Frederick Soddy

The English chemist Frederick Soddy (1877-1956) shared in the discoveries of atomic disintegration and of helium production during radioactive decay and introduced the term "isotope" to nuclear science.

Frederick Soddy was born at Eastbourne, Sussex, on Sept. 2, 1877. He studied at Eastbourne College; University College, Aberystwyth; and Merton College, Oxford, where in 1898 he received his degree in chemistry.

Radioactivity Studies at Montreal

Having accepted a demonstratorship in chemistry at McGill University, Montreal, Soddy found himself increasingly attracted by the work being done by Ernest Rutherford, then research professor of physics at the university. He joined Rutherford's team and brought to it his valuable experience as a chemist.

In a study of the radioactivity of thorium, Rutherford and Soddy added ammonia to a solution of a thorium salt, so precipitating out thorium hydroxide. When the insoluble material had been filtered off, the remaining solution still showed radioactivity. They established that this was due to a highly radioactive substance which they called thorium-X. Detailed measurements were made of the radioactivity of solution and precipitate over a number of weeks, and it became clear that different chemical species were involved in the process of radioactive decay over the period studied.

Further evidence for a strangely new kind of disintegration came from Rutherford's and Soddy's examination of the behavior of uranium, which when pure, emitted alpha particles only. The beta emission often encountered must therefore come from some other substance. Rutherford had already noted a gaseous emanation from thorium; now, with Soddy, he suggested that it belonged to the inert gas family. Also, they removed all doubts about the existence of a similar emanation from radium by condensing it with liquid air.

Soddy, who had long been interested in the historical problem of alchemy, now used the alchemical term "transmutation" to describe the changes that are accompanied by radioactive emission. Rutherford adopted the concept, and in 1903 they announced the general theory of radioactive disintegration. They proposed that radioactivity was an atomic phenomenon and that radiation was an accompaniment of chemical transmutations of the atoms themselves. This theory, though often bearing Rutherford's name alone, was in fact a product of the joint activity of Rutherford and Soddy.

Helium Studies at London

In 1903 Soddy left Montreal for London, drawn by the reputation of Sir William Ramsay at University College. Soddy was anxious to study further the gases associated with radioactive materials. Ramsay's laboratory, internationally acclaimed for the addition of the inert gases to the periodic table, was almost the only place where minute quantities of rare gases could be successfully examined.

Ramsay had recently acquired a small amount of radium bromide, and he and Soddy examined the gaseous emanations which were pumped off. After removal of oxygen and other common gases, the residue was examined spectroscopically. It was found to give the same spectrum as helium. When the gas was cooled by liquid air to remove the helium, the residue, as expected, gave no helium spectrum; but after a few days the helium line reappeared. Clearly helium had formed as a product of radioactive decay. Soddy concluded that the helium originated with the alpha particles, which were thus helium nuclei--a view later confirmed by Rutherford. Ramsay and Soddy showed that the other gaseous emanations were true inert gases.

Defining the Isotope at Glasgow

In 1904 Soddy moved to the University of Glasgow to take up a special appointment as lecturer in physical chemistry (including radiochemistry). During his first few years he made steady progress in purifying radioactive materials. In 1908 he married Winifred Beilby, only daughter of George Beilby of the Cassell Gold Extracting Company, which provided financial support for a research program in which Soddy was engaged involving methods of extraction for possible substitutes for radium. This project yielded few results of importance.

In 1910 Soddy turned his attention to the short-lived radioelements, collaborating with Alexander Fleck. They decided to establish the chemical characteristics of every known member of the disintegration series. They showed that in several cases a number of intermediates were chemically identical and inseparable from one another, yet underwent radioactive decay in quite different ways. Thus identical chemical properties were shown by radium-B, thorium-B, actinium-B, and lead.

Soddy's first generalization on these mysteries came in his rule that loss of an alpha particle from an atom with an even number in the periodic table produces an atom with the next lower even number. In subsequent changes, however, when alpha emission does not take place, a reversion to the original "family" may occur, and the products will be chemically inseparable from the starting material, even though the atomic weights vary. Complementary to this alpha-particle rule is the one that in beta emissions an atom moves up one place in the periodic table. In 1913 Soddy combined the alpha and beta rules into the group displacement law: one alpha emission causes a shift two places back in the periodic table, and one beta emission causes a shift one place further on. Hence a sequence of alpha-beta-beta emissions would mean a return to the original place in the table.

The underlying concept, that more than one kind of atom might be assignable to the same chemical "space," was daring and revolutionary. In December 1913 Soddy brought matters to a head by writing a letter to Nature in which he proposed that such chemically inseparable species should be termed "isotopes." In modern parlance, they differ from each other in mass but not in overall nuclear charge. The group displacement law and the related concept of isotopy were soon confirmed.

In 1914 Soddy became professor of chemistry at Aberdeen. His fortunes here were immediately and drastically affected by the war. He was able to complete some of the work begun in Glasgow, but his radiochemical researches were brought to a premature end by the special wartime demands made upon his laboratories.

Oxford and Retirement

With the ending of the war the future held great promise for radiochemical studies in Britain. In 1919 Soddy was appointed Lee's professor of inorganic and physical chemistry at the University of Oxford. Two years later he received the Nobel Prize in chemistry for his contributions to radiochemistry and, particularly, to the concept of isotopes.

It was widely hoped that, under Soddy's leadership, a British school of radiochemistry would emerge at Oxford that would complement the work of the atomic physicists at the Cavendish Laboratory in Cambridge. Unfortunately this was not to be, for his output of original work in science was negligible. In 1936 he resigned his chair. The death of his wife no doubt contributed to his discontent, but this cannot explain the full measure of his apparent disenchantment with experimental work.

Soddy was an extremely talented writer, and to some extent his literary gifts may have interfered with his laboratory research. His first book, Radioactivity, appeared in 1904. For many years, beginning in 1904, he contributed articles on radioactivity to the Annual Reports of the Chemical Society. The Interpretation of Radium (1909) was a popular treatise deriving much from his Glasgow lectures. The Chemistry of the Radioelements (1910) was a concise and reliable summary of the contemporary position. Later works included The Interpretation of the Atom (1932) and The Story of Atomic Energy (1947). He also wrote several books on economic theory. He died on Sept. 21, 1956, in Brighton.

Further Reading

• Muriel Howarth, Pioneer Research on the Atom (1958), contains a biography of Soddy. The Royal Society of London, Biographical Memoirs of Fellows of the Royal Society, vol. 3 (1957), has a biography of Soddy by Sir Alexander Fleck. Additional material is in Henry A. Boorse and Lloyd Motz, eds., The World of the Atom (2 vols., 1966).
• Frederick Soddy (1877-1956): early pioneer in radiochemistry, Dordrecht; Boston: D. Reidel Pub. Co.; Hingham, MA, U.S.A.: Sold and distributed in the U.S.A. and Canada by Kluwer Academic Publishers, 1986.
• Merricks, Linda, The world made new: Frederick Soddy, science, politics, and environment, New York: Oxford University Press, 1996.