of bowling pins. Thomson had envisioned the atom to be a small sphere in which equal amounts of positive and negative charge were distributed evenly. The incident massive alpha particles would suffer only small deflections in such a model. Instead, Rutherford and his collaborators found that alpha particles occasionally were scattered to large angles, some even back in the direction from which they came! Detailed analysis using conservation of momentum and energy—particularly of the small number that came straight back—implied that gold nuclei are very small compared with the size of a gold atom, contain almost all of the atom’s mass, and are tightly bound. Since the gold nucleus is several times more massive than the alpha particle, a head-on collision would scatter the alpha particle straight back toward the source. In addition, the smaller the nucleus, the fewer alpha particles that would hit one head on.

Although the results of the experiment were published by his colleagues in 1909, it took Rutherford two years to convince himself of their meaning. Like Thomson before him, Rutherford was reluctant to accept such radical results. Nature on a small scale is so unlike our classical world that even those at the forefront of discovery are sometimes surprised. Rutherford later wrote: “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. On consideration, I realized that this scattering backwards ... [meant] ... the greatest part of the mass of the atom was concentrated in a tiny nucleus.” In 1911, Rutherford published his analysis together with a proposed model of the atom. The size of the nucleus was determined to be about 10 − 15 m 10 − 15 m , or 100,000 times smaller than the atom. This implies a huge density, on the order of 10 15 g/cm 3 10 15 g/cm 3 , vastly unlike any macroscopic matter. Also implied is the existence of previously unknown nuclear forces to counteract the huge repulsive Coulomb forces among the positive charges in the nucleus. Huge forces would also be consistent with the large energies emitted in nuclear radiation.

The small size of the nucleus also implies that the atom is mostly empty inside. In fact, in Rutherford’s experiment, most alphas went straight through the gold foil with very little scattering, since electrons have such small masses and since the atom was mostly empty with nothing for the alpha to hit. There were already hints of this at the time Rutherford performed his experiments, since energetic electrons had been observed to penetrate thin foils more easily than expected. Figure 30.11 shows a schematic of the atoms in a thin foil with circles representing the size of the atoms (about 10 − 10 m 10 − 10 m ) and dots representing the nuclei. (The dots are not to scale—if they were, you would need a microscope to see them.) Most alpha particles miss the small nuclei and are only slightly scattered by electrons. Occasionally, (about once in 8000 times in Rutherford’s experiment), an alpha hits a nucleus head-on and is scattered straight backward.

Based on the size and mass of the nucleus revealed by his experiment, as well as the mass of electrons, Rutherford proposed the planetary model of the atom . The planetary model of the atom pictures low-mass electrons orbiting a large-mass nucleus. The sizes of the electron orbits are large compared with the size of the nucleus, with mostly vacuum inside the atom. This picture is analogous to how low-mass planets in our solar system orbit the large-mass Sun at distances large compared with the size of the sun. In the atom, the attractive Coulomb force is analogous to gravitation in the planetary system. (See Figure 30.12 .) Note that a model or mental picture is needed to explain experimental results, since the atom is too small to be directly observed with visible light.

Rutherford’s planetary model of the atom was crucial to understanding the characteristics of atoms, and their interactions and energies, as we shall see in the next few sections. Also, it was an indication of how different nature is from the familiar classical world on the small, quantum mechanical scale. The discovery of a substructure to all matter in the form of atoms and molecules was now being taken a step further to reveal a substructure of atoms that was simpler than the 92 elements then known. We have continued to search for deeper substructures, such as those inside the nucleus, with some success. In later chapters, we will follow this quest in the discussion of quarks and other elementary particles, and we will look at the direction the search seems now to be heading.

PhET Explorations

Rutherford scattering.

How did Rutherford figure out the structure of the atom without being able to see it? Simulate the famous experiment in which he disproved the Plum Pudding model of the atom by observing alpha particles bouncing off atoms and determining that they must have a small core.

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1.6: The Discovery of the Electron

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Learning Objectives

• To become familiar with the feature of an electron.
• Summarize and interpret the results of the experiments of Thomson and Millikan.

