An Introduction to Environmental Chemistry

oxygen (O), and to a lesser extent hydrogen (H), are superabundant in most Earth
surface materials such as air, water, organic matter and silicate minerals. In the
lithosphere, silicon (Si) and aluminium (Al) are next most abundant forming the
silicate minerals feldspar and quartz (see Chapter 4). In the hydrosphere it is the
dissolved ions in seawater (see Chapter 6) that dominate the chemistry, particu-
larly chloride (Cl-) and sodium (Na+), while the main atmospheric gases are nitro-
gen (N 2 ), oxygen (O 2 ), argon (Ar) and carbon dioxide (CO 2 ), along with water
vapour (see Chapter 3). The organic matter of the biosphere is made principally
of carbon and hydrogen bonded in various combinations (Section 2.7), along with
lesser amounts of oxygen and the nutrient elements nitrogen (N) and phosphorus
(P). Based on the information in this diagram it might be tempting to conclude
that we need only understand the behaviour of these elements in nature to under-
stand environmental chemistry. In fact the reverse is true. Paradoxically, it is often
the elements present in trace amounts in the solids and fluids of the environment
that tell us most about chemical processes.

2.3 Bonding

Many elements do not normally exist as atoms, but are bonded together to form
molecules. The major components of air, nitrogen and oxygen for example, are
present in the lower atmosphere as the molecules N 2 and O 2. By contrast, argon
is rather unusual because as an inert element (or noble gas—Section 2.2) it is
found uncombined as single argon atoms. Inert elements are exceptions and most
substances in the environment are in the form of molecules.

2.3.1 Covalent bonds

Molecular bonds are formed from the electrostatic interactions between electrons
and the nuclei of atoms. There are many different electronic arrangements that
lead to bond formation and the type of bond formed influences the properties of
the compound that results. It is the outermost electrons of an atom that are
involved in bond formation. The archetypical chemical bond is the covalent bond
and we can probably best imagine this as formed from outer electrons shared
between two atoms. Take the example of two fluorine atoms that form the fluo-
rine molecule:

eqn. 2.1

In this representation of bonding, the electrons are shown by dots. In reality the
bonding electrons are smeared out over the entire molecules, but their most prob-
able position is between the nuclei. The bond is shown as the two electrons
between the atoms. The bond is created from the two electrons shared between
the atoms. In simple terms it can be argued that this arrangement of electrons
achieves a structure similar to that of argon, i.e.:

:Ar..

..

:

:F.. ̇ ̇.+:F ̇ ̇...ª: ̇ ̇F..:F ̇ ̇..:

Environmental Chemist’s Toolbox 19