Page 446 - Schaum's Outline of Theory and Problems of Applied Physics

CHAPTER 35

The Solid State

CHEMICAL BONDS

When a compound is formed, atoms of the elements present are linked by chemical bonds. It is customary
to classify chemical bonds as ionic or covalent, although actual bonds are often intermediate between the two
extremes. In an ionic bond, one or more electrons from one atom are transferred to another atom, and the resulting
positive and negative ions then attract each other. In a covalent bond, one or more pairs of electrons are shared
by two adjacent atoms. As these electrons move about, they spend more time between the atoms than elsewhere,
which results in an attractive electric force that holds the atoms together.
A molecule is a group of atoms that are held together tightly enough by covalent bonds to behave as a single
particle. A molecule always has a definite composition and structure and has little tendency to gain or lose atoms.
Ionic bonds usually result in crystalline solids, not in molecules; such solids consist of aggregates of positive
and negative ions in a stable arrangement characteristic of the compound involved. Some crystalline solids are
covalent rather than ionic, as discussed below.

CRYSTALS
Most solids are crystalline, with the ions, atoms, or molecules of which they consist arranged in a regular pattern.
Four kinds of bonds are found in crystals: ionic, covalent, metallic, and van der Waals.
+
−
A crystal of ordinary salt, NaCl, is an example of an ionic solid, with Na and Cl ions in alternate positions
in a simple lattice (Fig. 35-1).
An example of a covalent solid is diamond, each of whose carbon atoms is joined by covalent bonds to four
other carbon atoms in a structure that is repeated throughout the crystal (Fig. 35-2). Both ionic and covalent
solids are hard and have high melting points, which are reflections of the strength of the bonds. Ionic solids are
much more common than covalent ones.
In a metal, the outermost electrons of each atom are shared by the entire assembly, so that a “gas” or “sea”
of electrons moves relatively freely throughout. The interaction between this electron sea and the positive metal
ions leads to a cohesive force, much as in the case of the shared electrons in a covalent bond but on a larger scale.
The presence of the free electrons accounts for such typical properties of metals as their opacity, surface luster,
and high electric and heat conductivities.
All molecules, and even inert-gas atoms such as those of helium, exhibit weak, short-range electric attractions
for one another due to van der Waals forces. These forces are responsible for the condensation of gases into liquids
and the freezing of liquids into solids even in the absence of ionic, covalent, or metallic bonds between the atoms
or molecules involved. Such familiar aspects of the behavior of matter as friction, viscosity, and adhesion are due
to van der Waals forces. Van der Waals forces arise from the lack of symmetry in the momentary distributions of
the electrons in a molecule. When two molecules are close together, these momentary charge asymmetries tend
to shift together, with the positive part of one molecule always near the negative part of the other even though
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