STRUCTURE AND BONDING 27
although considerable heating is often required. It can be con-
cluded, therefore, that strong forces exist between the particles
comprising the crystals, these being usually intermediate in strength
between those found in a metal and those found, for example, in
diamond. Although the solid crystals do not conduct electricity, the
melt does, indicating that the lattice is comprised of charged species,
i.e. ions. These ions carry the current and are discharged at the
oppositely charged electrode where the products can be identified.
X-ray diffraction studies indicate that the ions form a regular lattice,
each ion being surrounded by a number of ions of the opposite
charge; this number depends on the sizes of the ions concerned and
is not dictated by directed forces of attraction*. We can correctly
assume the non-directional forces of attraction holding the ions
together to be electrostatic in nature.
MOLECULAR CRYSTALS
This is a very large group comprising mainly crystalline organic
materials, but a number of inorganic substances, for example iodine,
also come under this heading. These substances melt easily, and may
even sublime, indicating the presence of relatively weak forces. They
do not conduct electricity in the solid or fused state indicating that
the electrons present are localised in strong bonds. These bonds,
however, do not permeate the entire structure, as in diamond,
and the crystal is comprised of molecules with strong forces between
the constituent atoms, but the intermolecular forces are weak.
In substances which are liquid or gaseous at ordinary tempera-
ture, the forces of attraction between the particles are so weak that
thermal vibration is sufficient for them to be broken. These sub-
stances can be converted into solids by cooling to reduce the thermal
energy.
The above classification of structures is made primarily for
convenience. In fact, the structures of many compounds cannot be
precisely described under any of these classes, which represent
limiting, or ideal cases. However, we shall use these classes to
examine further the limiting types of bonding found in them.
- Many ions can, of course, contain more than one atom (for example NO 3 , SOj )
and directed forces hold together the individual atoms within each of these ionic
species.