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Stories About Chemistry


32. Mortality and Immortality
in the World of Elements

There came a time when chemists became archeologists in a way. They learned to measure the age of various minerals in the Earth�s crust, much like an archeologist determines how many centuries ago a bronze adornment or an earthenware vessel was made.

Some minerals were found to be more than four and a half billion years old. They are as old as the planet Earth itself. But minerals are chemical compounds. They consist of elements. Therefore the elements are practically immortal. Isn�t it absurd to ask whether an element can die? Death is the sad fate of living creatures. No, this question is not so pointless as it seems at first glance.

There is a physical phenomenon called radioactivity. It consists in elements (rather, atomic nuclei) decaying spontaneously. Some nuclei emit electrons from their depths. Others throw up what are known as alpha-particles (helium nuclei). Still others break up into two approximately equal halves, this process being known as spontaneous fission.

Are all the elements radioactive? No, not all, but mainly those at the end of the Periodic System, starting from polonium.

In decaying, a radioactive element does not vanish altogether. It changes into another. The chain of radioactive transformations may be very long. For example, thorium and uranium finally change into stable lead. But along their route a good dozen of radioactive elements are born and perish.

Radioactive elements possess different vitalities. Some of them may exist tens of billions of years before vanishing entirely. The lifetime of others is a matter of minutes or even seconds. Scientists assess the vitality of radioactive elements by means of a special quantity, called the half-life period or just half-life. During this period any quantity of the radioactive element taken decays to exactly half of its initial weight.

The half-lives of uranium and thorium are several billion years each.

It is entirely different with the elements that come before them in the Periodic Table, protactinium, actinium, radium and francium, radon, astatine, and polonium. Their lifetimes are much shorter, not more than a hundred thousand years in any case. And this brings up an unexpected puzzle.

How is it that these short-lived elements still exist on Earth seeing that our planet is something like 5 billion years old? This almost unimaginable time period should have been long enough for radium, actinium, and the other elements of their group to vanish a hundred times over.

Still, they do exist and have been concealed in terrestrial minerals for ages. It is as if nature has some elexir at its disposal which keeps them from perishing.

No, this is not the case. It is simply that they keep reappearing, because they are fed by an inexhaustible source, the terrestrial reserves of uranium and thorium. As long as these radioactive �patriarchs� keep travelling down their long and complex path of transformations, ultimately to form stable lead, they must continue changing into the intermediate elements. Thus, among the chemical elements we can distinguish two large groups, namely, the primary and the secondary elements.

The primary elements are all the nonradioactive elements and uranium and thorium whose half-life periods are greater than the Earth�s age. They witnessed the formation of the solar system. All the rest are secondary elements.

Still, there will come a time when the Periodic System will find itself lacking several elements. These will be uranium and thorium, the eternal source of secondary elements which are, however, eternal only relatively.

At some future time they will also disappear from the face of the Earth, in a matter of a few hundred billion years. And together with them will vanish the products of their radioactive transformations.

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by Ian Ellis
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