Can you help me with this exothermic lattice energy problem?

Question

How do I determine which of the following pairs of ionic substances has the most exothermic lattice energy?

 

A. LiF, CsF 

B. NaBr, NaI 

C. BaCl2, BaO 

D. Na2SO4, CaSO4 

E. KF, K2O 

F. Li2O, Na2S 

Answer 1

Lattice energy (LE) will decrease as you go down a group because the atomic radii increase as you go down in a group.

LE will increase as the magnitude of the charge increases.

Or put another way:
The greater the charge, the higher the lattice energy.
The smaller the size of the ion, the higher the lattice energy.

Using these concepts, you should be able to figure out which has the greater lattice energy.

a) Li⁺ is smaller in size than Cs⁺, so lattice energy of LiF is greater than for CsF
b) Br^- is smaller than I^- so NaBr will have greater LE
c) O^2- has a greater charge than Cl^- 
d) Ca²⁺ has a greater charge than Na⁺
e) O^2- has a greater charge than F^-
f) Li⁺ is smaller than Na⁺ and O is smaller than S