Question

Choose the correct order of negative electron gain enthalpy?

• A. Cl < K < Br < Ca
• B. Cl < Br < K < Ca
• C. K < Ca < Br < Cl
• D. Ca < K < Br < Cl

Answer 1

The correct option is D Ca < K < Br < Cl

Put differently, we are looking at the electron affinity $E_{ea}$ values. The electron affinity of an atom or molecule is defined as the amount of energy released or spent when an electron is added to a neutral atom or molecule in the gaseous state to form a negative ion.

The more the added electron stabilizes the atom, the more is the negative electron gain enthalpy. Hence negative electron gain enthalpy is highest for $Cl$ and least for $Ca$. Again,
Electron gain enthalpy $\propto \frac{\text{Zeff}}{\text{Atomic size}}$
Since atomic size of $Br$ is higher than $Cl$, hence negative electron gain enthalpy for $Cl$ is more.

Also, adding an electron to $K$ affords a more stable $n{s}^2$ configuration which is why $K$ has a higher magnitude of energy released than $Ca$, which already has a fully-filled $n{s}^2$ configuration to start with.