Question

what is oxidation number of Zn and S in ZnSO4 note that 4 is for oxygen only. pls explain

Answer 1

Zn is a metal and thus forms a cation with +2 charge.

Zn⁺² is the only stable cation of Zinc.

Sulphate (SO₄⁻²) is a polyatomic anion with overall charge -2.
oxygen always has -2 charge except when in compounds O₂F₂ and OF₂ .

In SO₄⁻², let 'x' be the oxidation state of sulphur, then
                x  + 4(-2)   =   -2
                   x  -  8      =  -2
                       x         = +6
                 Thus the oxidation state on sulphur here is +6