Chapter 3 Chemical Bonds
3.4 Interactions Between Ions
A key contribution to the formation of ionic bonds is the strength of the interaction
between ions in a solid: it must be strong enough to overcome the energy
investment needed to make the ions. However, a very important point to note is
that an ionic solid is not held together by bonds between specific pairs of ions: all
the cations interact to a greater or lesser extent with all the anions, all the cations
repel one another, and all the anions repel one another. An ionic bond is a “global”
interaction characteristic of the entire crystal, a net lowering of energy of the entire
crystal relative to widely separated neutral atoms. By assessing this interaction
quantitatively we can see what determines the lattice energy of the solid. High lattice
energy indicates that the ions interact strongly with one another to give a
tightly bonded solid.
The strong electrostatic interactions between charged ions account for the typical
properties of ionic solids, such as their high melting points and their brittleness.
A high temperature is required before the ions are able to move past one another
to form a liquid. The same strong attractions and repulsions make it difficult for us
to push a block of ions in one region of the crystal past the ions in a neighboring
region: when we strike an ionic solid, ions with like charges come into contact and
repel one another. The resulting repulsions make it brittle and it shatters into fragments
(FIG. 3.5).
Our starting point for understanding the interaction between ions in a solid is
the expression for the Coulomb potential energy of the interaction of two individual
ions (Fundamentals, Section A):