Question

why NO2 is better electrophile as compared to NH4?

Answer 1

Hence an unpaired electron remains on the nitrogen atom which is then ionised to get a +ve charge (formal charge) on nitrogen. As nitrogen doesnt have an octet around it, hence it always craves for an electron and also the resonance between the N-O bonds make it a strong electrophile.
Theoretically NH4+ and H3O+ can not gain electrons due to unavailability of vacant orbitals in their valence shell. ...NH4+ splits to give NH3 and H+. H+ can gain electron pair and hence act aselectrophile .