British physicist J.J. Thomson announces the discovery of electrons

British physicist J.J. Thomson announces the discovery of electrons

On April 30, 1897, British physicist J.J. Thomson announced his discovery that atoms were made up of smaller components. This finding revolutionized the way scientists thought about the atom and had major ramifications for the field of physics. Though Thompson referred to them as "corpuscles," what he found is more commonly known today as the electron.

Mankind had already discovered electric current and harnessed it to great effect, but scientists had not yet observed the makeup of atoms. Thomson, a highly-respected professor at Cambridge, determined the existence of electrons by studying cathode rays. He concluded that the 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.

The plum pudding analogy was disproved by Ernest Rutherford, a student and collaborator of Thomson’s, in Thomson's lab at Cambridge in 1910. Rutherford's conclusion that the positive charge of an atom resides in its nucleus established the model of the atom as we know it today. In addition to winning his own Nobel Prize, Thomson employed six research assistants who went on to win Nobel Prizes in physics and two, including Rutherford, who won Nobel Prizes for chemistry. His son, George Paget Thomson, also won a Nobel Prize for his study of electrons. Combined with his own research, the network of atomic researchers Thomson cultivated gave humanity a new and detailed understanding of the smallest building-blocks of the universe.

Discovery of electrons

During the 1880s and ’90s scientists searched cathode rays for the carrier of the electrical properties in matter. Their work culminated in the discovery by English physicist J.J. Thomson of the electron in 1897. The existence of the electron showed that the 2,000-year-old conception of the atom as a homogeneous particle was wrong and that in fact the atom has a complex structure.

Cathode-ray studies began in 1854 when Heinrich Geissler, a glassblower and technical assistant to German physicist Julius Plücker, improved the vacuum tube. Plücker discovered cathode rays in 1858 by sealing two electrodes inside the tube, evacuating the air, and forcing electric current between the electrodes. He found a green glow on the wall of his glass tube and attributed it to rays emanating from the cathode. In 1869, with better vacuums, Plücker’s pupil Johann W. Hittorf saw a shadow cast by an object placed in front of the cathode. The shadow proved that the cathode rays originated from the cathode. English physicist and chemist William Crookes investigated cathode rays in 1879 and found that they were bent by a magnetic field the direction of deflection suggested that they were negatively charged particles. As the luminescence did not depend on what gas had been in the vacuum or what metal the electrodes were made of, he surmised that the rays were a property of the electric current itself. As a result of Crookes’s work, cathode rays were widely studied, and the tubes came to be called Crookes tubes.

Although Crookes believed that the particles were electrified charged particles, his work did not settle the issue of whether cathode rays were particles or radiation similar to light. By the late 1880s the controversy over the nature of cathode rays had divided the physics community into two camps. Most French and British physicists, influenced by Crookes, thought that cathode rays were electrically charged particles because they were affected by magnets. Most German physicists, on the other hand, believed that the rays were waves because they traveled in straight lines and were unaffected by gravity. A crucial test of the nature of the cathode rays was how they would be affected by electric fields. Heinrich Hertz, the aforementioned German physicist, reported that the cathode rays were not deflected when they passed between two oppositely charged plates in an 1892 experiment. In England J.J. Thomson thought Hertz’s vacuum might have been faulty and that residual gas might have reduced the effect of the electric field on the cathode rays.

Thomson repeated Hertz’s experiment with a better vacuum in 1897. He directed the cathode rays between two parallel aluminum plates to the end of a tube where they were observed as luminescence on the glass. When the top aluminum plate was negative, the rays moved down when the upper plate was positive, the rays moved up. The deflection was proportional to the difference in potential between the plates. With both magnetic and electric deflections observed, it was clear that cathode rays were negatively charged particles. Thomson’s discovery established the particulate nature of electricity. Accordingly, he called his particles electrons.

From the magnitude of the electrical and magnetic deflections, Thomson could calculate the ratio of mass to charge for the electrons. This ratio was known for atoms from electrochemical studies. Measuring and comparing it with the number for an atom, he discovered that the mass of the electron was very small, merely 1/1,836 that of a hydrogen ion. When scientists realized that an electron was virtually 1,000 times lighter than the smallest atom, they understood how cathode rays could penetrate metal sheets and how electric current could flow through copper wires. In deriving the mass-to-charge ratio, Thomson had calculated the electron’s velocity. It was 1 /10 the speed of light, thus amounting to roughly 30,000 km (18,000 miles) per second. Thomson emphasized that

we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state a state in which all matter, that is, matter derived from different sources such as hydrogen, oxygen, etc., is of one and the same kind this matter being the substance from which all the chemical elements are built up.

