Question

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

• 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 3rd 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: $O<F<Cl$