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4 LIQUIDS AND SOLIDS eLearn.Punjab

Properties of Covalent Crystals

The bonding in covalent crystals extend in three dimensions. They contain a network of atoms.
The valencies of atoms are directed in definite directions, so the packing of atoms in these crystals
is looser than those of ionic and metallic crystals. Thus covalent crystals have open structure.
These crystals are very hard and considerable amount of energy is required to break them.
They have high melting points and their volatility is very low.
Due to the absence of free electrons and ions they are bad conductors of electricity.
However, graphite has a layered structure and the electrons are available in between the layers.
These electrons are delocalised and conductivity becomes possible. Graphite is not a conductor
perpendicular to the layers.
Mostly covalent crystalline solids are insoluble in polar solvents like water but they are readily
soluble in non-polar solvents like benzene and carbon tetrachloride. The covalent crystals having
giant molecules like diamond and silicon carbide are insoluble in all the solvents. Because of their
big size, they do not interact with the solvent molecules. The chemical reactions of such crystalline
solids are very slow.
Let us try to understand the structure of diamond, which is a well known covalent solid.

Structure of Diamond

Diamond is one of the allotropic modifications of
carbon. It is best understood by taking into consideration
the number of electrons in the outermost shell of carbon,
which are four. The four atomic orbitals (one 2s and three
2p) undergo sp3 hybridization to give four sp3 hybridized
orbitals. They are directed in space along the four corners
of a tetrahedron Fig. (4.18 a).
This is the unit cell of diamond and a large number of
such unit cells undergo sp3-sp3 overlapping to form a huge
structure. Each carbon atom is linked with four other carbon
atoms. The bonds between carbon atoms are covalent
which run through the crystal in three-dimensions. All the
bond angles are 109.5° and the bond lengths are 154 pm.

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