Atoms with four electrons in their outer shell are midway between having a filled outer shell by either losing four electrons to another atom or gaining four electrons from another atom.  Thus, it is not a strong electron donor (an atom that tends to give away electrons) or a strong electron acceptor (an atom that tends to gain electrons). Such an atom tends to SHARE electrons with other atoms.   The sharing of a pair of electrons between atoms is a covalent bond.  Carbon, with an atomic number of 6, has four electrons in its outer shell and in the vast majority of cases forms covalent bonds with other atoms.