Carbon has 4 electrons in its outermost shell, and needs to gain or lose 4 electrons to attain noble gas configuration. Losing or gaining 4 electrons is not possible due to energy, considerations, hence it shares electrons to form covalent bonds.
Two reasons for large number of carbon compounds:
(i) Catenation: The unique ability of carbon to form bonds with other atoms of carbon giving rise to long chains of different types of compounds.
(ii) Tetravalency: Since carbon has a valency of 4,it is capable of bonding with four other atoms of carbon or atoms of elements like oxygen, hydrogen, nitrogen, sulphur, chlorine, etc.
Carbon forms strong bonds with most other elements because of its small size which enables the nucleus to hold on to the shared pairs of electrons strongly.
