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Stories About Chemistry

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25. Fourteen Twins

They are called lanthanides. Such is their name because all of them - fourteen in all - are “lanthanum-like,” that is, resemble lanthanum and one another almost like so many drops of water. Because of this astonishing chemical similarity they are all situated in a single box, the box of lanthanum whose number in the table is 57.

Isn’t this some terrible misunderstanding?

Mendeleyev himself and many other scientists reasoned that each element had a single quite definite place in the Periodic Table.

But here fourteen inhabitants of the system have crowded into the same box, all of them elements of the third group and of the sixth period.

Why not try sorting them out among the other groups?

Many chemists have tried, among them Mendeleyev. They placed cerium in the fourth group, praseodymium in the fifth, neodymium in the sixth, and so on. But this distribution defied all logic. The main and secondary subgroups of the Mendeleyev Table contain similar elements. But cerium had very little in common with zirconium, praseodymium and neodymium were strangers to niobium and molybdenum. Nor could the other rare-earth elements (such is the general name for lanthanum and the lanthanides) find relatives in the corresponding groups. On the other hand, they resembled each other like twin brothers.

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?

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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