107 Stories About Chemistry
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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.
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