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Stories About Chemistry


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.

Later, a little over ten years ago, the physicists discovered an error. More precise calculation showed that the electron would only crash down on a nucleus if its charge were about 150 or so. See how bright the prospects of completing the Big House!

How many new elements, how many unexpected discoveries await chemists! Over forty future inhabitants await permission to move into the house founded by Mendeleyev. Alas, today this is no more than a dream, an alluring but unrealizable fancy. In calculating the atomic number of the last element scientists had omitted something very important. Not that they forgot about it - they just wanted to see what would be if�

If there were no radioactivity. If nuclei with very large charges were as stable as those of the numerous elements existing on the Earth. Radioactivity is the absolute ruler of the elements heavier than bismuth. But it deals out long lifetimes to some and allows others to live only a few instants.

The hundred and fourth element, kurchatovium, has a half-life of only three-tenths of a second. And what about the hundred and fifth, and the hundred and sixth? Their half-lives are in all probability still shorter. And not far off we come to the deadline where the nucleus of the new element breaks up almost before it is born. We would be lucky to get as far as the hundred and tenth.

Nature itself and its strict physical laws are to blame for the Mendeleyev Table being incomplete. Still, how many a time has man conquered nature?

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by Ian Ellis
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