ATOMIC STRUCTURE 225

C,3 Si,

N, P,

O, S, Sc,

Fl, Cl, Br,

Ne, Arg,
two, three, four, five, six, seven and eight respectively.

Let us consider the bearing of this on the existence of
multiply charged positive ions, i.e. ions which have lost
more than one electron. Those electrons will have come
from the outer layer, as the electrons in this layer are much
more easily detached from the atom than those in the inner
layers; thus we should not expect to find atoms carrying
multiple charges unless there were more electrons than one
in the outer layer. Thus if this theory is true we should
expect to find the atoms of the elements of the first group
characterized by their inability to receive a double charge ;
this is a striking feature of the hydrogen atom. Mr. G. P.
Thomson has got by the anode ray method positive ray
photographs of the lines corresponding to lithium, sodium
and potassium, but has not detected the existence of double
charges on the atoms* of any of these elements.

Thus, as far as it goes, the evidence from multiply charged
atoms in the positive rays is consistent with this theory.

Let us next consider the question of negatively charged
atoms; these are atoms which have received an additional
electron. On this theory, however, eight is the maximum
number of electrons that can be on stable equilibrium on the
outer layer, hence atoms like those of neon and argon which
have already eight electrons in the outer layer have no room
for more electrons and hence cannot receive a negative charge;
the atoms of the elements in the other groups might be
expected to get negatively charged. The positive ray
photographs never give any indications of the lines due to thus be a limit