Ionic solids, such as NaCl and KNO3, are often recognized by their brittleness because
the electrons made available by cation formation are localized on a neighbouring anion
instead of contributing to an adaptable, mobile electron sea. Ionic solids also commonly
have high melting points and most are soluble in polar solvents, particularly water. However,
there are exceptions: CaF2, for example, is a high-melting ionic solid but it is insoluble
in water. Ammonium nitrate, NH4NO3, is ionic in terms of its interactions between the
ammonium and nitrate ions, but melts at 170°C. Binary ionic materials are typical of elements
with large electronegativity differences, typically Δ 3, and such compounds are
therefore likely to be found at the top corner of a Ketelaar triangle (Fig. 3.27).
The classification of a solid as ionic is based on comparison of its properties with those of
the ionic model, which treats the solid as an assembly of oppositely charged, hard spheres
that interact primarily by nondirectional electrostatic forces (Coulombic forces) and repulsions
between complete shells in contact. If the thermodynamic properties of the solid calculated
on this model agree with experiment, then the solid may be ionic. However, it should
be noted that many examples of coincidental agreement with the ionic model are known,
so numerical agreement alone does not imply ionic bonding. The nondirectional nature of
electrostatic interactions between ions in an ionic solid contrast with those present in a covalent
solid, where the symmetries of the atomic orbitals play a strong role in determining
the geometry of the structure. However, the assumption that ions can be treated as perfectly
hard spheres (of fixed radius for a particular ion type) that have no directionality in their
bonding, is far from true for real ions. For example, with halide anions some directionality
might be expected in their bonding that results from the orientations of their p orbitals, and
large ions, such as Cs and I, are easily polarizable so do not behave as hard spheres. Even
so, the ionic model is a useful starting point for describing many simple structures.
We start by describing some common ionic structures in terms of the packing of hard
spheres of different sizes and opposite charges. After that, we see how to rationalize the
structures in terms of the energetics of crystal formation. The structures described have
been obtained by using X-ray diffraction (Section 8.1), and were among the first substances
to be examined in this way.