Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [86]
The chemical properties of these three groups of elements formed part of the evidence that led Bohr to propose that the atom of each element in a row of the periodic table is built up from the previous element by the addition of another electron to the outer electron shell. Each row would end with a noble gas in which the outer shell was full. Since only electrons outside the closed shells, called valence electrons, took part in chemical reactions, atoms with the same number of valence electrons shared similar chemical properties and occupied the same column in the periodic table. The halogens all have seven electrons in the outermost shell, requiring just one more electron to close it and acquire an electron configuration of a noble gas. The alkalis, on the other hand, all have one valence electron.
It was these ideas that Pauli heard Bohr outline during the Göttingen lectures in June 1922. Sommerfeld had greeted the shell model as 'the greatest advance in atomic structure since 1913'.26 If he could mathematically reconstruct the numbers 2, 8, 18 … of the elements in the rows of the periodic table, then it would be, Sommerfeld told Bohr, 'the fulfilment of the boldest hopes of physics'.27 In truth, there was no hard mathematical reasoning to back up the new electron shell model. Even Rutherford told Bohr that he was struggling 'to form an idea of how you arrive at your conclusions'.28 Nevertheless, Bohr's ideas had to be taken seriously, especially after the announcement in his Nobel lecture in December 1922 that the unknown element with atomic number 72, later called hafnium, did not belong to the 'rare earth' group of elements was later confirmed to be correct. However, there was no organising principle or criteria behind Bohr's shell model. It was an ingenious improvisation based on an array of chemical and physical data that could in large part explain the chemical properties of the various groupings of elements in the periodic table. Its crowning glory was hafnium.
As he continued to fret over the anomalous Zeeman effect and the shortcomings of the electron shell model, Pauli's time in Copenhagen came to an end. In September 1923 he returned to Hamburg, where the following year he was promoted from assistant to privatdozent. But with Copenhagen a short train journey and a ferry across the Baltic Sea, Pauli was still a regular visitor to the institute. He concluded that Bohr's model could work only if there was a restriction on how many electrons could occupy any given shell. Otherwise, in contradiction of the results of atomic spectra, there seemed nothing to prevent all the electrons in any atom from occupying the same stationary state, the same energy level. At the end of 1924 Pauli discovered the fundamental organising rule, the 'exclusion principle', that provided the theoretical justification that had been missing in Bohr's empirically devised electron shell atomic model.
Pauli was inspired by the work of a Cambridge postgraduate student. Edmund Stoner, 35, was still working on his doctorate under Rutherford when in October 1924 his paper 'The Distribution of Electrons Among Atomic Levels' was published in the Philosophical Magazine. Stoner argued that the outermost or valence electron of an alkali atom has as many energy states to choose from as there are electrons in the last closed shell of the first inert noble gas that follows it in the periodic table. For example, lithium's valence electron could occupy any one