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89. The Chemistry of Single Atoms

There were times when chemists bewailed the difficulty of studying the properties of a new element available only in milligrams.

Since then the “criterion of smallness” has had to be revised more than once. In 1937 the Italian scientists Perrier and Segre quite successfully studied the properties of the just then obtained element No. 43, called technetium, having at their disposal only one ten-thousand-millionth of a gram of this new representative of the Periodic Table.

Their experience was of benefit to others.

When working with the transuranium elements chemists had to forget entirely such weight units as grams, milligrams or even micrograms. “Imponderable, invisible amounts”—such were the terms that appeared on the pages of scientific papers devoted to the transuranium elements. The deeper investigators delved into this region of the Periodic System, the greater were the difficulties they encountered.

Finally came the turn of the hundred and first element, called mendelevium in honour of the great Russian chemist.

Since the new transuranium element received a name, scientists must have been quite convinced that it had actually been obtained.

It had been comparatively easy to calculate the conditions under which one could hope to synthesize element No. 101. There was not much difficulty in writing the equation of the corresponding nuclear reaction. Which isotope of the new transuranium element would form could also be foreseen.

Such was theory. But what had been obtained in practice needed confirmation, to prove that isotopes of precisely the hundred and first element, and none other, had formed as a result of the nuclear process.

What followed was fantastic. “During one experiment of synthesis of the hundred and first more than one atom of the new element cannot be expected to form”—such was the verdict of strict physical and mathematical reasoning, and so it turned out in reality. Only one single atom, an unknown atom, announced its birth. But was it an atom of the hundred and first element?

Sensitive radiometric instruments enabled determination of the half-life of the atom, but not of its chemical nature.

And in general, is it possible to study even the chief chemical properties of a single atom?

Chromatography came to the rescue.

Now follow our reasoning closely. The hundred and first element must belong to the actinide family. In many of their properties the actinides resemble the elements of another similar family of elements called the lanthanides. Separation of the lanthanides was accomplished by ion-exchange chiromatography, the individual lanthanides coming out of the mixture in a strict sequence, the heavier ones first, and then the lighter ones.

In the actinide series the hundred and first element was to follow einsteinium (No. 99) and fermium (No. 100). If we wish to separate einsteinium, fermium and element No. 101 chromatographically, the last-named element should appear in the first drops of the liquid issuing from the chromatographic column.

Seventeen times scientists repeated the experiment of mendelevium synthesis. Seventeen times they used ion-exchange chromatography to establish the chemical nature of the new man-made atom. And in each case the mendelevium atom appeared exactly in the drop of solution in which it was to have appeared according to theory. Formerly only fermium and einsteinium came out in these drops.

Hence, the atomic number of mendelevium is 101 and it is a typical actinide in properties.

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by Ian Ellis
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