Thus, the electron was the first subatomic particle identified, the smallest and the fastest bit of matter known at the time.

In 1909 American physicist Robert Andrews Millikan greatly improved a method employed by Thomson for measuring the electron charge directly. In Millikan’s oil-drop experiment, he produced microscopic oil droplets and observed them falling in the space between two electrically charged plates. Some of the droplets became charged and could be suspended by a delicate adjustment of the electric field. Millikan knew the weight of the droplets from their rate of fall when the electric field was turned off. From the balance of the gravitational and electrical forces, he could determine the charge on the droplets. All the measured charges were integral multiples of a quantity that in contemporary units is 1.602 × 10 −19 coulomb. Millikan’s electron-charge experiment was the first to detect and measure the effect of an individual subatomic particle. Besides confirming the particulate nature of electricity, his experiment also supported previous determinations of Avogadro’s number. Avogadro’s number times the unit of charge gives Faraday’s constant, the amount of charge required to electrolyze one mole of a chemical ion.

April 30, 1897: J.J. Thomson Announces the Electron . Sort Of

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Joseph John Thomson didn't give us the name or the structure or the exact mass, but he was the first to identify a subatomic particle. Reproduction of steel engraving from The Electrician, 1896

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1897: Physicist J.J. Thomson tells a startled scientific audience that he's discovered something smaller than an atom, a particle with a minuscule mass and a negative charge.

Some in the audience at the Royal Institution of Great Britain that Friday evening later told Thomson they thought he was "pulling their legs." The atom, after all, was known to be indivisible. That's what its name meant.

As director of Cavendish Laboratory at Cambridge University, Thomson was researching electrical currents inside cathode ray tubes. He observed that the rays are deflected by an electric field.

Researchers had been puzzled by cathode rays until Thomson theorized that the rays were in fact streams of small subatomic particles, the first known. He called them "corpuscles," the Latin for "small bodies."

Thomson figured his negatively charged corpuscles accounted for about one-thousandth of the mass of a hydrogen atom (1/1836 or 1/1837 is the accepted ratio today), matched by a positive charge elsewhere in the atom. Thomson was vague in 1897 but later theorized that the negative electrons swarmed around in a "sphere of uniform positive electrification." (Establishing the nuclear-orbital model of the atom would fall to Ernest Rutherford and Niels Bohr in later decades.)

In a commentary on the published version of Thomson's lecture, Irish physicist George F. FitzGerald suggested that the corpuscles were actually free electrons.

Other scientists had proposed that cathode rays were composed of particles and had attempted to establish their relative mass and charge. Thomson's great contribution was estimating that ratio and recognizing that the ratio was universal and didn't depend on the specific materials. That led him to postulate that the particles were one of the building blocks of the atom itself, even though he hadn't fully proved that at the time of his epochal lecture.

Thomson was awarded the 1906 Nobel Prize "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases." He was knighted in 1908.

His 1907 book was titled The Corpuscular Theory of Matter, and he continued to call his discovery "corpuscles" until 1913.

J.J. Thomson was born in 1856 in Cheetham Hill, Manchester in England, of Scottish parentage. In 1870 he studied engineering at University of Manchester known as Owens College at that time, and moved on to Trinity College, Cambridge in 1876. In 1880, he obtained his BA in mathematics ( Second Wrangler and 2nd Smith's prize) and MA (with Adams Prize) in 1883. In 1884 he became Cavendish Professor of Physics. One of his students was Ernest Rutherford, who would later succeed him in the post. In 1890 he married Rose Elisabeth Paget, daughter of Sir George Edward Paget, KCB, a physician and then Regius Professor of Physic at Cambridge. He fathered one son, George Paget Thomson, and one daughter, Joan Paget Thomson, with her. One of Thomson's greatest contributions to modern science was in his role as a highly gifted teacher, as seven of his research assistants and his aforementioned son won Nobel Prizes in physics. His son won the Nobel Prize in 1937 for proving the wavelike properties of electrons.

