Since covalent bond is formed by mutual sharing of electrons, therefore, the factors which favour the formation of covalent bonds are as follows
1. Electron Affinity. A covalent bond is generally favoured between the two atoms if both the atoms have high electron affinity.
2. Ionisation Energy. Ionisation energy of both the atoms participating in bonding should be high.
3. Atomic Size. Atomic size of the atoms forming covalent bond should be smaller. Smaller the atomic radii of atoms, more stronger the covalent bond will be. For example, H-H bond is more stronger than Cl-Cl bond which in tum is more stronger than Br-Br bond.
4. Electronegativity. The electronegativities of both then atoms should be high. The difference of electronegativities between the two atoms should be minimum.