Joseph John Thomson was the first to isolate isotopes of stable elements.
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.
In 1803, John Dalton
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,
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,
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.
After the discovery of the electron, Thomson directed his research efforts toward discovering the nature of “positive electricity.”
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)
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.
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.
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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.
Becquerel Wins the Nobel Prize for Discovering Natural Radioactivity
Barkla Discovers the Characteristic X Rays of the Elements
Thomson Wins the Nobel Prize for Discovering the Electron
Bohr Uses Quantum Theory to Identify Atomic Structure
Rutherford Describes the Atomic Nucleus
Aston Builds the First Mass Spectrograph and Discovers Isotopes
Discovery of the Compton Effect