High ionisation energy
Elements that have higher ionisation energies cannot form ionic bonds, as they cannot lose electrons easily. Instead, they acquire stability by mutual sharing of electrons, and therefore form covalent bonds.
Equal electron affinity
The atoms of the two elements, which have equal or nearly equal electron affinities tend to complete their outer shells by mutual sharing of electrons.
This is because electron transfer in such cases will not be possible. Atoms tend to share the electrons and form a covalent bond when equal or nearly equal electronegativity of the two combining elements exists. This state does not permit the transfer of electron(s) from one atom to another.High nuclear charge and small atomic size
High nuclear charge and smaller atomic size leads to the increase in Ionisation energy of the combining elements.Therefore it favours covalent bond formation, because the transfer of electrons in such cases will not be possible.
