107 Stories About Chemistry
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When chemists were asked what boxes of the table to place the lanthanides in, they shrugged their shoulders in bewilderment. Indeed, what could they say when they did not know the reason for the astonishing similarity of the lanthanides? But the explanation proved quite simple. The Periodic System has curious groups of elements whose atoms have quite a peculiar constitution. The last electron added to form these atoms does not settle in their outermost, or even in their second-last shells, but penetrates, in conformance with strict physical laws, right through to the third-last shell. They feel quite cosy there and have no inclination to abandon their places under any circumstances. They participate in chemical reactions only in very rare cases. Now since all the lanthanides have three electrons in their outer shells, they are trivalent, as a rule. Nor is it accidental that the number of lanthanides is fourteen, neither more nor less. This is because there are exactly fourteen vacancies in the third-last shell of their atoms, the one that is being filled. That is why chemists found it possible to place all the lanthanides in one single box together with lanthanum
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