Chemical Bonding
Chemical bonding is the process by which atoms combine to form compounds. There
are four types of chemical bond, of which the ionic bond and valency bond are,
probably, of most interest to geologists and mineralogists.
The ionic bond
An ionic bond is created when electrons pass betweem atoms creating cations
(positively charged ions) and anions (negatively charged ions). Ions of opposite
charge attract each other. The attracting force is equal in all directions and
increases as the distance between the ions decreases. The ions therefore tend to
pack together into a lattice whose shape is determined by the sizes, and therefore
packing, of the ions involved. An example, the lattice for sodium chloride,
is shown in Atoms, Electrons and Compounds.
Most good books on minerals show the lattice structure for each group of minerals.
The lattice unit cell determines a material's properties. The cell shape and
dimensions determine the crystal system; a major feature in identifying minerals
in the hand. The more closely the ions can pack, the greater is the bonding force
and the greater is the hardness of the mineral. Some lattice arrangements have
weak internal links that result in easy cleavage in one particular plane. A well
known example is the Mica family.
In practice minerals are seldom formed by 100% ionic bond. They are usually
part ionic/part covalent.
The covalent bond
A covalent bond is created when two atoms share outer shell electrons so that
the electrons orbit around both atoms giving each a full complement of electrons
in its outer shell. In a covalent bond no electrons are given up or acquired so
no ions are formed. A covalent bond is therefore possible between two atoms of the
same type and is the means by which molecules are formed.
The metallic bond
The outer electrons of metals are only loosely attracted to the nucleus and are
therefore easily detached. The resulting metal cations tend to cluster together
with the detached electrons surrounding the cations as an anionic cloud. This
electron cloud is the source of the electric current that flows in a metal connected
between the terminals of a battery. It is the reason why metals are such good
electrical conductors.
The Van der Waal bond
This is a weak force that binds the noble gases and some elements such as Sulphur
and Graphite. It is associated with a low melting temperature.
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