An Introduction to Environmental Chemistry

usually depending on oxidation state (see Box 4.3), these lower orbital electrons

act as additional valence electrons. For example, the element iron (Fe) exists in

compounds in a reduced (Fe^2 +or ferrous iron) and oxidized (Fe^3 +or ferric iron)

state. In general, the transition metals are less regular in their atomic properties

when compared to the main groups, which also makes their behaviour more com-

plicated to predict in nature.

It is clear from the discussion above, and by looking at the Periodic Table (Fig.

2.2) that some elements are classed as metals, some as semi-metals and some as

non-metals. In each row of the Periodic Table the degree of metallic character

decreases progressively from left to right, i.e. up the energy ramps of Fig. 2.1b.

In essence this is because those elements with low ionization energy hold elec-

trons loosely. In an applied electrical voltage these excited electrons will flow,

conducting the electricity, whereas in non-metals there is a gap in the electron

configuration that will not allow passage of excited electrons. In the case of semi-

metals the gap in electron configuration is small enough that excited electrons

can jump through, but only when activated by an external energy source. In effect

the semi-metal flips between being an insulator (when not stimulated by exter-

nal energy) and a conductor (when stimulated by external energy). Semi-metals

such as silicon are also known as semi-conductors, and are used in various indus-

trial applications to speed up electrical processes, most famously as the key com-

ponent of the ‘silicon chip’ in computer microprocessors.

There have been many attempts to further classify the elements geologically

and environmentally. In Fig. 2.3 we show the most abundant elements in four of

the main environmental materials of the Earth. A glance at this figure shows that

18 Chapter Two

Oceans

Atmosphere

Biosphere

C – H O, N, P, S

Lithosphere Rivers

Fe, Ca, Na, K,

Mg, Ti

Cl–, Na+, SO2–, Mg2+,

Ca2+, K+, HCO 3 , Br–

Alx Six O 8 ± (OH)

SiO 2 H 2 O

N 2 , O 2 , H 2 O, Ar CO 2

4

Hydrosphere

Fig. 2.3Distribution of elements in the four main environmental materials, lithosphere,

hydrosphere, atmosphere and biosphere. The elements are shown in their actual form as

compounds, ions or molecules as appropriate. The main components of each material are

shown in boxes, other major constituents are shown outside the boxes.