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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?

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