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their individual struggles with the quantum. Both were beginning to feel the strain. 'I suppose it's a good thing that I have so much to distract me,' Einstein wrote to Ehrenfest in March 1922, 'else the quantum problem would have got me into a lunatic asylum.'73 A month later, Bohr confessed to Sommerfeld: 'In the last few years, I have often felt myself scientifically very lonesome, under the impression that my effort to develop the principles of the quantum theory systematically to the best of my ability has been received with very little understanding.'74 His feelings of isolation were about to end. In June 1922, he travelled to Germany and gave a celebrated series of seven lectures spread over eleven days at Göttingen University that became known as the 'Bohr Festspiele'.

More than a hundred physicists, old and young, came from all over the country to hear Bohr explain his electron shell model of the atom. It was his new theory about the arrangement of electrons inside atoms that explained the placing and grouping of elements within the periodic table. He proposed that orbital shells, like layers of an onion, surrounded an atomic nucleus. Each such shell was actually made up of a set or subset of electron orbits and was able to accommodate only a certain maximum number of electrons.75 Elements that shared the same chemical properties, Bohr argued, did so because they had the same numbers of electrons in their outermost shell.

According to Bohr's model, sodium's eleven electrons are arranged 2, 8 and 1. Caesium's 55 electrons are arranged in a 2, 8, 18, 18, 8, 1 configuration. It is because the outer shell of each element has a single electron that sodium and caesium share similar chemical properties. During the lectures Bohr used his theory to make a prediction. The unknown element with atomic number 72 would be chemically similar to zirconium, atomic number 40, and titanium, atomic number 22, the two elements in the same column of the periodic table. It would not, Bohr said, belong to the 'rare earth' group of elements that were on either side of it in the table, as predicted by others.

Einstein did not attend Bohr's Göttingen lectures, as he feared for his life following the murder of Germany's Jewish foreign minister. Walther Rathenau, a leading industrialist, had been in office only a few short months when he was gunned down in broad daylight on 24 June 1922 to become the 354th political assassination by the right since the end of the war. Einstein was one of those who had urged Rathenau not to take such a high-profile post within government. When he did, it was deemed 'an absolutely unheard of provocation of the people!' by the right-wing press.76

'Here our daily lives have been nerve-racking since the shameful assassination of Rathenau', Einstein wrote to Maurice Solovine.77 'I am always on the alert; I have stopped my lectures and am officially absent, though I am actually here all the time.' Warned by reliable sources that he was a prime target for assassination, Einstein confided to Marie Curie that he was thinking about giving up his post at the Prussian Academy to find a quiet place to settle down as a private citizen.78 For the man who in his youth had hated authority had now become a figure of authority. He was no longer simply a physicist, but was a symbol of German science and of Jewish identity.

Despite the turmoil, Einstein read Bohr's published papers, including 'The Structure of the Atoms and the Physical and Chemical Properties of the Elements', which appeared in the Zeitschrift für Physik in March 1922. He recalled nearly half a century later how Bohr's 'electron-shells of the atoms together with their significance for chemistry appeared to me like a miracle – and appears to me as a miracle even today'.79 It was, Einstein said, 'the highest form of musicality in the sphere of thought'. What Bohr had done was indeed as much art as science. Using evidence gathered from a variety of different sources such as atomic spectra and chemistry, Bohr had built up a particular atom, one electron shell