Structure and
bonding
THE NATURE OF THE PROBLEM
A very superficial examination of a large number of chemical sub-
stances enables us to see that they differ widely in both physical and
chemical properties. Any acceptable theory of bonding must
explain these differences and enable us to predict the properties of
new materials. As a first step towards solving the problem we need
to know something of the arrangement of atoms in chemical sub-
stances. The structure of a solid can be investigated using a beam of
X-rays or neutrons. From the diffraction patterns obtained it is
possible to find the arrangement of the particles of which it is com-
posed. Measurement of the amount of heat needed to melt the solid
yields information concerning the forces of attraction between these
particles, whilst the effect of an electric current and simple chemical
tests on the solid may tell if it is a metal or a non-metal. Should the
material be a non-conducting solid, we can determine whether it is
composed of ions by investigating the effect of an electric current on
the molten material.
Results of such investigations suggest that there are four limiting
kinds of structure and these will be briefly considered.
THE METALLIC LATTICE
In a pure metal the atoms of the solid are arranged in closely packed
layers. There is more than one way of achieving close packing but it
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