The increasing order of electron affinity for the given electronic configurations of elements is:
$I$. $1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^5$ $(Cl)$
$II$. $1s^2 \ 2s^2 \ 2p^3$ $(N)$
$III$. $1s^2 \ 2s^2 \ 2p^5$ $(F)$
$IV$. $1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^1$ $(Al)$

The order of electron affinity for $F, Cl, Br,$ and $I$ is .....

The formation of the oxide ion,$O^{2-}_{(g)}$,from oxygen atom requires first an exothermic and then an endothermic step as shown below:
$O_{(g)} + e^- \to O^{-}_{(g)} ; \Delta_f H^{\Theta} = -141 \ kJ \ mol^{-1}$
$O^{-}_{(g)} + e^- \to O^{2-}_{(g)} ; \Delta_f H^{\Theta} = +780 \ kJ \ mol^{-1}$
Thus,the process of formation of $O^{2-}$ in gas phase is unfavourable even though $O^{2-}$ is isoelectronic with neon. It is due to the fact that,

Consider the following conversions:
$(i) \ O_{(g)} + e^- \to O^{-}_{(g)} ; \Delta H_1$
$(ii) \ F_{(g)} + e^- \to F^{-}_{(g)} ; \Delta H_2$
$(iii) \ Cl_{(g)} + e^- \to Cl^{-}_{(g)} ; \Delta H_3$
$(iv) \ Na_{(g)} \to Na^{+}_{(g)} + e^- ; \Delta H_4$
Which of the following statements is incorrect?

The increasing order of electron affinity for $Cl, Br$ and $I$ is .....

The set containing the elements with positive electron gain enthalpies is