Question

Which of the following represents the correct order of increasing electron gain enthalpy with negative sign for the elements $O,S,F$ and $Cl$ ?

• A. Cl < F < O < S
• B. O < S < F < Cl
• C. F < S < O < Cl
• D. S < O < Cl < F

Answer 1

Answer: (B)

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 somewhat higher electron gain enthalpy as compared to the corresponding members of second period, because of their small size.
$O$ and $S$ belong to $VIA(16)$ group and $Cl$ and $F$ belong to $VIIA(17)$ group. Thus, the electron gain enthalpy of $Cl$ and $F$ is higher as compared to $O$ and $S$.
$Cl$ and $F>O$ and $S$.
Between $Cl$ and $F,Cl$ has higher electron gain enthalpy then the $F$, since the incoming electron experiences a greater force of repulsion because of small size of $F$-atom. Similar is true in case of $O$ and $S$, i.e. the electron gain enthalpy of $S$ is higher as compared to $0$ due to its small size. Thus, the correct order of electron gain enthalpy of given elements is
$O<S<F<Cl$