Arrange MgO, CaO, NaF and KBr in order of decreasing lattice enthalpy. Give a reason for each step.

Step 1. Charges. MgO and CaO have ions with charges of 2. NaF and KBr have charges of 1. The first pair has much larger lattice enthalpies.

Step 2. MgO and CaO. $\text{Mg}^{2+}$ is smaller than $\text{Ca}^{2+}$, so MgO comes first.

Step 3. NaF and KBr. $\text{Na}^+$ and $\text{F}^-$ are both smaller than $\text{K}^+$ and $\text{Br}^-$, so NaF comes before KBr.