CHAPTER IX
STATIC MODELS OF ATOMS AND MOLECULES
In ultimate logic any physical representation is in fact a mental
construction or analogy, designed to relieve the mind from the in
tangible and elusive character of a complex of abstract relations.
Larmor, Aether and Matter, p. 334, 1900
1. Some Advantages of Static Models
T HE results of the X-ray analysis of crystals lend some
support to the view that the electrons concerned in bind
ing together the atoms of a metal in a crystal occupy more or
less definite positions with reference to the atoms in the crystal
lattice. In polar compounds such as rock salt (NaCl) the binding
forces between the atoms can be regarded as due to the attraction
of oppositely charged ions. The sodium ion resembles the neon
atom, but possesses a nuclear charge of 11 instead of 10 units.
The chlorine atom has one electron less than argon, but if it
acquire an additional electron (say from a neutral sodium atom)
it becomes a univalent chlorine ion, which resembles in its
structure the argon atom. In the crystal of rock salt each sodium
atom is surrounded by six chlorine atoms arranged symmetrically
round it, and in the same way each chlorine atom is surrounded
by six sodium atoms. We find, in fact, two interpenetrating
face-centred cubic lattices. This may be interpreted by regard
ing the crystal as a pattern in three dimensions formed of sodium
and chlorine ions of opposite signs, held together by electrostatic
forces. According to W. H. and W. L. Bragg * the interatomic
forces in metals may possibly be like those in polar compounds,
the electrons playing the part of the negative ions. For example,
metallic silver and silver chloride both have the same type of
structure as rock salt, the length of the side of the elementary
face-centred cube is in Angstrom units 4*060 for the metal and
5*56 for the salt. This suggests that an electron in the metal
takes the place of a chlorine ion in the salt.
It is of interest to note that in nearly all cases the lattice
constant, i.e. the length of the side of the elementary cube or
* W. H. and W. L. Bragg, X-rays and Crystal Structure.
