[physicsforums]

Q: How can we compare the strength of ionic bond of two compounds? Example: Which is more ionic, NaCl or LiCl?

A: As I read on some online resources, LiCl will be more ionic. This is because Li atom is smaller than Na, and hence separation between ions of LiCl is less than NaCl, making LiCl more ionic.

My doubt: The Fajan's rules summarize the extent of Covalent nature in an ionic bond. It states that as the size of the cation decreases, the covalent charater of the ionic compound increases. But in the example written above, this fact contradicts that LiCl is more ionic.

Please help me out here!

[Rutherford]

The strength of the ionic bond can be expressed by Coulomb's law: F=k·q₁q₂/r² (with increase of distance, the strength is lowered). Besides this, Fajans' rules have to be taken into consideration. Which of these two will prevail, depends on the specific case. I think that in this case, both ions are still small enough, so the Coulomb's law determines the strength (the difference in their size has a bigger impact on the coulomb force than on polarization change).