How many pairs are, in which first species has lower ionization energy than second species?
(i) N and O (ii) Br and K (iii) Be and B (iv) I and I−
(v) Li and Li+ (vi) O and S (vii) Ba and Sr
(v) Li<Li+; Li+ has a noble gas configuration which is stable and hence difficult to disturb. Hence, it has a higher ionization energy than Li.
(vii) Ba < Sr; As we go down the group, ease of ionization increases. This means that Ba is easier to ionise than Sr, hence it will have a lower ionization energy.