Man's physical universe



determined by this method, which has been found to give values agreeing

with those obtained by chemical means within 1

part to 1000 or


The mass spectrograph showed that many of the elements have

atomic weights which are the mean of the atomic weights of two or

more isotopes. {Isotopes are atoms having different atomic weights hut

the same atomic number.) The existence of these isotopes is best shown

by the mass spectograph, but there is other evidence for their existence,

as explained in the next paragraph.

The Majority of the Elements Are Mixtures of Isotopes.

We have already mentioned the fact that the different atoms of an

element are not always of the same weight but that most elements are

composed of atoms of two or more different weights, having the same

atomic number. The chief characteristic of an element, therefore, is

its atomic number. The reason that the chemical atomic weight of

most elements is the same, regardless of their source, is that these

isotopes are intermingled in nature in very nearly constant proportions.

It was found that forty or more different kinds of atoms were produced

by radioactive decomposition of natural radioactive elements

and that in many cases these different kinds of atoms showed the same

chemical properties and were therefore atoms of the same element.

For example, it was found that lead (average atomic weight = 207.2),

with atomic weights of 214, 212, 210, 208, 206, and 204, was obtained

by the decomposition of uranium, thorium, and protoactinium.

The discovery of the existence of isotopes naturally led chemists to

try to separate possible isotopes of nonradioactive elements by chemical

or physical means. Such attempts met with considerable success.

For example, isotopes of chlorine were partially separated by Harkins

and his co-workers by use of diffusion methods.

In 1932 an isotope of hydrogen with the atomic weight of 2 was discovered

by Harold C. Urey and his co-workers by examining spectroscopically

the least volatile portion of some liquid hydrogen which had

been subjected to fractional distillation. This new hydrogen is called

deuterium, and water prepared from it is called deuterium oxide, or

heavy water. Deuterium is now separated by electrolysis, fractional

distillation of water, and other methods. There is about one part of

deuterium in 6000 parts of ordinary hydrogen. The discovery of

deuterium made possible some recent great advances in the transmutation

of elements.

In 1940 heavy sulfur, atomic weight 34, as compared with ordinary

sulfur of atomic weight 32.06, was separated by Urey and his workers

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