Question

The radii of the lithium and magnesium ions are 76 pm and 72 pm, respectively. which compound has stronger ionic attraction, lithium oxide or magnesium oxide

Answer 1

 From the Born–Landé equation the lattice energy U ∝ (Z+ × Z-) / (r+ + r-) where Z+ and Z- are the charges on the cation and anion, respectively; r+ and r- are radii of the cation and anion, respectively.

The Z+×Z- term dominates. MgO has 2×2 (4) so it will have the higher U than Li2O Z+ × Z- = 2.
You only consider (r+ + r-) term when the Z+×Z- term is the same; smallest the (r+ + r-),  the larger is the U. 
U(MgO) = 3795 kJ mol^-1
U(Li2O) = 2799 kJ mol^-1 

MgO has larger.