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fact that radioactivity was a nuclear and not an atomic phenomenon. It allowed him to explain the process of radioactive disintegration in which one radioelement decayed into another with the emission of alpha, beta or gamma radiation as a nuclear event. Bohr realised that if radioactivity originated in the nucleus, then a uranium nucleus with a charge of plus 92 transmuting into uranium-X by emitting an alpha particle lost two units of positive charge, leaving behind a nucleus with a charge of plus 90. This new nucleus could not hold on to all of the original 92 atomic electrons, quickly losing two to form a new neutral atom. Every new atom formed as the product of radioactive decay immediately either acquires or loses electrons so as regain its neutrality. Uranium-X with a positive nuclear charge of 90 is an isotope of thorium. They both 'possessed the same nuclear charge and differed only in the mass and intrinsic structure of the nucleus', explained Bohr.71 It was the reason why those who tried, failed to separate thorium, with an atomic weight of 232, and 'uranium-X', thorium-234.

His theory of what was happening at the nuclear level in radioactive disintegration implied, Bohr said later, 'that by radioactive decay the element, quite independently of any change in its atomic weight, would shift its place in the periodic table by two steps down or one step up, corresponding to the decrease or increase in the nuclear charge accompanying the emission of alpha or beta rays, respectively'.72 Uranium decaying with the emission of an alpha particle into thorium-234 ended up two places further back in the periodic table.

Beta particles, being fast-moving electrons, have a negative charge of minus one. If a nucleus emits a beta particle, its positive charge increases by one – as if two particles, one positive and the other negative, that existed in harmony as a neutral pair had been ripped apart with the ejection of the electron, leaving behind its positive partner. The new atom produced by beta decay has a nuclear charge that is one greater than the disintegrating atom, moving it one place to the right in the periodic table.

When Bohr took his ideas to Rutherford he was warned about the danger of 'extrapolating from comparatively meagre experimental evidence'.73 Surprised by this muted reception, he attempted to convince Rutherford 'that it would be the final proof of his atom'.74 He failed. Part of the problem lay in Bohr's inability to express his ideas clearly. Rutherford, preoccupied with writing a book, did not make the time to fully grasp the significance of what Bohr had done. Rutherford believed that although alpha particles were emitted from the nucleus, beta particles were just atomic electrons somehow ejected from a radioactive atom. Despite Bohr's trying on five separate occasions to persuade him, Rutherford hesitated in following his logic all the way to its conclusion.75 Sensing that Rutherford was by now becoming 'a bit impatient' with him and his ideas, Bohr decided to let the matter rest.76 Others did not.

Frederick Soddy soon spotted the same 'displacement laws' as Bohr, but unlike the young Dane, he was able to publish his research without first having to seek approval of a superior. Nobody was surprised that Soddy was at the forefront of these breakthroughs. But no one could have guessed that an eccentric 42-year-old Dutch lawyer would introduce an idea of fundamental importance. In July 1911, in a short letter to the journal Nature, Antonius Johannes van den Broek speculated that the nuclear charge of a particular element is determined by its place in the periodic table, its atomic number, not its atomic weight. Inspired by Rutherford's atomic model, van den Broek's idea was based upon various assumptions that turned out to be wrong, such as nuclear charge being equal to half the atomic weight of the element. Rutherford was suitably annoyed that a lawyer should publish 'a lot of guesses for fun without sufficient foundation'.77

Having failed to gain any support, on 27 November 1913 in another letter