Frederick Soddy and the World Set Free

by Nicholas Mee on September 13, 2020

Frederick Soddy was a pioneer of nuclear physics and radio-chemistry. He was appointed Lecturer in Physical Chemistry at Glasgow University in 1904. Five years later he published The Interpretation of Radium, a best-selling and very influential book based on six public lectures delivered at the university. In one lecture Soddy addresses the audience holding aloft a bottle containing a uranium oxide solution:

Frederick Soddy’s Plaque at Glasgow University.

Is it not wonderful to reflect that in this tiny bottle there lies asleep and waiting to be evolved the energy of at least one hundred and sixty tons of coal? The energy in a ton of uranium would be sufficient to light London for a year.

The radioactive decay of uranium releases energy at a slow and steady pace. But Soddy considered what would happen

if only it were under our control and could be harnessed to do the world’s work in the same way as the energy in coal has been harnessed and controlled.

He believed that if a way were found to speed up the release of energy from radioactive decay, then the world would be transformed. Soddy’s visionary musings would inspire a remarkable and prophetic novel.

The World Set Free

The cover of the first American edition of The World Set Free by H.G. Wells.

H.G. Wells wrote The World Set Free in 1913 on the eve of the First World War. Wells speculated about where future developments in physics might lead. He imagined that physicists would meet the challenge set by Soddy by developing ways to extract nuclear energy at will, and invented a character named Holsten who would make the critical breakthrough. In the words of the novel:

The problem which was already being mooted by such scientific men as Ramsay, Rutherford, and Soddy, in the very beginning of the twentieth century, the problem of inducing radio-activity in the heavier elements and so tapping the internal energy of atoms, was solved by a wonderful combination of induction, intuition, and luck by Holsten so soon as the year 1933.

Holsten’s great discovery is a new element named carolinium that could be induced to release the energy locked within its atoms. This opened the door to military applications, bombs of hitherto inconceivable destructive power, atomic bombs, as Wells calls them in the first ever use of this term. Although Wells’s method of delivery for the new weapons was of its time—they were lobbed over the side of aircraft by intrepid airmen, as depicted on the front cover of the American edition—the novel would prove uncannily accurate about future developments of nuclear weapons and their military consequences. But that was still some decades ahead.

Decay Chains

Just a few years earlier, Soddy and Rutherford had discovered that atoms change their identity when they emit radiation. They realised that radioactive atoms often change into other radioactive atoms and this transmutation continues until eventually a stable atom is formed that persists indefinitely. Soddy and his colleagues now began the laborious process of mapping out the identity of each atom in these radioactive decay chains. 

Soddy’s example of a disintegration series or decay chain in his book The Interpretation of Radium.

In The Interpretation of Radium, Soddy presented the still incomplete example of a decay chain shown above. The atoms in each chain could be identified by their distinct radioactive decay patterns. They were labelled with names such as uranium II, uranium X, emanation, mesothorium, ionium and radium A to radium G. Eventually, over forty radioactive species were identified. But clearly they could not all be new elements.


Soddy found the answer in 1913, when he conclusively demonstrated that although radium G, thorium D and actinium D might each be the stable endpoints for three different radioactive decay chains, they were all chemically identical. All three of these different atomic species were, in fact, atoms of the same rather familiar element lead, but with different atomic masses. We know them today as lead-206, lead-207 and lead-208. It is perhaps ironic that whereas the alchemists famously aimed to create gold as the culmination of a process beginning with lead, the radio-chemists had discovered that lead is actually the endpoint of these natural radioactive decay series.

At a family dinner party, Soddy described his research to a very learned family friend Margaret Todd who was a medical doctor. She suggested that chemically identical atoms with different masses might be called isotopes from the Greek ‘iso’ meaning ‘the same’ and ‘topos’ meaning ‘place’, because they would all share the same place in the Periodic Table. Soddy gratefully accepted her suggestion and this is still the term that is used today.

Protons and Neutrons

We now know that the atomic nucleus is composed of two types of particle: positively-charged protons and electrically-neutral neutrons. (These two types of particle are collectively known as nucleons.) Chadwick’s discovery of the neutron in 1932 provided the explanation for how elements can have different isotopes. It would also bring about the possibility of nuclear power and nuclear weapons, as envisaged by H.G. Wells.

