7 4 THE QUANTUM [ v . 3
the periodic properties of the elements on the basis of the stability
of such configurations. " Although Thomson’s assumption re
garding the distribution of the positive electricity in the atom is
not consistent with more recent experimental evidence, never
theless his work has exerted great influence upon the later
developments of the atomic theory on account of the many
original ideas which it contained ” (Bohr).
The atomic model now generally accepted supposes that the
resultant positive charge is concentrated in an extremely minute
nucleus which is surrounded by the negative electrons. In
1904 Professor Nagaoka discussed an atomic model of this kind,
which may be termed a saturnian atom. He imagined an
attracting centre and a large number of equal particles repelling
one another inversely as the square of the distance moving
round the circumference of the circle. Very similar is the
“ nuclear atom ” developed by Sir Ernest Rutherford in 1911. It
is to Rutherford that we owe the experimental evidence in favour
of this model, for he showed that the positive charge is con
centrated in a nucleus of extremely small dimensions, containing
practically the whole mass of the atom, whilst electrons are
arranged round about the nucleus in certain orbits or in certain
definite rings.
The theory developed by Bohr on the basis of the old quantum
theory makes use of this atomic model, and a particular electron
is supposed to describe an orbit which may be specified by
one or more quantum numbers. Kossel on the one hand, and
Lewis and Langmuir on the other, have emphasized the fact
that in the periodic system there are certain elements which
must correspond to especially stable configurations of electrons.
These are the inactive gases helium, neon, argon, krypton, xenon
and niton (radon).
Table of Electron Configurations
Langmuir.
Element.
Bohr.
2
2 8
288
2 8 8 18
2 8 8 18 18
2 8 8 18 18 32
Helium
Neon
Argon
Krypton
Xenon
Niton
2i
2i 8 2
2j 8 2 83
2i 8 2 183 8 4
2j 8 2 183 I84 8 5
2 X 8 2 183 324 185 8 6
The small subscript figures which appear on the right indicate
the character of the orbit in Bohr’s theory. It will be noticed