Thomson Confirms the Possibility of Isotopes

Joseph John Thomson was the first to isolate isotopes of stable elements.

Summary of Event

In the history of science, isotopes remain one of the unpredicted facets of nature. Isotopes are one or more forms of a chemical element and act similarly in chemical or physical reactions. Isotopes differ in their radioactive transformations, and they possess different atomic weights. Isotopes
[kw]Thomson Confirms the Possibility of Isotopes (1910)
[kw]Isotopes, Thomson Confirms the Possibility of (1910)
[g]England;1910: Thomson Confirms the Possibility of Isotopes[02570]
[c]Science and technology;1910: Thomson Confirms the Possibility of Isotopes[02570]
[c]Physics;1910: Thomson Confirms the Possibility of Isotopes[02570]
Thomson, Joseph John
Aston, Francis William
Prout, William
Hertz, Heinrich
Rutherford, Ernest

In 1803, John Dalton Dalton, John proposed a new atomic theory of chemistry that stated that chemical elements in a compound combine by weight in whole-number proportions to one another. Whole-number rule[Whole number rule] By 1815, William Prout took Dalton’s hypothesis one step further and stated that the atomic weights of elements are integral multiples of the hydrogen atom. For example, if the weight of hydrogen is 1, then the weight of carbon is 12 and that of oxygen is 16. Over the next decade, a large number of carefully controlled experiments were conducted to determine the atomic weights of several elements. These results did not support Prout’s hypothesis. For example, the atomic weight of chlorine was found to be 35.5. It took the discovery of isotopes in the early part of the twentieth century to justify Prout’s original theory.

Sir Joseph John Thomson was appointed professor of physics at the University of Cambridge in 1884, but his career is generally associated with the Cavendish Laboratory, Cavendish Laboratory where he was one of the leading scientists in addition to serving as director. Thomson’s initial area of research concerned electrical discharges through gases; he used the cathode-ray discharge tube as an instrument for the exploration of matter. Cathode-ray tubes[Cathode ray tubes]

One area of controversy and competition between English and German scientists at that time was whether the cathode-ray discharge was composed of waves or particles. The first scientist to prove the case conclusively would gain both prestige and national honor. Thomson did not fit the public’s idea of a gifted scientist. His mathematical abilities were not highly refined, he possessed poor hand-eye coordination, and he was regarded as a clumsy experimenter. His genius lay in his ability to visualize mentally the experimental parameters necessary to produce intricate experiments. He also possessed the ability to create models that explained experimental results. Thomson accepted the challenge of creating an experiment that would prove that discharges from the cathode-ray tube were composed of particles.

Beginning in 1884, when he became director of the Cavendish Laboratory, Thomson pursued a series of experiments on electrical discharges. By the early 1890’s, he realized that the research of Heinrich Hertz and his student Philipp Lenard, Lenard, Philipp which showed the penetration of discharges through metal foil, was, in fact, a partial validation of his own theory of particles. In addition, Thomson’s research provided evidence that cathode-ray discharges traveled at half the speed of light, again supporting a particle theory rather than a wave theory (in which waves traveled at the speed of light). Other physicists in England also demonstrated that cathode-ray discharges, acting as particles, traveled in a curved pattern in the presence of a magnetic field. None of these experiments offered definitive proof of the particle theory, however.

The stalemate between the two competing theories changed with the arrival at the Cavendish Laboratory of Ernest Rutherford. Thomson and Rutherford began a series of X-ray experiments, and the results substantiated their theory because the radiated gases retained their conductivity after the radiation had been shut off. This condition could be explained only if either positive or negative charges were produced by radiation.

In 1899, Thomson compiled the evidence of his research and created a model of the atom with negatively charged particles on the outside. By 1904, Thomson was further able to refine the model of the atom with electrons accelerating in concentric rings surrounding the atom. Atoms;structure This later model proposed an inner ring containing the smallest number of electrons and outer rings containing progressively more electrons. The beginning of particle and nuclear physics can be dated from this moment, and all future work must credit Thomson for his contributions.

After the discovery of the electron, Thomson directed his research efforts toward discovering the nature of “positive electricity.” Positive electricity As a result, without specifically looking for isotopes, Thomson became one of the first to isolate isotopes of elements. This chance event occurred as a result of the phenomenon of positive electricity, which was first identified by Wilhelm Wien. Wien, Wilhelm Thomson undertook the next stage by developing an instrument sensitive enough to analyze the positive electron.

One person who was pivotal to the discovery of isotopes was Francis William Aston, a gifted experimenter who possessed the capacity for infinitely refining an instrument until it produced the desired results. In addition to his mechanical skills, he was an expert glassblower, and he created the discharge tubes that Thomson needed for atomic research. Aston was known for his patience while working through a series of experimental procedures many times, making minute adjustments on the instruments. He had become interested in atomic research when he read Thomson’s book Conduction of Electricity Through Gases (1903) Conduction of Electricity Through Gases (Thomson) and wanted to pursue research in cathode rays and X rays.

Aston came to the Cavendish Laboratory in 1910, and his collaboration with Thomson began at the moment when Thomson turned his attention to the positive rays generated by the cathode of the discharge tube. When the electrons are stripped from an atom, the atom becomes positively charged. By using magnetic and electrical fields, it is possible to channel these positive rays in parabolic tracks. Thomson was able to identify the atoms of different elements by examining photographic plates of such tracks. Aston’s first contribution at Cavendish was to improve this instrument by blowing a spherical discharge tube with a finer cathode and by making a better pump to create the vacuum. He also devised a camera that could take sharper photographs of the parabolic tracks.

