What are the essential requirements for the formation of covalent bond?

The essential requirements for the formation of a covalent bond are,
(i) An atomic orbital of one atom containing one electron overlaps with an atomic orbital of other atom containing an electron of opposite spin.
(ii) Properly oriented atomic orbitals of different atoms can overlap together.
(iii) Greater the overlapping of orbitals, stronger will be the bond.
(iv) Overlapping of orbitals causes delocalization of electrons which in turn lowers the enthalpy and increases the stability.