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15
The Atomic Theory

Si

28     P 31
S 32     Cl 35
Ar 40     K 39

The differences in the atomic weights are the same in the two series, so that each series may be supposed to grow by the addition of the same kind of primordial atom, but one series starts from one kind of atom, the other from another. The question is, should we not expect the number of electrons in the atom of an element to be connected with the number which represents the order of the element in the series to which it belongs when the elements are divided into two series, rather than with its order in a series which contains the whole of the elements without any rearrangement? As a matter of fact the difference between the numbers given by these views for the electrons in an atom of one of the heavier elements would be too small to be detected by any experiment at present within our powers. With the lighter elements, however, it ought to be possible to distinguish between these views, and experiments with this object are at present being made in the Cavendish Laboratory.

The number of electrons in an atom is such a fundamental quantity that its determination throws a good deal of light on some of the most keenly discussed problems in Physics and Chemistry, such as the transmutation of the elements and the relation between mass and weight. Let us begin by considering its connexion with the first of these questions.


Transmutation of the Elements

The constant difference between the number of electrons in the atom of one element and that in the atom of the element next in the series is strong evidence in favour