In the solid state, both metallic and ionic compounds
possess ordered arrays of atoms or ions and form crystalline
materials with lattice structures. Studies of their
structures may conveniently be considered as related
topics because both are concerned with the packing of
spherical atoms or ions. However, differences in bonding
result in quite distinct properties for metallic and ionic
solids. In metals, the bonding is essentially covalent. The
bonding electrons are delocalized over the whole crystal,
giving rise to the high electrical conductivity that is
characteristic of metals. Ionic bonding in the solid state
arises from electrostatic interactions between charged
species (ions), e.g. Naþ and Cl in rock salt. Ionic solids
are insulators.