NATURE OF MATERIALS - Materials for Civil and Construction Engineers - page 55

The electrons in the outermost filled shell are the valence electrons.
These are the electrons that participate in the formation of primary bonds
between atoms. The eight electrons needed to fill the s and p subshells are
particularly important. If these subshells are completely filled, the atoms are
very stable and virtually nonreactive, as is the case for the noble gases neon
and argon. In many cases atoms will release, attract, or share electrons to
complete these subshells and reach a stable configuration. Calcium and
chromium are examples of atoms with electrons that fill the 4s subshell,
while the 3d subshell is incomplete or empty, as would be expected from
Figure 2.2. Copper demonstrates that there are exceptions to the energy rule.
One would expect copper to have two electrons in the 4s subshell, leaving
nine electrons for the 3d subshell. However, it has ten electrons in the 3d
subshell and only one in the 4s subshell. A similar disparity exists for
chromium. Note that iron has two electrons in the 4s subshell; thus, it has
two electrons more than it needs for a stable configuration. These two elec-
trons are readily released to form iron molecules. Aluminum is also an
exception, since it has an excess of three electrons.

■ 2.1.2 Bonding

As two atoms are brought together, both attractive and repulsive forces
develop. The effects of these forces are additive, as shown in Figure 2.3,
such that once the atoms are close enough to interact, they will reach a
point at which the attractive and repulsive forces are balanced and an equi-
librium is reached. Energy is required either to bring the atoms closer together

Attraction

Sum

Interatomic
spacing

0

Repulsion

(a)

Equilibrium spacing

Interatomic
spacing

0

Bonding energy

(b)

FIGURE 2.3 Attractive and repulsive (a) forces, and (b)
energies between atoms. (© Pearson Education, Inc. Used by
permission.)