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CHAPTER 5

Chemical

Bonding

5.1. INTRODUCTION
The vast bulk of materials found in nature are compounds or mixtures of compounds rather than free elements.
On or near the earth’s surface, the nonmetallic elements oxygen, nitrogen, sulfur, and carbon are sometimes found
in the uncombined state, and the noble gases are always found in nature uncombined. Also, the metals copper,
silver, mercury, and gold sometimes occur in the free state. It is thought that, except for the noble gases, these
elements have been liberated from their compounds somewhat recently (compared with the age of the earth)
by geological or biological processes. It is a rule of nature that the state which is most probably encountered
corresponds to the state of lowest energy. For example, water flows downhill under the influence of gravity,
and iron rusts when exposed to air. Since compounds are encountered more often than free elements, it can be
inferred that the combined state must be the state of low energy compared with the state of the corresponding
free elements. Indeed, those elements that do occur naturally as free elements must possess some characteristics
that correspond to a relatively low energy state.
In this chapter, some aspects of chemical bonding will be discussed. It will be shown that chemical combi-
nation corresponds to the tendency of atoms to assume the most stable electronic configuration possible. Before
we start to study the forces holding the particles together in a compound, however, we must first understand the
meaning of a chemical formula (Sec. 5.2). Next we learn why ionic compounds have the formulas they have, for
example, why sodium chloride is NaCl and not NaCl 2 or NaCl 3 .

5.2. CHEMICAL FORMULAS
Writing a formula implies that the atoms in the formula are bonded together in some way. The relative
numbers of atoms of the elements in a compound are shown in a chemical formula by writing the symbols of the
elements followed by appropriate subscripts to denote how many atoms of each element there are in the formula
unit. A subscript following the symbol gives the number of atoms of that element per formula unit. If there is no
subscript, one atom per formula unit is implied. For example, the formula H 3 PO 4 describes a molecule containing
three atoms of hydrogen and four atoms of oxygen, along with one atom of phosphorus. Sometimes groups of
atoms which are bonded together within a molecule or within an ionic compound are grouped in the formula
within parentheses. The number of such groups is indicated by a subscript following the closing parenthesis. For
example, the 2 in (NH 4 ) 2 SO 4 states that there are two NH 4 groups present per formula unit. There is only one
SO 4 group; therefore parentheses are not necessary around it.
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