The charges and sizes of the ions in an ionic compound affect the strength of the electrostatic interaction between the ions and thus the strength of the lattice energy of the ionic compound.

Question

Arrange the compounds according to the magnitudes of their lattice energies based on the relative ion charges and sizes.

: KBr

MgS

MgCl2

NaCl

Answer 1

Right off the bat, you see that the cations have either a 1+ charge (Na and K) or a 2+ charge (Mg). So, MgS and MgCl2 will have greater LE than either KBr or NaCl based on charge alone. Between MgS and MgCl2, S has a 2+ and Cl has a 1- and S is also smaller than Cl so MgS will have a greater LE than MgCl2.

So far we have MgS > MgCl2.

Now looking at KBr and NaCl, we see that Na is smaller than K and Cl is smaller than Br making the attraction between Na and Cl greater than that between K and Br. So LE for NaCl is greater than LE for KBr.

Final rank order would be:

MgS > MgCl2 > NaCl > KBr