Question

covalency of oxygen cannot exceef 2

Answer 1

Because in order to extend covalent ,the paired es are to be promoted to higher vacant orbitals in given atom and since in oxygen there is no vacant orbital available for promotion of es hence covalent of O cannot exceed 2

Answer 2

it has 6 valence electrons and only two electrons are needed to fill in its valence shell

therefore, oxygen cannot exceed the valency of two

hope that helped...