He was awarded a Nobel Prize in 1906, "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases." He was knighted in 1908 and appointed to the Order of Merit in 1912. In 1914 he gave the Romanes Lecture in Oxford on "The atomic theory". In 1918 he became Master of Trinity College, Cambridge, where he remained until his death. He died on August 30, 1940 and was buried in Westminster Abbey, close to Sir Isaac Newton.

Thomson was elected a Fellow of the Royal Society on June 12, 1884 and was subsequently President of the Royal Society from 1915 to 1920.


Cathode rays

Thomson conducted a series of experiments with cathode rays and cathode ray tubes leading him to the discovery of electrons and subatomic particles. Thomson used the cathode ray tube in three different experiments.

First experiment

In his first experiment, he investigated whether or not the negative charge could be separated from the cathode rays by means of magnetism. He constructed a cathode ray tube ending in a pair of cylinders with slits in them. These slits were in turn connected to an electrometer. Thomson found that if the rays were magnetically bent such that they could not enter the slit, the electrometer registered little charge. Thomson concluded that the negative charge was inseparable from the rays.

Second experiment

In his second experiment, he investigated whether or not the rays could be deflected by an electric field (something that is characteristic of charged particles). Previous experimenters had failed to observe this, but Thomson believed their experiments were flawed because they contained trace amounts of gas. Thomson constructed a cathode ray tube with a practically perfect vacuum, and coated one end with phosphorescent paint. Thomson found that the rays did indeed bend under the influence of an electric field, in a direction indicating a negative charge.

Third experiment

In his third experiment, Thomson measured the charge-to-mass ratio of the cathode rays by measuring how much they were deflected by a magnetic field and how much energy they carried. He found that the charge to mass ratio was over a thousand times higher than that of a hydrogen ion (H + ), suggesting either that the particles were very light or very highly charged.

Thomson's conclusions were bold: cathode rays were indeed made of particles which he called " corpuscles", and these corpuscles came from within the atoms of the electrodes themselves, meaning that atoms are in fact divisible. The "corpuscles" discovered by Thomson are identified with the electrons which had been proposed by G. Johnstone Stoney.

Thomson imagined the atom as being made up of these corpuscles swarming in a sea of positive charge this was his plum pudding model. This model was later proved incorrect when Ernest Rutherford showed that the positive charge is concentrated in the nucleus.

Thomson's discovery was made known in 1897, and caused a sensation in scientific circles, eventually resulting in him being awarded a Nobel Prize in Physics in 1906.

Isotopes and mass spectrometry

In 1913, as part of his exploration into the composition of canal rays, Thomson channelled a stream of ionized neon through a magnetic and an electric field and measured its deflection by placing a photographic plate in its path. Thomson observed two patches of light on the photographic plate (see image on right), which suggested two different parabolas of deflection. Thomson concluded that the neon gas was composed of atoms of two different atomic masses (neon-20 and neon-22).

This separation of neon isotopes by their mass was the first example of mass spectrometry, which was subsequently improved and developed into a general method by Thomson's student F. W. Aston and by A. J. Dempster.

Other work

In 1906 Thomson demonstrated that hydrogen had only a single electron per atom. Previous theories allowed various numbers of electrons.

Mass-to-charge ratio

Thomson pushed his experiments further to determine the mass-to-charge ratio of electrons by deflecting a beam of cathode rays by electric and magnetic fields. This more precise information about electrons’ properties allowed Thomson, in 1904, to draft a model of the atom known as the “plum pudding model.” It described a sphere of positive matter that embedded the “corpuscles” or electrons, distributed over a large sea of a positive charge.

Joseph John Thomson

Joseph John Thomson, better known as J. J. Thomson, was a British physicist who first theorized and offered experimental evidence that the atom was a divisible entity rather than the basic unit of matter, as was widely believed at the time.

Joseph John Thomson, better known as J. J. Thomson, was a British physicist who first theorized and offered experimental evidence that the atom is a divisible entity rather than the basic unit of matter, as was widely believed at the time. A series of experiments with cathode rays he carried out near the end of the 19th century led to his discovery of the electron, a negatively charged atomic particle with very little mass. Thomson received the Nobel Prize in Physics in 1906 for his work exploring the electrical conductivity of various gases.

