For each of the following pairs, predict which one has lower first ionisation enthalpy?
(i) N or 0
(ii) Na or ${{Na}^{+}}$
(iii) ${{Be}^{+}}$ or ${{Mg}^{2+}}$
(iv) I or ${{I}^{-}}$
(i) O has lower ∆iH1 than N because in case of O, loss of an electron gives ${{O}^{+}}$ which has stable exactly half-filled electronic configuration while in case of N, an electron has to be lost from a stable half-filled electronic configuration.
(ii) Na has lower ∆ iH1 than ${{Na}^{+}}$ because of the following two reasons
(a) In case of ${{Na}^{+}}$, an electron has to be lost from a stable inert gas configuration but in case of Na, loss of an electron gives stable inert gas configuration.
(b) Na is neutral but Na+ is positively charged.
(iii) Be+ has lower ∆iH1 than Mg2+ because in case of Be+, the loss of one electron gives a stable inert gas configuration but in case of Mg2+, the electron has to be lost from the stable inert gas configuration.
(iv) I has lower ∆iH1* than I- because in case of I-, an electron has to be lost from a stable inert gas configuration.