Francis William Aston invented the first mass spectrograph to measure the mass of atoms and discovered that the atomic mass of elements is a combination of isotopes.
Isotopes are one or more forms of a chemical element and act similarly in chemical or physical reactions. Isotopes differ in two ways: They possess different atomic weights and different radioactive transformations. In 1803, John Dalton
After his discovery of the electron, Sir Joseph John Thomson, the leading physicist at Cavendish Laboratory, devoted much of his remaining research years to determining the nature of “positive electricity.” As a result, without specifically looking for isotopes, Thomson became one of the first to confirm the possibility of isotopes of elements. The phenomenon of positive electricity was first identified by Wilhelm Wien, and Thomson then began developing an instrument sensitive enough to analyze the positive electron. Francis William Aston, as a result of the combination of these events, was pivotal to the discovery of isotopes.
Francis William Aston.
Aston was ideally suited for the line of research that would eventually win for him a Nobel Prize.
Thomson had a reputation for inviting gifted scientists to work at the Cavendish Laboratory. The invitation to Aston in 1910 was such a case. Recommended by J. H. Poynting, who had taught Aston physics at Mason College, Aston began a lifelong association at Cavendish, and Trinity College became his home. By 1906, Thomson had discovered the electron and turned his attention to positive rays generated by the cathode of the discharge tube. When the electrons are stripped from an atom, the atom becomes positively charged. Through the use of magnetic and electrical fields, it is possible to channel these positive rays into parabolic tracks. By examining photographic plates of these tracks, Thomson was able to identify the atoms of different elements.
Aston’s first contribution at Cavendish was to improve this instrument by blowing a spherical discharge tube with a finer cathode; he developed a more efficient pump to create the vacuum. He also devised a camera for sharper photographs of the parabolic tracks. By 1912, the improvements to this 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. Aston was given the task of resolving this mystery. To separate the two constituents of neon, Aston decided to try fractional distillation and diffusion. The fractions of the distillation were in such minute quantities, however, that he had to invent a new balance to measure the differences. The quartz microbalance used a sealed quartz bulb balanced on a liquid of known density. By measuring the degree to which the bulb floated or sank, he could compare the density of a known gas with that of an unknown one. Aston started with a quantity of neon and after several thousand operations was able to extract a minute amount of the heavier element of neon. The change in density was barely outside the range of experimental error, however, and hence too doubtful to prove his case.
During World War I, Aston worked as a chemist at the Royal Aircraft Factory at Farnborough, improving the substance used to coat the canvas that covered the aircraft. These were productive years; a number of scientists, housed at the same facility, were able to discuss the latest developments in physics and chemistry. Aston, who was normally shy, was encouraged to discuss his work. In addition, neon discharge tubes were a focus of research in the factory because they were an excellent source of light for the stroboscope. After the war, Aston continued his attempts to distill the heavier part of neon but failed to achieve the necessary results.
In 1919, Aston began to build a mass spectrograph, an idea he had developed while at Farnborough. The idea was to treat ionized or positive atoms like light. He reasoned that just as light can be dispersed into a spectrum and analyzed through its constituent colors, the same could be achieved with atoms of an element such as neon. By using magnetic fields to focus the stream of particles, he was able to create a mass spectrum and record this on a photographic plate. The heavier mass of neon was collected on one part of a spectrum and the lighter neon showed up on another. This was a magnificent apparatus: The masses could be analyzed without reference to the velocity of the particles, which was a problem with the parabola method devised by Thomson. Neon turned out to possess two isotopes: one with a mass of 20 and the other with a mass of 22 in a ratio of 10:1. When combined, this gave the atomic weight 20.20, which was the accepted weight of neon.
Aston’s accomplishment in developing the mass spectrograph was recognized immediately by the scientific community. It was a simple device and one capable of accomplishing a large amount of research quickly. Following Aston’s pioneering work, the field of isotope research played an important part in other areas of physics.
The years following 1919 were highly charged with excitement, as month after month new isotopes were announced. Chlorine had two; bromine had isotopes of 79 and 81, which gave an almost exact atomic weight of 80; krypton had six isotopes; xenon had even more. In addition to the discovery of nonradioactive isotopes, the “whole-number rule” for chemistry was verified: Protons were the basic building blocks of different atoms, and these occurred in whole numbers. In 1920, Aston found that the mass of the hydrogen atom was about 1 percent greater than a whole number. At the time, it was thought reasonably certain that when four atoms of hydrogen were brought together, they would produce one atom of helium. This means that in “packing” the nucleus, about 1 percent of the mass would be lost.
Aston’s original mass spectrograph had an accuracy of one in one thousand. In 1927, he built an instrument that was capable of ten times greater accuracy. Using this instrument, he was able to determine the packing fractions of a large number of elements. The apparatus was also sensitive enough to measure Albert Einstein’s law of mass-energy conversion during a nuclear reaction. Between 1927 and 1935, Aston reviewed all the elements he had worked on earlier and published updated results. He also began to build an instrument with another ten times greater accuracy; this instrument proved to be of great value to nuclear chemistry.
The discovery of isotopes opened the way to further research in nuclear physics and completed those speculations begun by Prout during the previous century. Even though radioactivity was discovered separately, isotopes played a central role in the field of nuclear physics and chain reaction.
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Aston, F. W. Mass-Spectra and Isotopes. Rev. ed. New York: Longmans, Green, 1941. Expanded and reedited edition of Aston’s earlier work Isotopes. Describes the physics in terms that can be understood by general readers, although much of the content is aimed at readers with technical knowledge. Particularly valuable for insights into Aston’s precise and clearly focused mind. -
Ball, Philip. The Ingredients: A Guided Tour of the Elements. New York: Oxford University Press, 2003. Entertaining volume aimed at the general reader explains the chemical elements, offering both historical and practical perspectives. Includes illustrations and index. -
Chalmers, T. W. Historic Researches: Chapters in the History of Physical and Chemical Discovery. New York: Charles Scribner’s Sons, 1952. Provides an overview of the experimental work done through the 1920’s in physics and chemistry. -
Crowther, J. G. The Cavendish Laboratory, 1874-1974. New York: Science History Publications, 1974. History of the laboratory includes extensive sections on the working environment in which Aston conducted his studies. -
Ihde, Aaron J. The Development of Modern Chemistry. New York: Harper & Row, 1964. Comprehensive history of the different aspects of chemistry includes brief coverage of Aston’s contribution. Chapter 18 covers the developments in radiochemistry that led directly to Aston’s work. -
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 work of individual scientists and technologists. Chapter 6, which is devoted to the work of Thomson, briefly discusses Aston’s contribution. Includes bibliography and index. -
Thomson, George Paget. J. J. Thomson and the Cavendish Laboratory in His Day. London: Thomas Nelson, 1964. Details the work done by Thomson and his associates. Describes experiments in great detail and provides excellent drawings of the experimental equipment and photographs of the results.
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