The son of a bookseller, Thomson was born on December 18, 1856, in Cheetham Hill, located just north of Manchester, England. He entered Owens College when he was 14 years old, where he became interested in experimental physics, though he had initially intended to pursue a career in engineering. Thomson’s father died only a few years into his college studies, making it financially difficult for Thomson to remain in school. However, through the efforts of his family and scholarships he continued at Owens College until 1876. He then transferred to Trinity College, Cambridge, on a mathematics scholarship. He remained associated with Cambridge University in varying capacities the rest of his life. In 1880, Thomson received a bachelor’s degree in mathematics and became second wrangler, a title bestowed on the second highest-scoring individual on the Cambridge mathematics exams.

Following graduation, Thomson became a Fellow at Trinity College and began work at the Cavendish Laboratory, part of the Cambridge Physics Department. In 1883, he became a lecturer at Cambridge and the following year was appointed Cavendish Professor of Experimental Physics, becoming the successor to Lord Rayleigh. Also in 1884, the Royal Society of London elected Thomson as a Fellow. The receipt of such considerable honors by so young a scientist was highly unusual, but was largely the result of Thomson’s significant early work expanding James Clerk Maxwell’s theories of electromagnetism. Coverage of these efforts, which continued over many years, appeared in Thomson’s 1892 treatise Notes on Recent Researches in Electricity and Magnetism.

In the early 1890s, much of Thomson’s research focused on electrical conduction in gases. During a visit to the United States in 1896, he gave a series of lectures discussing his findings. In 1897, the lectures were published as Discharge of Electricity through Gases. That same year, when Thomson returned to Cambridge, he made his most significant scientific discovery, that of the electron (which he initially referred to as the corpuscle). On April 30, 1897, Thomson made his discovery public while giving a lecture to the Royal Institution. The evidence he produced in support of his theoretical claims was culled from a series of innovative experiments with cathode ray tubes. In one experiment, Thomson attempted to use magnetism to see if negative charge could be segregated from cathode rays, in another he tried to deflect the rays with an electric field, and in a third he assessed the charge-to-mass ratio of the rays. These and additional studies carried out by Thomson and others quickly led to widespread acceptance of Thomson’s discovery.

Once the existence of the electron was accepted, the next step was to consider how the particles were incorporated into the atom. Thomson was initially a strong proponent of what is commonly called the plum-pudding atomic model or the Thomson atomic model, although many other representations of the atom were suggested by his contemporaries. According to Thomson’s view, each atom was a positively charged sphere with electrons scattered throughout (like bits of fruit in a plum pudding). He maintained this notion until experimental research and theoretical work indicated that the atomic model described in 1911 by Ernest Rutherford, a former student of Thomson, was much more likely. The Rutherford atomic model described the structure of the atom as a positively charged nucleus around which negatively charged electrons circulated. Research since that time has resulted in the abandonment of the Rutherford model in favor of other atomic models.

Throughout most of his life, Thomson was a leading scientific figure in Britain. He held a variety of administrative positions and received many prestigious awards in addition to the Nobel Prize. Thomson served as president of the Royal Society from 1915 to 1920, and was awarded several medals by the organization, including the Royal Medal (1894), the Hughes Medal (1902) and their highest honor, the Copley Medal (1914). In 1908, the royal family honored Thomson with knighthood, and the following year he was elected president of the British Association for the Advancement of Science. His contributions were further recognized with the Order of Merit (1912), election as a master of Trinity College (1918) and honorary degrees from universities around the globe.

Thomson married in 1890. His wife was Rose Elisabeth Paget, daughter of Sir George E. Paget, Regius Professor of Physic at Cambridge. The couple had two children. Their son, George Paget Thomson, followed in his father’s footsteps, winning the Nobel Prize in Physics for work involving the electron.

Discovery of the Electron Took Decades and Multiple Scientists

In this installment on the history of atom theory, physics professor (and my dad) Dean Zollman discusses the discovery of the electron. Although one gifted scientist got the credit, he had help. – Kim

By Dean Zollman
In the last decade of the 19th century, discoveries began new directions in our thinking about the composition of matter. Two of these discoveries – radioactivity and x-rays – were somewhat accidental. We will look at each of them in future posts. Another — the identification of the electron as a component of matter – was the result of careful research and the development of improved technologies. In this post, I will discuss the electron, how it was discovered, and some of the recent views about whether this research was really a discovery.

The generally accepted year for the “discovery” of the electron is 1897. However, this discovery had its roots in research and development that date to the first half of the 19th century. Because research such as this is always built on previous work, I have a difficult time knowing how far back to go. I have chosen to start the story with Heinrich Geissler (1814-1879).

