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  4. Among halogens, the correct order of amount of energy released in electron gain (electron gain enthalpy) is

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Q. Among halogens, the correct order of amount of energy released in electron gain (electron gain enthalpy) is

Classification of Elements and Periodicity in Properties

Solution:

Electron gain enthalpy becomes less negative on moving down a group due to increase in atomic size. However, the electron gain enthalpy of F is less negative than that of Cl due to electron-electron repulsion in small sized F atom. Thus, the correct order is :
$\Delta_{eg} H \left(kJ \,mol^{-1}\right) : \begin{matrix}F&<&Cl&>&Br&>&I\\ -328&&-349&&-325&&-295\end{matrix}$