Which ionic compound would be expected to have the highest lattice energy?

a. CCl4 b. MgCl2 c. AlCl3 d. NaCl

Lattice energy can be calculated using the Born-Lande equation.

http://en.wikipedia.org/wiki/Born-Land%C3%A9_equat…

Because we are only looking at it from a qualitative perspective, and no crystal information is given (I assume that’s not something you have to take into account, we extrapolate the following observation from the formula:

E is proportional k * Z_cation * Z_anion / r. Where k is a combined constant, Z’s are the charges, and r is the interionic distance.

A primary note: although your question assumes all of the options listed are ionic compounds, CCl4, carbon tetrachloride is NOT a ionic compound, given that the electronegativity difference between carbon and chlorine is about .1. Wikipedia will back me up:
http://en.wikipedia.org/wiki/Carbon_tetrachloride

That said, between the other three, it is clear that AlCl3 has the highest lattice energy, as Al has a positive charge of 3 (giving a product of 3, as opposed to 2 or 1), and that Al’s greater ionization gives AlCl3 a smaller interionic radius.

Source(s): Science background and wikipedia

Alcl3 Lattice Energy