Geissler was an instrument maker who, in 1857, created electric discharge tubes. These tubes were long, sealed glass cylinders and had metal electrodes at each end. Geissler connected a high voltage across the two electrodes and used another of his inventions, the vacuum pump, to decrease the pressure inside the tube. He found that the gas inside the tube would glow, with the color depending on what gas was trapped inside. The picture below shows a drawing, published in 1869, of several different Geissler tubes. These instruments might remind you of neon lights and they should. The tubes used in neon lights are modern variations on Geissler tubes.

By M. Rapine (public domain)

Geissler conducted research to improve the tubes. He provided many tubes to other researchers and sold them to nonscientists for entertainment and decorative purposes.

The Cathode Ray

Sir William Crookes as drawn by Sir Leslie Ward in 1902 (public domain)

One of the researchers was William Crookes (1832-1919). Crookes improved on the tube and conducted many experiments. One of his conclusions was that something was being emitted from the negative electrode and was moving in a straight line to the positive end of the tube. Whatever was moving seemed to behave somewhat like rays of light. The negative end of an electrical device was called the cathode, so these “things” became known as cathode rays and the vessels were called cathode-ray tubes. (If you wish to see several photographs of Geissler and Crookes tubes, you should visit the Cathode Ray Tube Site.)

Just like light, the cathode rays could cast a shadow. In a famous experiment. Crookes inserted a Maltese cross in a tube. He saw that a shadow of the cross was cast on the end of the tube. However, the cathode rays in some ways acted differently from light. For example, they could be deflected by a magnetic field.

By D-Kuru, used under the terms of Creative Commons BY-SA 2.0 license

Two different views of cathode rays developed. Most British physicists concluded that the experiments indicated that the “rays” were some type of particle. Crookes proposed that they were negatively charged molecules. On the European continent, primarily in Germany, the light-like behavior led physicists to the conclusion that the rays were disturbances in the ether.

Each side had some experimental evidence to support its view. At the time, light was “known” to be a wave that traversed the ether, and all waves and other disturbances in the ether were assumed to travel at the same speed as light. However, cathode rays moved at much slower speeds. So this fact was an indication that the rays were not disturbances in the ether but particles. Also, on the particle side, the rays were deflected by a magnetic field, indicating that they had an electrical charge.

Atoms Have ‘Corpuscles’

Many of the prominent scientists who were involved in this debate also conducted work related to the model of the atom as a vortex in the ether that we discussed last month. One was John Joseph Thomson (1856-1940). Last month, I mentioned that he had written a theoretical paper on vortices in the ether. In 1884, he became the Cavendish Professor of Physics at Cambridge University where he undertook many experimental studies. In 1895, x-rays were discovered to be coming from a Crookes tube (more on that discovery next time). This result piqued Thomson’s interest in cathode rays. He set about to measure the ratio of the mass of a cathode ray to its electrical charge. A drawing and a picture of his apparatus are shown below.

By J.J. Thomson (Philosophical Magazine, 44, 293 (1897, Public domain)

Photo by Science Museum London/ Science and Society Picture Library (used under the terms of Creative Commons BY-SA 2 license)

In the drawing the cathode is labeled C. That is where the cathode rays are emitted. The item marked A is the positive electrode (anode) so the cathode rays are attracted toward it. But there is a slit in the anode, so some of the cathode rays go through the slit and continue their journey. Object B narrows the beam of rays that pass into the next region. D and E are metal plates that can be connected to a battery. Not shown in the drawing but visible in the picture are two coils of wire that can be used to create a magnetic field. With all of this equipment Thomson and his assistants could deflect the cathode rays up or down. Connect the positive side of the battery to D and negative to E and the rays move up. Reverse it and they move down. The magnetic field is a little more complex, but up and down motion can be created by the direction of the electrical current in the coils.

Thomson’s plan was to balance the electrical and magnetic forces so that the cathode rays went straight through his apparatus even though they were subject to both electric and magnetic forces. From voltages and currents, he could determine the magnitude of these forces. Then, doing some algebra with equations that had been developed during the 19th century he could come up with a value for the ratio of the mass of a cathode ray to its charge. He could not determine either the charge by itself or the mass by itself. His measurements allowed only a determination of the ratio.

