Why Is the Enthalpy of Formation for NaCl Less Negative Than for KCl?

[babita]

Homework Statement

In my book it is given that ΔH_ffor NaCl is less negative than that for KCl.
I'm not able to understand that...I think that electrostatic attractions between Na⁺ and Cl^- will be stronger than K⁺ & Cl^- as Na has smaller size than K...in its formation more energy should be released and ΔH_ffor NaCl should be more negative.

I also noticed that in case of F my reasoning seems correct.

Homework Equations

The Attempt at a Solution

 

[babita]

please reply someone...thanks