Soddy, born the son of a corn merchant in the coastal town of Eastbourne, was educated at the University College of Aberystwyth and Oxford University. After working with Ernest Rutherford in Canada and William Ramsay in London, Soddy took up an appointment at Glasgow University in 1904. He moved to take the chair of chemistry at Aberdeen University in 1914 where he remained until 1919 when he accepted the post of Dr. Lee's Professor of Chemistry at Oxford.

Soddy's work was quite revolutionary in that he succeeded in overthrowing two deep assumptions of traditional chemistry. The first arose out of his period of collaboration with Rutherford, from 1901 to 1903. Together they established that radioactive elements could change into other elements through a series of stages.

Soddy's next major achievement was to make some kind of sense of the bewildering variety of new elements that had been found as decay products of radium, thorium, and uranium. The books of the period refer to such strange entities as mesothorium, ionium, radium A, B, C, D, E, and F, and uranium X. Such entities were clearly distinct for they had markedly different half-lives. But what they were and, more significantly, where they fitted in the periodic table were difficult questions. There were gaps in the periodic table but far too few to accommodate so many new elements. One further difficulty soon forced itself on chemists. Attempts to separate thorium from radiothorium by Otto Hahn in 1905 and radium D from lead by Georg von Hevesy a few years later had failed, as had numerous other attempts to separate various radioactive elements by chemical means.

Finally Soddy made the bold claim that the reason such substances could not be separated was because they were in fact identical. Consequently some kind of modification of the periodic table was called for. In his view (1913) “it would not be surprising if the elements…were mixtures of several homogeneous elements of similar but not completely identical atomic weights.” He called such chemically identical elements, with slightly differing atomic weights, isotopes (from the Greek words meaning in the same place). He could thus assert that both radium D and thorium C were in fact isotopes of lead. Radium D has a half-life of 24 years and an atomic weight of 210 while thorium C has a half-life of 87 minutes and an atomic weight of 212; but, although they have different half-lives and slightly differing weights, they were both chemically indistinguishable as lead.

Until the discovery of the neutron by James Chadwick a complete understanding of this enormously fruitful idea was not available to Soddy. All he could propose, somewhat vaguely, as an explanation was different numbers of positive and negative charges in the nucleus. As yet, nobody seemed to suspect the existence of a neutral particle, without a charge.

He did, however, go on to explain the transformation of atoms by his displacement law. In this, the emission of an alpha particle, a helium nucleus of two protons and two neutrons, lowers the atomic weight by four while the emission of a beta particle, an electron, raises the atomic number by one. Given these rules Soddy could show how, for example, uranium and thorium could both decay, by different paths, to different isotopes of lead (Casimir Fajans independently suggested the same law).

Despite the award of the 1921 Nobel Prize for chemistry for his work on the origins and nature of isotopes, Soddy became disillusioned with science and his place in it. After 1919 Soddy took no further part in creative science. He wrote a good deal, mainly in the fields of economic and social questions, which raised little interest or support. On the issue of energy, however, he was remarkably perceptive. As early as 1912 he could comment that “the still unrecognized ‘energy problem’ … awaits the future,” continuing with the by now familiar refrain of our profligate use of hydrocarbons, “a legacy from the remote past,” and concluding with what he saw as our only hope, atomic energy, which “could provide anyone who wanted it with a private sun of his own.”