However, that ratio was enough to indicate that cathode rays were something quite different from any known object. First, they were particles. A disturbance in the ether could not have been deflected in this way. Second, the ratio that Thomson measured was about 1,000 times different than he would have expected if cathode rays were atoms. However, he could not with the experiment determine which was different. The mass could have been 1,000 times smaller or the electric charge could have been 1,000 times bigger. (Thomson measurements were not all that good. Today we know the electron is about 1,800 times less massive than the hydrogen nucleus. But nothing anywhere close to this small had ever been measured, so being off by a large amount did not matter.)

Thomson bet on the mass being smaller. On April 30, 1897, at a public lecture he announced the discovery of “corpuscles,” which he said were very small constituents of all atoms. Over the next few years, he completed several other experiments, including one that enabled him to determine the mass of the corpuscles. Eventually, he built a model of the atom which included the corpuscles. But, I will save that for a later post.

In an earlier post, I mentioned George Stoney (1826 –1911) who coined the word electron in 1891. Others began to use that label for the cathode ray corpuscles, but Thomson did not. In his Nobel Prize acceptance speech (1906), Thomson referred to “carriers of negative electricity” as “corpuscles.” Eventually however, “electron” became the commonly accepted name.

So, Who Deserves the Credit?

Historians and philosophers of science have lots of discussion about the discovery of the electron. Lots of experiments led up to Thomson’s. And others were doing similar experiments at about the same time. So, should Thomson deserve credit for the “discovery” when it was just one step in a many-step process? Further the impact of Thomson’s announcement was not immediate. It took a while to soak in.

Some philosophers will use this example to debate what it means to discover something new. I don’t want to go there. Clearly Thomson’s work was an important step in our understanding of the structure of matter. It built on other people’s work, and others built on it. Some people were doing similar work at the same time. That’s the way science happens.

Next time we will look at something which was clearly a discovery – x-rays.

Post Scripts

  • In addition to being an excellent scientist, Thomson was also a gifted mentor. Seven of his research assistants and his son received Noble Prizes.
  • The cathode-ray tube may seem like an esoteric device. However, until very recently almost all of us had at least one in our homes. Before flat screen televisions, the picture tube on our TVs was an advanced version of a cathode-ray tube. At the back was a device that accelerated electrons. It was quite similar to parts A and C in Thomson’s diagram. Then magnetic coils similar to the coils of wire in Thomson’s apparatus applied forces to steer the electrons to various locations on the front of the screen. Of course, doing this is such a way that we saw a picture required some technology that was not available until the 20th century. But the basic principles are much the same as they were when J.J. Thomson identified cathode rays as corpuscles that eventually came to be called electrons.
  • You can try a virtual version of Thomson’s experiment. This one shows a drawing of modern equipment such as students would use today. In this simulation, changing the current changes the magnetic force while changing the voltage changes the electric force. Another has the same experiment, but it is set in apparatus similar to that of Thomson. For this one, E is the electric field, and B is the magnetic field.

Images via Wikimedia Commons.

Dean Zollman is university distinguished professor of physics at Kansas State University where he has been a faculty member for more than 40 years. During his career he has received four major awards — the American Association of Physics Teachers’ Oersted Medal (2014), the National Science Foundation Director’s Award for Distinguished Teacher Scholars (2004), the Carnegie Foundation for the Advancement of Teaching Doctoral University Professor of the Year (1996), and AAPT’s Robert A. Millikan Medal (1995). His present research concentrates on the teaching and learning of physics and on science teacher preparation.

Discovery of the Electron

Thomson continued to investigate the cathode rays, and he calculated the velocity of the rays by balancing the opposing deflection caused by magnet and electric fields in a cathode ray tube. By knowing the velocity of the cathode rays and using a deflection from one of the fields, he was able to determine the ratio of electric charge (e) to the mass (m) of the cathode rays. He continued this line of experimentation and introduced various gases into the cathode tube and found that the ratio of the charge to mass (e/m) didn’t depend on the type of gas in the tube or the type of metal used in the cathode. He also determined that the cathode rays were about a thousand times lighter than the value already obtained for hydrogen ions. In further investigations, he measured the charge of electricity carried by various negative ions and found it to be the same in gaseous discharge as in electrolysis.

From his work with the cathode tube and comparison with results derived from electrolysis, he was able to conclude that cathode rays were negatively charged particles, fundamental to matter, and much smaller than the smallest known atom. He called these particles 𠇌orpuscles.” It would be a few years later before the name 𠇎lectron” would come into common usage.

