A New Kind of Hydrogen

D. P. Mellor


For about a century after Dalton first proposed his theory of atoms chemists believed that all the atoms of a given element possessed the same mass. This belief was, of course, quite consistent with all the facts known at the time. During the present century, however, the belief was shaken by the discovery of chemically similar but chemically inseparable radioactive atoms whose masses were undoubtedly different from one another. At first it seemed that such atoms, known as isotopes, were to be found only among the radioactive elements, but it was not long before it became quite clear that nearly every element is a mixture of atoms differing slightly from one another in mass, but identical in chemical behaviour. This discovery was perhaps a little disconcerting to chemists at first. It was not so troublesome as it appeared, since it was found that in nature the proportions of atoms of different weights in any element were always constant. Oxygen, the most abundant element in the earth's crust, consists almost entirely of atoms of mass 16; about one atom in ten thousand has a mass of 17, and this proportion is, as far as we know, quite invariable.

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