common example of covalent crystals is diamond, which is a network of carbon atoms, each of
which is bonded to four others in a tetrahedral arrangement.
Metallic Crystals
Metallic solids consist of metal atoms packed together. They have high melting and boiling points as
a result of their strong covalent attractions. They differ from normal covalent solids in that the
valence orbitals of the constituent atoms interact to form molecular orbitals delocalized over the
entire crystal and through which electrons can move. Because of the ability of electrons to move
freely within the such solids, metallic crystals conduct electricity and heat very well.
Molecular Crystals
The molecules or atoms making up these solids are held together by intermolecular forces that tend
to be weaker than ionic and covalent forces. They thus tend to have low melting points and are
easily deformable. The structures of a lot of biological molecules (such as proteins) are determined
by performing X-ray crystallography on the molecular crystals that they form.
AMORPHOUS SOLIDS
An amorphous solid, such as glass, does not possess the long-range, periodic structure that
crystalline solids do, although the molecules are also fixed in place.