This only occurs in chemical which contain only non-metals
molecules have covalent bonding
Molecules conain 2 or more non-metal atoms

A:B
Nucleus of A is attracted to the electron pair
Nucleus of B is also attracted to the electron pair

We show a covalent bond which is one shaired pair of electrons by

Molecules of an element are also covalent.
Covalent molecules usually only contain a few atoms.
Most covalent substances have very small molecules.

There are a few covalent substances that are giant.

Diamond uses all four of its outer shell electrons to form 4 covalent bonds with 4 other Carbon atoms. Each form 4 more bonds and so it forms a giant molecule. This formation is called Tetrahedral. Diamond is very strong and has a hign mp, because to melt it you need to break thousands of covalent bonds

In Graphite each Carbon is only bonded to three other Carbon atoms. This leaves one electron in each Carbon atom. This extra electron leaves the Carbon atom and becomes delocalised between the layers of Carbon atoms. Graphite can conduct because it has delocalised mobile electrons. Graphite is soft and easily rubbed off because there are only weak forces between the layers of Carbon