Ionic bonds become stronger as the ions get closer and their charges increase.
The formula for the force between two ions is
Thus, the attractive force or bond strength increases when the charges increase. It decreases when the ions are closer together. The charges are more important than the distances.
Small ions with 2 and 3 charges form strong bonds. Large ions with only +1 and -1 charges form weaker bonds.
Arrange the following compounds in order of increasing ionic bond strengths: AlN, CsI, LiF, MgO, Na₂O.
AlN > MgO > Na₂O > LiF > CsI
AlN has 3 charges on each ion.
MgO has 2 charges on each ion.
Na₂O has 1 charge on Na and 2 charges on O.
LiF has 1 charge on each ion.
CsI has 1 charge on each ion. Cs and I are larger than Li and F, so the ions are further apart.