Question

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

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

Answer 1

The correct option is B $O<F<Cl$

In general, the electron gain enthalpy becomes more negative on going from left to right in a period while it becomes less negative on going from top to bottom in a group.

But $F$ has a less electron gain enthalpy than $Cl$ because in $F$, on adding another electron externally to its valence shell, it causes severe electron-electron repulsion due to which the system becomes comparatively unstable. Whereas, adding an electron to $Cl$ is quiet convenient because of the larger size which is due to the large size of the $3$rd shell.
Whereas, $O$ = $1s^{2}2s^{2}2p^4$ has a less negative value than $F$ because, on going from left to right in the period, nuclear charge increases.
Therefore, the correct order of increasing electron gain enthalpy is: $O<F<Cl$