Luckily for the progress of atomic science, radioactivity occurs naturally in the heavy elements. It was the attempt to unravel the mysteries of natural radioactivity that led to the rapid development of atomic understanding around the turn of the twentieth century.1 Although natural radioactivity contains a store of valuable information, one basic fact about it made particularly difficult the task of decoding its message about submicroscopic nature: It occurs in long chains of successive transformation and transmutation. In a natural sample of uranium are juxtaposed 18 different radioactive isotopes of 10 elements with half lives ranging from 164 microseconds to 4 billion years, all undergoing simultaneously their individual processes of decay. It was a triumph of human intellect, as well as of international cooperation and communication, that only fifteen years elapsed from Becquerel’s discovery of radioactivity to Rutherford’s discovery of the nucleus.
Among the new insights gained from the study of radioactive decay chains, two stand out as dramatic revolutions in thinking about the structure of matter: (1) Transmutation accompanies radioactivity; one element can transform itself into another. (2) There exist different versions of the same element; isotopes are identical chemically, but differ in atomic weight and in radioactive properties. This pair of discoveries toppled two nineteenth-century axioms of chemistry—that each element is composed of identical atoms of a single kind, and that atoms are immutable. It is ironic that after alchemists had expended so much fruitless effort in trying to transmute elements, transmutation was finally discovered to be a spontaneous process in nature.
The large number of radioactive isotopes among the heavy elements fit into three series, each based on a very long-lived parent isotope. The uranium series springs from 92U238, whose half life is 4.5 billion years. The thorium series springs from 90Th232, whose half life is 14 billion years. The actinium series (named for one of its members, not for its parent) springs from 92U235, whose half life is 713 million years. Because of its shorter half life, most of the uranium 235 that was present when the Earth was formed has since decayed away, but enough remains—one atom of U235 for every 138 atoms of U238—to give rise to a significant chain of decay. For illustrative purposes, it is sufficient to focus attention on one decay chain. The uranium series is illustrated in the figure below. This diagram is a selected part of the upper right end of a chart of the nuclei, with proton number (atomic number) plotted vertically, neutron number plotted horizontally.
The key discoveries of transmutation and of isotopes, which flowed from the analysis of radioactive decay chains, were closely tied to the discovery of several new elements. When radioactivity was discovered, only two elements heavier than bismuth (now known to be element number 83) were known, thorium and uranium. Since the atomic numbers of these elements were unknown, the number of missing elements between bismuth and uranium was also unknown. After radioactivity became available as a new tool of analysis, the missing pieces began to fill in rapidly. In 1898, Marie and Pierre Curie isolated, identified, and named two new elements, polonium and radium. Although they did not know it at the time, they actually isolated particular isotopes of these elements, those produced by the uranium decay chain: 84Po210, whose half life is 138 days, and 88Ra226, whose half life is 1,620 years. (This isotope of radium used to be used in the luminous paint of watch dials.) In the following year, André Debierne identified another new element, actinium (now known to have atomic number 89). At about the same time, Rutherford in Canada and Ernst Dorn in Germany were finding evidence for new radioactive gases. Rutherford named his new substance emanation. It came from thorium, had a half-life of 1 minute, and, as he and Frederick Soddy later learned, was chemically inert. Dorn’s gas, also chemically inert, came from radium and had a half life of 3.8 days. He called it radon. Now both are recognized as isotopes of radon (Z = 86), the heaviest of the rare gases. Rutherford’s emanation (also known for a time as thoron) is 86Rn220. Dorn’s “radon” is 86Rn222. With the discovery of these isotopes began a decade of confusing multiplicity in the apparent number of elements.
A century of progress in chemistry had seemingly made clear that all atoms of a given element are identical. Therefore scientists engaged in the study of radioactivity quite naturally assumed that if two substances, both identifiable as elements, differed in any way at all—for instance, in half life—they must be different elements. As radioactive decay chains were gradually untangled, new elements seemed to proliferate remarkably. Some of the names assigned to isotopes in the uranium series are shown in the figure above. At one time, six isotopes of thorium bore six different names, as shown in the table below.
Before long, the proliferation of “elements” began to frustrate the chemists. In 1907, Herbert McCoy and William Ross in Chicago found thorium and radiothorium to be chemically inseparable. In 1908, Otto Hahn in Berlin found it impossible to separate ionium and thorium by chemical means. Finally, in 1910, totally frustrated in his efforts to separate these substances, Soddy took a courageous step. He suggested that a single element might exist in two or more forms, different in mass and in radioactive properties, but identical chemically. Later he named these separate forms isotopes. Clinching evidence for isotopes came from two quite different experiments in 1913. In that year, Soddy showed the spectra of ionium and thorium to be identical. At about the same time, J. J. Thomson magnetically deflected a beam of neon ions moving in an evacuated space, and discovered that some of the ions were of atomic mass about 20, others of atomic mass 22. He thereby demonstrated that multiple isotopes are not exclusively an attribute of radioactive elements.
Working with Rutherford, Soddy had also figured in another courageous conclusion in 1902. Stimulated by a suggestion of Becquerel, Rutherford and Soddy studied a radioactive “impurity” in thorium, which they named thorium X.2 Finding that thorium and thorium X were undeniably distinct chemically, and that thorium X was a product created by thorium, they concluded that transmutation accompanies radioactivity. The Curies, less willing than Rutherford to abandon the solid rock of immutability of atoms, were at first reluctant to accept this revolutionary proposal. But in 1903 (the year Marie Curie received her doctorate and a Nobel Prize), they too accepted the idea of transmutation. Before long it was established beyond question by the combined evidence of chemistry and radioactivity.
Following up their proposal of transmutation, Rutherford and Soddy drew two other related conclusions about the nature of radioactivity that stand as landmarks of discovery in this period. First, they proposed the “one-step” theory of transmutation, that an atom does not gradually evolve into another as it releases radioactive energy, but instead transforms itself instantaneously at the moment of radioactive decay. In this proposal, they were coming very close to the idea of probability working at a fundamental level, since radioactive decay was known to follow rules of probability. But in the absence of forcing evidence, to abandon certainty in science was inconceivable. The hint provided by radioactive decay was ignored for many years. Second, Rutherford and Soddy, analyzing the energy released by radioactivity, concluded that radioactive transmutation must involve at least 100,000 times as much energy per atom as does chemical change. From their work emerged the picture of radioactive decay that is still valid—a violently disruptive explosion of a single atom.
Most of what is in this Essay comes from a charming and authoritative small book by Alfred Romer, The Restless Atom (Doubleday Anchor Books,1960).
1 Often the outcome of scientific inquiry is hard to foresee. In an interesting twist of history, the radioactivity of the heaviest natural element, uranium, launched a series of discoveries that culminated in a successful theory of the lightest element, hydrogen.
2 Thorium X is now recognized as an isotope of radium, 88Ra224.