Although the preceding treatment has dealt with an ideal situation involving
only two atoms, a similar yet more complex condition exists for solid materials because
force and energy interactions among many atoms must be considered. Nevertheless,
a bonding energy, analogous to E0 above, may be associated with each
atom. The magnitude of this bonding energy and the shape of the energy-versusinteratomic
separation curve vary from material to material, and they both depend
on the type of atomic bonding. Furthermore, a number of material properties depend
on E0, the curve shape, and bonding type. For example, materials having large
bonding energies typically also have high melting temperatures; at room temperature,
solid substances are formed for large bonding energies, whereas for small
energies the gaseous state is favored; liquids prevail when the energies are of intermediate
magnitude. In addition, as discussed in Section 6.3, the mechanical stiffness
(or modulus of elasticity) of a material is dependent on the shape of its force-versusinteratomic
separation curve (Figure 6.7).The slope for a relatively stiff material at
the r r0 position on the curve will be quite steep; slopes are shallower for more
flexible materials. Furthermore, how much a material expands upon heating or contracts
upon cooling (that is, its linear coefficient of thermal expansion) is related to
the shape of its E0-versus-r0 curve (see Section 19.3). A deep and narrow “trough,”
which typically occurs for materials having large bonding energies, normally correlates
with a low coefficient of thermal expansion and relatively small dimensional
alterations for changes in temperature.