Covalent Bonds

Bonding often occurs between atoms that have high ionization energies and high electron affinities. In other words, neither atom loses electrons easily, but both atoms attract electrcms. In such cases, there can be no transfer of electrons between atoms. What there can be is a sharing of electrons. Bonding in which electrons are shared rather than transferred is called covalent bonding.
By sharing electrons, each atom fills up its outermost energy level. So the shared electrons are in the outermost energy level of both atoms at the same time.