Geology Reference
In-Depth Information
many ore metals in saline aqueous solutions such as
hydrothermal fluids (Chapter 4).
no vacancies exist, except in the upper band at unat-
tainable energy levels. The essential characteristic of
such insulators is therefore a filled band, separated
from vacant higher levels by a large energy gap.
In a metal, valence electrons can fill only the lower
part of the band. Upper levels remain vacant and,
together with other overlapping bands which are read-
ily accessible (Figure  7.6a), serve as a conduction band
that enables electrons to migrate to new positions in
the crystal. Electrons are not stable in these higher lev-
els, although thermal excitation from the filled levels
below is sufficient to ensure that a proportion of such
levels is always occupied. A voltage applied across the
crystal will produce a net flow of conduction-band
electrons that constitutes an electric current. Similar
energy bands in the π -bonding network lie behind the
electrical conductivity of graphite (Box 7.4).
This leads to the notion of a metal crystal as a regular
array of 'cations' (although in this context ionic radii are
not applicable) immersed in a fluid of mobile, ' de localized '
valence electrons. Owing to the mobility of the valence
electrons, the bonding in metals is essentially non-direc-
tional. Most metals have close-packed atomic structures
determined by the stacking rules for spheres of equal
size. Twelve-fold co-ordination is therefore the com-
monest configuration, although a number of metals
show eight-fold co-ordination instead.
Hydrogen was cited above as an example of a non-
metallic element, as it occurs in the elemental state on
Earth solely in the diatomic form H 2 . Experiments at
extremely high pressure have shown, however, that
highly compressed hydrogen can develop the electronic
band structure characteristic of a metal (Box 7.6), and it
is believed to exist in this metallic form in the deep int-
eriors of Jupiter and Saturn. The strong magnetic fields
of Jupiter and Saturn have been attributed to electric
currents flowing in such a metallic hydrogen fluid.
Metals and semiconductors
In a crystal of pure copper, no electronegativity differ-
ence exists between neighbouring atoms, so we expect
the Cu-Cu bonding to be covalent. Yet the characteris-
tic properties of metals - lustre, opaqueness, electrical
and thermal conductivity, and ductility - make them
quite different from the covalent solids considered so
far. The explanation of these differences lies in the
molecular orbitals by which a metal crystal is held
together.
When a hydrogen atom forms a covalent bond, its
valence shell becomes in effect fully occupied through
electron sharing. The electron configuration in the hydro-
gen molecule (1s σ 2 ), for example, is equivalent from each
atom's point of view to the configuration of helium. A
sulfur atom participating in two bonds similarly attains
a noble-gas configuration (Ar). The picture in a metal,
however, differs from these elements in two ways:
(a) Metals have lower electronegativities and ioniza-
tion energies than hydrogen and sulfur. Electrons
in their valence shells are more loosely held.
(b) The formation of covalent bonds does not lead to a
noble-gas configuration in a metal atom. Vacant
energy levels remain available in the metal's
valence shell.
In any covalent crystal, the overlap of valence orbit-
als between neighbouring atoms leads to a system of
molecular orbitals extending throughout the crystal.
The electrons occupying them are nominally shared by
all of the atoms present. The molecular energy levels
available to these electrons are grouped in bands
(Box  7.6), and the physical properties of a crystal
depend on how these bands are arranged in energy
space and how they are populated.
Figure 7.6 shows three alternative situations. In dia-
mond (b) - and also in sulfur (not shown) - the valence
electrons completely fill the lower band (equivalent to
filled valence shells in all of the atoms). Each electron
has a molecular wave function that confines it to a par-
ticular location in the crystal. In order to migrate it
must find a vacancy in another molecular orbital, but
Semiconductors
Germanium (Figure  7.6c) illustrates a technologically
important intermediate case, the semiconductor. If a
small gap exists between a filled band and an empty
conduction band, the material will behave as an insul-
ator except when activated by an energy impulse from
outside. One of the uses of germanium, for example, is
as a detector of γ -ray photons (Box 6.3), such as those
emitted by certain trace elements in geological mater-
ials when irradiated by neutrons in a nuclear reactor
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