NATURE OF MATERIALS - Materials for Civil and Construction Engineers - page 56

( compression ) or to separate them ( tension ). The distance at which the net
force is zero corresponds to the minimum energy level and is called the
equilibrium spacing. The minimum energy is represented by a negative
sign. The largest negative value is defined as the bonding energy. The bond-
ing energy can be computed from equations for the attractive and repulsive
forces. Based on the strength of the bonds, the theoretical strength of a
material can be estimated. However, this theoretical strength grossly over-
estimates the actual strength due to flaws in the molecular structure (Van
Vlack 1964, 1989).
The bonding energy depends on the molecular mechanism holding the
atoms together. There are two basic categories of bonds: primary and sec-
ondary. Primary bonds form when atoms interact to change the number of
electrons in their outer shells so as to achieve a stable and nonreactive elec-
tron structure similar to that of a noble gas. Secondary bonds are formed
when the physical arrangement of the atoms in the molecule results in an
imbalanced electric charge; one side is positive and the other is negative.
The molecules are then bonded together through electrostatic force.

Primary Bonds Three types of primary bonds are defined, based on the man-
ner in which the valence electrons interact with other atoms:

1. ionic bonds —transfer of electrons from one elemental atom to another
(Figure 2.4)
2. covalent bonds —sharing of electrons between specific atoms (Figure 2.5)
3. metallic bonds —mass sharing of electrons among several atoms
(Figure 2.6)

Ionic bonds are the result of one atom releasing electrons to other atoms
that accept the electrons. Each of the elements reaches a stable electron con-
figuration of the outer s and p subshells. All of the atoms are ions, since they
have an electrical charge. When an atom releases an electron, the atom
becomes positively charged; the atom receiving the electron becomes negatively

FIGURE 2.4 Ionic bonding.
(© Pearson Education, Inc. Used
by permission.)

Na Cl

H H

FIGURE 2.5 Covalent bonding.
(© Pearson Education, Inc. Used by
permission.)