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Stories About Chemistry

INDEX

37. Where is Thy Place, Uranium?

There are no elements in Mendeleyev’s System which have no place at all. There are elements with no definite place.

Such, for instance, is the very first of them, hydrogen. Investigators are still at odds as to whether element No. 1 should be in the first, or in the seventh group of the Periodic Table.

Uranium is in much the same predicament. But had not Mendeleyev determined its position once and for all?

Cartoon showing a letter addressed to Mr. Uranium, held by a uniformed deliveryman looking  at a signpost topped with a U. On it are various direction arrows, showing 6th Period, Group 3, Group 6 and 7th Perios.

For decades no one questioned uranium’s being in the sixth group of the Periodic Table as the heaviest member of the family including also chromium, molybdenum and tungsten, and its position seemed quite infallible.

But times changed, and uranium was no longer the last in the series of elements. A whole “cohort” of man-made transuranium elements ranged out at its right, and they all had to be placed in the Mendeleyev Table.

What groups and what boxes were to be assigned to the transuranium elements? After much controversy a large number of scientists came to the conclusion that they should all be placed together in a single group and in a single box.

This decision did not fall from the moon, something of the sort had happened once before in the Periodic Table. The lanthanides, a total of 14 elements of the sixth period were all placed together with lanthanum in a single box of Group III.

The physicists had long since predicted that a similar phenomenon would recur in the next period. They stated that a family of elements resembling the lanthanides should exist in the seventh period. The name of this family would be actinides, because it would begin right after actinium which is situated just below lanthanum in the table.

Hence, all the transuranium elements are members of this family. And not only they, but also uranium and its nearest left-hand neighbours, namely, protactinium and thorium. They all had to leave their old, familiar places in the sixth, fifth and fourth groups, and move into the third.

Almost one hundred years ago Mendeleyev had moved uranium out of this group. Now it was back in it, but this time with “fuller rights.” See what curious things may happen in the life of the Periodic System.

The physicists agreed to such a state of affairs, but not all the chemists, and not without reserve, because as regards properties uranium is just as much a stranger to Group III as it was in Mendeleyev’s time. Nor is the third group quite suitable for thorium and protactinium.

Where is thy place, uranium? It remains a point of controversy among scientists.


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