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

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

AFMCClassification of Elements and Periodicity in Properties

Solution:

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 $VI\, A (16)$ group and $Cl$ and $F$ belong to $VII\, A\, (17)$ 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
$O < S < F < Cl$