Thomson first announced his idea that cathode rays were corpuscles at a Friday evening meeting of the Royal Institution in late April 1897. The suggestion put forth by Thomson that the corpuscles were about one thousand times smaller than the size of the then smallest particle known, the hydrogen atom, caused a stir in the scientific community. Also, the idea that all matter was made up of these small corpuscles was a real change in the view of the inner workings of the atom. The notion of the electron, or the smallest unit of negative charge, was not new however, Thomson’s assumption that the corpuscle was a fundamental building block of the atom was radical indeed. He is credited with the discovery of the electron since he provided experimental evidence of the existence of this very small fundamental particle—of which all matter consists. His work would not go unnoticed by the world, and in 1906 he was awarded the Nobel Prize in physics "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases." Two years later, he was knighted.

Thomson&aposs Plum Pudding model of the atom.


J. J. Thomson's identification of the electron in 1897 focused new attention on questions of atomic structure. Thomson conjectured that the electron was a fundamental building block of matter or atoms, and along with his colleagues at Cambridge attempted to build upon his discovery in order to model atomic structure with theoretical speculations and extensive experimental investigations, particularly scattering experiments. They struggled to explain many observations, such as the nature of positive charge, the relation between number of electrons and atomic weight, and the mechanical stability and chemical properties of atoms. While the Cambridge scientists and others working within the framework they had established came up with models of the atom that successfully accounted for many of these phenomena, the behavior of atoms came to be explained much more effectively as physicists adopted the ideas of quantum science beginning about 1912.

Other investigations also built upon Thomson's discovery. Further research by Thomson, as well as work by Henri Becquerel (1852-1908), Lenard, Ernst Rutherford (1871-1937), and others, helped to show that the electron identified by Thomson was the same as the negatively charged particles observed in phenomena such as radioactivity and the photoelectric effect. American scientist Robert Millikan (1868-1953) improved upon Thomson's measurement of the charge on the electron by observing the motion of charged oil drops. By the 1920s, scientists were studying electrons within the framework of quantum physics, and began to explore the theory that electrons behaved not only as particles but also as waves. Several Nobel Prizes were given for early research related to the discovery and study of the electron, including one to Thomson in 1906 and to Millikan in 1923. As testimony to Thomson's influence as a teacher, seven of his research assistants also went on to win Nobel Prizes for physical research.

The impact of the discovery of the electron extended far beyond science. Throughout the nineteenth century, research into electrical phenomena had been intertwined with efforts to advance practical uses of electricity such as the telegraph and electrical power. The investigations of Thomson's era helped bring about the rapid invention and development of "wireless telegraphy," or radio, and led to the invention of television and later the development of microwave technologies such as radar. Radio arose in part from investigations into the nature of the electromagnetic "ether" or atmosphere, a subject that Thomson also addressed in his research. The invention of television is more directly indebted to the discovery of the electron, as electronic television is based on cathode ray tubes in which a beam of electrons is aimed at a screen. While Thomson's experiments and theories did not result directly in any of these inventions, his contributions advanced understanding of the nature and behavior of electrical processes and atomic structure, making such technological developments easier and faster.

Problem #5: Describe the Atom [Solved]

Describe the atom, giving a brief historical background development that led to the present description.

As said in the previous problem, an atom is the smallest part of an element that can exist chemically. But the atom is not the ultimate particle of matter it is itself made of smaller particles called sub-atomic particles and it has a structure.

The atom has been an object of discussions and studies since antiquity. Greek Philosophers asked the question: What are the ultimate constituents of matter? Among the ideas proposed at that time is Democritus’s suggestion that matter discontinuous, i.e. matter is composed of atoms (in Greek, atomos means indivisible).

But it is at the beginning of 19 th Century that real scientific research about the nature and structure of the atom started. From that time up to now, many atomic models have been proposed:

I. Dalton’s Atomic Model

In 1803, the British chemist and physicist advanced a first scientific proposition of an atomic model or atomic theory. In his theory:

  1. Elements consist of indivisible small particles called “atoms”
  2. All atoms of the element are identical. Different elements have different types of atoms
  3. An atom can neither be created nor destroyed
  4. Compounds are formed when atoms of different elements join in simple ratios to form molecules
Source: Analist Chemistry Blog

Although this model constituted the cornerstone in the study of matter, it was discovered later on that some of the statements were right (4), others half-truth (2), others wrong (1, 3).

