Covalent Bonds of Hydrocarbon

When atoms combine to form molecules, atoms generally each lose, gain, or share valence electrons until they attain or share eight.

A valence electron is the outer shell electron that is associated with an atom. In a covalent bond, A valence electron participate the formation of covalent bond.
In a single covalent bond, both atoms in the bond contribute one valence electron in order to form a shared pair.