Long before the end of the 19th century, it was well known that applying a high voltage to a gas contained at low pressure in a sealed tube (called a gas discharge tube) caused electricity to flow through the gas, which then emitted light (Figure \(\PageIndex{1}\)). Researchers trying to understand this phenomenon found that an unusual form of energy was also emitted from the cathode, or negatively charged electrode; this form of energy was called a "cathode ray". In 1897, the British physicist J. J. Thomson (1856–1940) proved that atoms were not the most basic form of matter. He demonstrated that cathode rays could be deflected, or bent, by magnetic or electric fields, which indicated that cathode rays consist of charged particles.

More important, by measuring the extent of the deflection of the cathode rays in magnetic or electric fields of various strengths, Thomson was able to calculate the mass-to-charge ratio of the particles. These particles were emitted by the negatively charged cathode and repelled by the negative terminal of an electric field. Because like charges repel each other and opposite charges attract, Thomson concluded that the particles had a net negative charge; these particles are now called electrons. Most relevant to the field of chemistry, Thomson found that the mass-to-charge ratio of cathode rays is independent of the nature of the metal electrodes or the gas, which suggested that electrons were fundamental components of all atoms.

Aside: Electrostatic Forces

If two objects each have electric charge, then they exert an exert an electric force on each other. The magnitude of the force is linearly proportional the charge on each object and inversely proportional to square distance between each other. The magnitude of this electrostatic force is linearly proportional the distance between them and involves the existence of two types of charge, the observation that like charges repel, unlike charges attract and the decrease of force with distance.

The SI unit of electric charge is the coulomb (C) named after French physicist Charles-Augustin de Coulomb.

The video below shows JJ Thompson used such tube to measure the ratio of charge over mass of an electron.

Millikan’s Oil Drop Experiment: Measuring the Charge of the Electron

The American scientist Robert Millikan (1868–1953) carried out a series of experiments using electrically charged oil droplets, which allowed him to calculate the charge on a single electron. Millikan created microscopic oil droplets, which could be electrically charged by friction as they formed or by using X-rays. These droplets initially fell due to gravity, but their downward progress cou ld be slowed or even reversed by an electric field lower in the apparatus. By adjusting the electric field strength and making careful measurements and appropriate calculations, Millikan was able to determine the charge on individual drops (Figure \(\PageIndex{2}\)).

Looking at the charge data that Millikan gathered, you may have recognized that the charge of an oil droplet is always a multiple of a specific charge, \(1.6 \times 10^{−19}\, C\). Millikan concluded that this value must therefore be a fundamental charge—the charge of a single electron—with his measured charges due to an excess of one electron (\(1 \times (1.6 \times 10^{−19}\, C)\)), two electrons (\(2 \times (1.6 \times 10^{−19}\, C)\)), three electrons (\(3 \times (1.6 \times 10^{−19}\, C)\)), and so on, on a given oil droplet.

Defintion: Elementary Charge

The charge of an electron is sometimes referred to as the elementary charge and usually denoted by \(e\). The elementary charge is a fundamental physical constant and as of May 2019, its value is defined to be exactly \(1.602176634 \times 10^{−19}\, C\).

Since the charge of an electron was now known due to Millikan’s research, and the charge-to-mass ratio was already known due to Thomson’s research (\(1.759 \times 10^{11}\, C/kg\)), it only required a simple calculation to determine the mass of the electron as well.

\[\begin{align*} \mathrm{Mass\: of\: electron} &= \mathrm{1.602\times 10^{-19}\:\cancel{C}\times \dfrac{1\: kg}{1.759\times 10^{11}\:\cancel{C}}} \\[4pt] &= \mathrm{9.107\times 10^{-31}\:kg} \end{align*}\]

Scientists had now established that the atom was not indivisible as Dalton's theory had postulated , and due to the work of Thomson, Millikan, and others, the charge and mass of the negative, subatomic particles—the electrons—were known. However, the positively charged part of an atom was not yet well understood.

Exercise \(\PageIndex{1}\)

In a Millikan's oil drop experiment done in alternate universe, the measured charges on drops are found to be \(8 \times 10^{-19}\, C\), \(12 \times 10^{-19}\, C \) and \(20 \times 10^{-19}\, C\). What is the elementary charge in this universe?

Millikan experiment involves confirm that the charges of the drops were all small integer multiples of the elementary charge. The charges on the drop are found to be multiple of 4. Hence the small charge are \(4 \times 10^{-19} C\).

Contributors and Attributions

Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors.  Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at http://cnx.org/contents/[email protected] ).