An Introduction to Environmental Chemistry

Similarly, non-metals whose atoms have one, two or three electrons less than an
inert gas structure form monovalent (e.g. bromine, Br-), divalent (e.g. sulphur,
S^2 - ) and trivalent (e.g. nitrogen, N^3 - ) anions. In general, the addition or loss of
more than three electrons is energetically unfavourable, and atoms requiring such
transfers generally bond covalently (Section 2.3.1).
The silicon ion Si^4 +is an interesting exception. The high charge and small
ionic radius make this cation polarizing or electronegative (see Box 4.2), such
that its bonds with the oxygen anion O^2 - in silicate minerals (see Section 4.2)
are distorted. This produces an appreciable degree of covalency in the Si–O
bond.

2.4 Using chemical equations

The chemical principles discussed in this book are often illustrated using equa-
tions. It is useful to know a few of the ground rules chemists have adopted to
construct these. Let us begin by looking at an equation depicting the process of
rusting metallic iron:

eqn. 2.6

Firstly, the arrow shows that the reaction is favoured in one direction (we will
demonstrate this later when discussing energy needed to drive reactions). Next
we can see that the reaction balances, i.e. we have four atoms of iron and six atoms
of oxygen on both sides of the equation. When chemical reactions take place, we
neither gain nor lose atoms. Finally, the subscripted characters in brackets rep-
resent the status of the chemical species. In this book l=liquid, g=gas, s=solid
and aq=an aqueous species, i.e. a component dissolved in water.
It is important to realize that these reactions are usually simplifications of
the actual chemical transformations that occur in nature. In equation 2.6 we are
representing rusted or oxidized iron as Fe 2 O 3 , the mineral haematite. In nature,
rusted metal is a complex mixture of iron hydroxides and water molecules. So
equation 2.6 summarizes a series of complicated reaction stages. It illustrates a
product we might reasonably expect to form without necessarily depicting the
stages of reaction or the complexity encountered in nature.
Many of the equations in this book are written with the reversible reaction
sign (two-way half-arrows; e.g. eqn. 2.5). This shows that the reaction can
proceed in either direction and this is fundamental to equilibrium-based chem-
istry (see Box 3.2). Reactions depicting dissolution of substances in water may or
may not show the water molecule involved, but dissolution is implied by the (aq)
status symbol. Equation 2.7, read from left to right, shows dissolution of rock salt
(halite).

eqn. 2.7

The reverse reaction (right to left) shows crystallization of salt from solution.
When writing chemical equations, the sum of charges on one side of the equa-
tion must balance the sum of charges on the other side. On the left side of equa-

NaCl()sªNa+()aq +Cl-()aq

4Fe()s+3O2g()Æ2F e 2 O3s()

Environmental Chemist’s Toolbox 21