The number of protons in the nucleus determines the chemical properties of the atom, so a nucleus containing two protons is a helium nucleus, for instance, whereas a nucleus containing ninety-two protons is a uranium nucleus. But the nucleus also contains neutrons. The helium-3 nucleus contains two protons and a neutron, whereas a helium-4 nucleus contains two protons and two neutrons. So nuclei with the same number of protons and different numbers of neutrons form atoms of the same element, but they have different atomic masses—they are different isotopes of that element.

Schematic diagram of the two stable helium isotopes. Left: helium-3. Right: helium-4. (In reality the diameter of the electron orbitals is vastly greater than the nuclear diameter.)

Alpha and Beta

Soddy’s decay chain shown above indicates whether each radioactive decay is due to the emission of an alpha particle or a beta particle. (All the atoms in the diagram emit alpha particles except the second which emits a beta particle.) Soddy was the first to understand how the emission of these particles affects the position of an atom in the Periodic Table. 

Alpha particles are composed of two protons and two neutrons—they are identical to helium-4 nuclei. So each alpha decay transforms the nucleus into one with two fewer protons and two fewer neutrons. On the chart below, alpha decays move the nucleus two steps diagonally to the left.

Beta decay is rather different. When a nucleus undergoes beta decay one of its neutrons changes into a proton and an electron is emitted, along with another particle known as an antineutrino. (The beta particle is the emitted electron.) The nucleus therefore retains the same total number of nucleons, but it has one more proton and one less neutron. So on the chart below, beta decays move the nucleus one step diagonally to the right. 

The Nuclear Chequerboard

The chequerboard diagram shown here plots a sequence of nuclear transmutations. It shows the same radioactive decay chain as illustrated in Soddy’s diagram above. Following the arrows we see that uranium-238 undergoes alpha decay and transforms into thorium-234, which then undergoes beta decay to form protactinium-234. This is followed by a further beta decay to form uranium-234, and a sequence of five alpha decays. Eventually, after a few more beta and alpha decays the nucleus that results is lead-206, which is stable. The chart can be used to complete Soddy’s diagram. For example, Soddy’s Uranium X is clearly thorium-234 and his parent of radium is thorium-230.

We now know there are eighty-eight naturally occurring elements, of which eighty are stable. These elements are formed of 339 naturally occurring isotopes, including 252 that are stable. If we include isotopes created artificially in particle accelerators over 3,300 have been identified, although many of these are so unstable that they decay within a fraction of a second.

The Misuse of Science

Soddy had seen the rapid advance of science as the way towards a wonderful future in which human civilisation would be transformed. The First World War had a devastating psychological effect on him. He was sickened by the mass slaughter resulting from the industrialisation of war and the use of science to kill as efficiently as possible. The death of Henry Moseley, the rising star of nuclear physics, hit him particularly hard. He wrote:

When Moseley was killed at Gallipoli I felt enraged. Sometimes I think that something snapped in my brain. . . . I felt that governments and politicians, or man in general, was not yet fitted to use science. 

Soddy believed that scientists should take responsibility for the profound social consequences that would follow from their discoveries.

In 1919 Soddy became Professor of Chemistry at Oxford University, and two years later he was awarded the Nobel Prize for Chemistry. But his attention had turned to politics and economics, topics that seemed more relevant for the future of humanity. He challenged the thinking of conventional economists and campaigned for a radical new approach to finance and international monetary relations. Although his physics-inspired approach to economic theory was never fully accepted by mainstream economists, many of his once contentious proposals are now standard practise in the management of modern economies.

During the interwar years Soddy was almost alone in calling for the scientific community to recognise the social implications of its research. Further advances in nuclear physics and the outbreak of another global war would bring these issues into sharp focus. Many physicists would now agonise over their role in the development of nuclear weapons, and the looming threat of annihilation such weapons pose, realising that the consequences of their use would be the total devastation depicted by H.G. Wells in The World Set Free.

Further Information

There is more about Frederick Soddy’s early work here: Rutherford this is Transmutation!

There is more about the work of Henry Moseley in this article: Henry Moseley and the Nuclear Treasure Chest.

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