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40. An Unfinished Building?

   We have said many nice things about the Periodic System and its great architect. But it suddenly occurred to us that the building is not complete. Its seventh storey is just slightly more than half finished. It should have 32 flats, but only 17 have been furnished so far. And then there is something strange about the flat owners: you can hardly tell straight away whether they actually live there or not. A real phantasmagoria.

   Chemists and physicists have long been debating the question whether there is a logical ending to the Periodic Table, or, to put it more simply, what the atomic number of the very last element would be.

   About forty years ago the number 137 began to appear on the pages of special serious papers and books in physics. A prominent scientist even ventured to write a booklet entitled "The Magic Number 137."

   Why is this number so remarkable?

   The electron shell closest to the nucleus in atoms is not always the same distance from it. The radius of the shell becomes smaller with increasing nuclear charge. Therefore in the uranium atom this shell is much closer to the nucleus than, say, in potassium. There should finally come a moment when the nucleus and the shell closest to it become the same size. What would then happen to the electrons on this shell?

   They would "fall" on the nucleus and be "swallowed up" by it. But penetration of a negative charge into the nucleus reduces the total positive charge of the nucleus by one unit. Hence, the atomic number of the newly formed element would be one unit smaller than that of the parent element. And so we have come to the ultimate number of elements. The last flat in the Big House is No. 137.

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