II. Thomson’s Atomic Model

Figure 1a: Cathode Ray Experiment

Figure 1b: JJ Thomson Atomic Model (Plum pudding atomic model)

III. Rutherford’s Atomic Model

In 1911, Ernest Rutherford, New Zealand-born British physicist, a former student of J.J. Thomson, after his gold foil experiment (Fig.2), proposed a new atomic model.

Figure 2: Gold Foil Experiment

In that model, almost the total mass of the atom is concentrated in the nucleus, surrounded by an empty space occupied by the tiny electrons revolving around the nucleus.

Figure 3: Rutherford's Atomic Model (Planetary Atomic Model)

But this model couldn’t explain why the electrons, negatively charged, wouldn’t be attracted by the positively charged nucleus and spiral into the nucleus. According to James C. Maxwell, “An electron that is accelerating radiates energy. As it loses energy, it spirals in to the nucleus”. Hence the atom proposed by Rutherford couldn’t be stable!

IV. Bohr’s Atomic Model

In 1913, in order to solve the problem raised by Rutherford’s atomic model, Niels Bohr introduced the concept of quantization of atomic energy levels.

Only if an electron receives the appropriate energy corresponding to the difference between two energy levels, ΔE = En2 – En1= hν, then it can jump (excited state) from n1 level to n2level. But since the excited state is not stable, the electron will return back to the non-excited state or ground state by emitting the absorbed energy.

Formation of the Absorption and Emission Spectra of Hydrogen

You have certainly observed the emission phenomenon when you drop willingly or accidentally some crystals of salt (NaCl) in a blue flame: a very brilliant yellow flame is observed, which is the emission flame of Sodium atom.

Emission Flame of Sodium

Flame Test

Although Bohr’s atomic model helped in explaining some phenomena and behaviors of the atom, it has its own weaknesses:

  • This model applies well for Hydrogen atom, the simplest atom of the chemical elements, made of 1 proton and 1 electron it couldn’t apply for multi-electron (more than 1 electron) species
  • It is against the Heisenberg’s Uncertainty Principle, since in this model, the electron can be localized at any point of the orbit. The uncertainty principle states that it is not possible to know with high accuracy both the position and momentum of a moving particle.

V. Quantum Mechanical Model of the Atom

This model was introduced by Erwin Schrodinger in 1926. Schrodinger’s model considers that an electron cannot be localized precisely on an orbit or point due to Heisenberg’s Uncertainty Principle. But only the probability of finding an electron in a certain region can be estimated.

In this model, an electron in an orbital is described by 4 quantum numbers:

  1. The principal quantum number: n = 1, 2, 3, 4. gives the main or principal energy level. Traditionally those energy levels have been named by letters: K, L, M, N .
  2. The orbital quantum number, l: with values: (n – 1), (n – 2) . 1, 0. It shows the angular moment of the electron. It gives the shape of the orbital. Traditionally the orbital quantum numbers have been given specific letters to identify them: s(l = 0), p(l = 1), d(l = 2), f(l = 3), etc.
  3. The magnetic quantum number, (ml): ml = + l, +(l – 1), . 1 , 0, -1, . –(l -1), -l, which governs the energies of electrons in external magnetic fields. It gives the orientation of the orbital.
  4. The spin of electron: an electron can spin around its axis, clock or anti-clockwise that quantum number, represented by the symbol s, indicates the spinning movement of the electron it can take two values: s = +1/2, -1/2, sometime represented by the signs ↑, ↓.

The first 3 quantum numbers describe an orbital in terms of the principal quantum number, its shape and its orientation.

s, p, and d Orbitals

VI. Chadwick

In May 1932, James Chadwick announced that the atomic nucleus contains a new uncharged particle, which he named the ‘neutron”. This discovery helped to explain the existence of Isotopes.

This discovery concluded more than 1 1/3 century of research on the composition and the structure of the atom, made of:

  • Nucleus: the center of the atoms where Protons(p) positively charged and Neutrons (n) with no charge are found.
  • Electrons(e), negatively charged that surround and move around the nucleus.
  • In an atom, the number of protons is equal to the number of electrons that is why an atom is neutral.

Representation of an atom:

X = Chemical symbol of the element

A = Mass number, equal to number of protons + number of neutrons

Z = Number of protons or atomic number, equal to the number of electrons