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As an example of ionic bonding, consider the reaction of
sodium (a soft reactive metal) with chlorine (a pale yellow-
green gas). When an atom of sodium and an atom of chlorine
collide, they react violently as the valence electron is transferred
from the sodium to the chlorine atom. This produces a sodium
ion and a chlorine ion. The reaction can be illustrated with elec-
tron dot symbols as follows:

O
O
S
S
Na N N ClS Na (Cl)
Q
Q
As you can see, the sodium ion transferred its valence
electron, and the resulting ion now has a stable electron con-
figuration. The chlorine atom accepted the electron in its
Sodium (Na) outer orbital to acquire a stable electron configuration. Thus,
a stable positive ion and a stable negative ion are formed.
Chlorine (Cl)
Because of opposite electrical charges, the ions attract each
FIGURE 9.6 Sodium chloride crystals are composed of sodium other to produce an ionic bond. When many ions are in-
+ –
and chlorine ions held together by electrostatic attraction. A crystal volved, each Na ion is surrounded by six Cl ions, and each
+
–
builds up, giving the sodium chloride crystal a cubic structure. Cl ion is surrounded by six Na ions. This gives the result-
ing solid NaCl its crystalline cubic structure, as shown in
Figure 9.7. In the solid state, all the sodium ions and all the
chlorine ions are bound together in one giant unit. Thus, the
IONIC BONDS term molecule is not really appropriate for ionic solids such
An ionic bond is defined as the chemical bond of electrostatic as sodium chloride. But the term is sometimes used anyway,
+
attraction between negative and positive ions. Ionic bonding since any given sample will have the same number of Na
–
occurs when an atom of a metal reacts with an atom of a non- ions as Cl ions.
metal. The reaction results in a transfer of one or more valence
electrons from the metal atom to the valence shell of the nonmetal Energy and Electrons in Ionic Bonding
atom. The atom that loses electrons becomes a positive ion, and
The sodium ions and chlorine ions in a crystal of sodium chlo-
the atom that gains electrons becomes a negative ion. Oppositely
ride can be formed from separated sodium and chlorine atoms.
charged ions attract one another, and when pulled together, they
The energy involved in such a sodium-chlorine reaction can be
form an ionic solid with the ions arranged in an orderly geomet-
assumed to consist of three separate reactions:
ric structure (Figure 9.6). This results in a crystalline solid that is
typical of salts such as sodium chloride (Figure 9.7).
1. energy Na N Na e
O
O
S

S
2. N ClS e (Cl) energy
Q
Q
O
O
S
S
S
S

3. Na (Cl) Na (Cl) energy
Q
Q
The overall effect is that energy is released and an ionic bond is
formed. The energy released is called the heat of formation. It
is also the amount of energy required to decompose the com-
pound (sodium chloride) into its elements. The reaction does
not take place in steps as described, however, but occurs all at
once. Note again, as in the photosynthesis-burning reactions
described earlier, that the total amount of chemical energy is
conserved. The energy released by the formation of the sodium
chloride compound is the same amount of energy needed to
decompose the compound.
Ionic bonds are formed by electron transfer, and electrons
are conserved in the process. This means that electrons are not
FIGURE 9.7 You can clearly see the cubic structure of these
ordinary table salt crystals because they have been magnified about created or destroyed in a chemical reaction. The same total
10 times. number of electrons exists after a reaction that existed before
234 CHAPTER 9 Chemical Bonds 9-6