What Does This Equation Tell Us? As before, the negative sign of the total potential
energy, the sum of Ep and Ep*, tells us that the ions have a lower potential
energy when they are present as a solid rather than widely separated as a gas.
The greatest stabilization is expected when the ions are highly charged (so
0 z1z2 0 is large) and small (so d is small, but not smaller than d*). ■
We can now see why nature has adopted an ionic solid, calcium phosphate, for
our skeletons: the doubly charged small Ca2 ions and the triply charged PO4
3
ions attract one another very strongly and clump together tightly to form a rigid,
insoluble solid (FIG. 3.8).
Ionic solids typically have high melting points and are brittle. The lattice energy
of an ionic solid is large when the ions are small and highly charged.