1. The atoms having no tendency to lose electrons form covalent bonds.
2. The two combining atoms should have equal attraction for electrons.
3. Each of the two combing atoms should have 5, 6 or 7 electrons in their valence shells or the outermost shells. In such cases, both the atoms achieve the stable octet by sharing 3,2 or 1 electron pair respectively. The nonmetals of groups VA, VIA and VIIA respectively satisfy this condition and hence they are capable of covalent-bond formation.
4. Covalent bond is favoured most when the combining atoms come from the electronegative elements. Thus, electronegative element + electronegative element—covalent bonding.