1. The atoms having no tendency to lose electrons form covalent bonds.
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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.
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4. Covalent bond is favoured most when the combining atoms come from the electronegative elements. Thus, electronegative element + electronegative element—covalent bonding.