promising area. The chemical bonding in many
sulfides makes them behave in some respects like
metals, giving them a greater capacity to conduct
electricity than the silicate rocks that host them
(Chapter 7). This property can be exploited by the
geophysicist to track down undiscovered ore bodies.
Once again we see the important influence of atomic
characteristics on the macroscopic properties of min-
erals and other geological materials. A basic grasp of
the nature of atoms and chemical bonding is there-
fore directly relevant to the work of the ordinary
Every atom consists of two parts:
(i) a nucleus at the centre, containing nearly all of the
mass of the atom but accounting for only one ten-
thousandth of its diameter;
(ii) a family of electrons gathered around the nucleus,
forming a three-dimensional 'cloud' that makes up
the volume of the atom.
The basic facts about these two atomic components
are given in Table 5.1. The nucleus represents an
extraordinarily dense state of matter, and it accomm-
odates the atom's positive charge (which is proportional
to Z , the number of protons it contains). Protons in
such close proximity exert a powerful electrostatic
repulsion on each other, but the nucleus is held
together by a still greater force called the strong force
In geochemistry we are more interested in the neg-
atively charged electrons gathered around the nucleus,
trapped in the electrostatic pull of its positive charge.
The number of electrons in a neutral atom is equal to
the number of protons in the nucleus. These captive
electrons provide the means by which atoms can
associate and bond together. They are the currency of
chemical reactions, being exchanged or shared
whenever atoms interact and form bonds. It is therefore
to the behaviour of electrons in atoms that the present
chapter is devoted.
Box 5.1 Units of atomic size
The size of an atom may be measured in nanometres
(nm) or picometres (pm):
1 nm = 10 -9 m or one thousand-millionth of a
metre = 1000 pm.
1 pm = 10 -12 m or one million-millionth of a metre.
Most atoms and ions have diameters in the range
0.1-0.3 nm or 100-300 pm.
The nanometre is the currently recognized SI unit
for atomic size (Appendix A). The traditional unit for
atomic dimensions was the 'Ångstrom unit' (1 Å =
0.1 nm = 10 -10 m) named after Swedish spectroscopist
Anders Jonas Ångström.
Table 5.1 Basic facts about atoms
10 −14 m
10 −10 m
Protons and neutrons*
Approximate mass of individual particles
1.7 × 10 −27 kg (≈1800 electron masses)
9 × 10 −31 kg
Relative numbers of particles
In most types of nucleus, there are slightly
more neutrons than protons.
The number of electrons in a neutral atom is
equal to Z , the number of protons*.
Approximate density of matter
(kg dm −3 = g cm −3 )
10 12 kg dm −3
1 kg dm −3
* Protons carry one unit of positive charge and neutrons are electrically neutral. Electrons each have one unit of negative charge. Z is called
the atomic number .