In 1910, as a result of these refinements, Thomson saw the first indication of isotopes, although he was not aware of their importance. Two years later, further improvements to the apparatus provided proof that the individual molecules of a substance have the same mass. While working on the element neon, Thomson obtained two parabolas, one with a mass of 20 and the other with a mass of 22. At first, he thought the heavier of the two isotopes was a new element, but he eventually came to the conclusion that he had separated the isotopes of an element.


The task of identifying the large families of isotopes was left for Aston to accomplish. In 1919, Aston created a new instrument called the mass spectrograph. Mass spectrograph The idea was to treat ionized or positive atoms much in the same manner as light rays. Aston reasoned that just as light can be dispersed into a spectrum and analyzed in terms of its constituent colors, similar results might be achieved with atoms. By using magnetic fields to focus the stream of particles, he was able to create a spectrum of the atomic mass and record the result on a photographic plate. He used the gas neon for his initial test of the mass spectrograph, and the apparatus was able to separate the spectrum of both the heavier and lighter masses of the gas. The mass spectrograph had a distinct advantage over Thomson’s parabola method, as it was independent of the velocity of the particles. Aston found that neon had two isotopes: one with a mass of 20 and the other with a mass of 22 in a ratio of 10:1. This result reflected exactly the accepted atomic weight of neon (20.20), which was a combination of the two isotopes.

The search for isotopes became a major area of concentration in physics in the decade after 1919. A new isotope was found almost every other month. Chlorine had 2; bromine had isotopes of 79 and 81, which gave an atomic weight of exactly 80; krypton had 6; and other elements possessed even more. These results produced not only an entire family of nonradioactive isotopes but also finally verified the “whole-number rule” of Prout’s hypothesis.

Nevertheless, a discrepancy remained. It appeared that the atomic weight of hydrogen was slightly greater than 1. When Aston attempted to resolve this problem in 1920, he postulated that although hydrogen had a mass of 1 percent greater than a whole number, this mass was lost when atoms were “packed” to produce other elements. For example, when four atoms of hydrogen are brought together to produce one atom of helium, about 1 percent of the mass is lost. Although a more sophisticated model of the atom was required to explain the “packing fractions,” the accuracy of Aston’s instrument remained untarnished.

In 1927, Aston began to refine his mass spectrograph. The original instrument had an accuracy of one in one thousand, whereas the new instrument had ten times that accuracy. Aston then began a series of experiments to discover the packing fractions of a large number of elements. Some time later, the mass spectrograph was found to be sensitive enough to measure Einstein’s law of mass-energy conversion during a nuclear reaction. Between 1927 and 1935, Aston published the updated results. In 1935, he attempted to refine his instrument for even greater accuracy, and the resulting instrument proved to be of critical importance to the new science of nuclear chemistry, as the accurate measurement of chemical masses was essential to the success of the discipline.

The discovery of isotopes opened the way to extensive research in nuclear physics and completed the speculations begun by Prout a century earlier. Also, in the field of radioactivity—which was discovered through a separate sequence of historical events—isotopes played a central role in the development of nuclear reaction. Isotopes

Further Reading

  • Crowther, J. G. The Cavendish Laboratory, 1874-1974. New York: Science History Publications, 1974. Describes the history of the Cavendish Laboratory. Covers Thomson’s early years at the laboratory, his work as the director, his assistants and students, and his work on the electron. Also discusses the work of Thomson’s predecessor, Lord Rayleigh, and his successor, Ernest Rutherford.
  • Laidler, Keith J. To Light Such a Candle: Chapters in the History of Science and Technology. New York: Oxford University Press, 2005. Discusses the progress of both pure science and applied science over the past two centuries through the specific contributions of individual scientists and technologists. Chapter 6 is devoted to the work of Thomson. Includes bibliography and index.
  • Segrè, Emilio. From X-Rays to Quarks: Modern Physicists and Their Discoveries. San Francisco: W. H. Freeman, 1980. Segrè was one of a handful of physicists who participated directly in nuclear physics and wrote on the history of physics. Early sections of this volume cover the discoveries and theories of those who produced a coherent picture of the atom.
  • Strutt, Robert John, Fourth Baron Rayleigh. The Life of Sir J. J. Thomson, O.M., Sometime Master of Trinity College. Cambridge, England: Cambridge University Press, 1942. Firsthand account of Thomson’s activities by a friend who was there at the time. Includes information on Thomson’s presidency of the Royal Society and mastership of Trinity College, his views on education, and his personal life.
  • Thomson, George Paget. J. J. Thomson and the Cavendish Laboratory in His Day. London: Thomas Nelson, 1964. Describes in detail experiments conducted by Thomson and his colleagues, providing excellent drawings of the experimental equipment and photographs of the results.
  • Thomson, J. J. Recollections and Reflections. 1936. Reprint. New York: Arno Press, 1975. Memoir includes a chapter on “psychical research” as well as several lengthy chapters on Thomson’s visits to the United States. Contains extensive sections on individuals who influenced his life and work.

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Rutherford Describes the Atomic Nucleus

Aston Builds the First Mass Spectrograph and Discovers Isotopes

Discovery of the Compton Effect