AIPMT 2010 Physics and Chemistry Paper

Electron gain enthalpy, generally, increases in a period from left to right and decreases in a group on moving downwards. However, members of III period have samewhat higher electron gain enthalpy as compared to the coressponding members of second period, because of their small size. $\mathrm{O}$ and $\mathrm{S}$ belong to $\mathrm{VI} \mathrm{A}(16)$ group and $\mathrm{Cl}$ and $\mathrm{F}$ belong to VII $\mathrm{A}(17)$ group. Thus, the electron gain enthalpy of $\mathrm{Cl}$ and $\mathrm{F}$ is higher as compared to $\mathrm{O}$ and $\mathrm{S}$. $\mathrm{Cl}$ and $\mathrm{F} > \mathrm{O}$ and $\mathrm{S}$ Between $\mathrm{Cl}$ and $\mathrm{F}, \mathrm{Cl}$ has higher electron gain enthalpy as in $\mathrm{F}$, the incoming electron experiences a greater force of repulsion because of small size of $\mathrm{F}$ atom. Similar is true in case of $\mathrm{O}$ and $\mathrm{S}$ ie, the electron gain enthalpy of $\mathrm{S}$ is higher as compared to $\mathrm{O}$ due to its small size. Thus, the correct order of electron gain enthalpy of given elements is $\mathrm{O} < \mathrm{S} < \mathrm{F} < \mathrm{Cl}$