Given two ionic compounds, for instance:
$$\ce{CaS} \quad \mathrm{or} \quad \ce{KCl}$$
What is the procedure to predict which of the two have the highest lattice energy (in absolute value)?
(Suppose that I do not know the values of radii of the atoms but I can only look at the periodic table).
I know that I must look at the ratio
$$\frac{Z_1 Z_2}{r_1 + r_2} \propto E_\text{lattice},$$
where $r_1 + r_2$ is the distance between the two atoms in each molecule of the compound, $Z_1 Z_2$ is the product of atomic numbers of each element in the molecule.
But, for example, in the case above I have both $r_1 + r_2$ and $Z_1 Z_2$ bigger for $\ce{KCl}$ then $\ce{CaS}$, so I cannot say (without knowing the numerical values) if one of the two ratios would be higher.
Am I missing something?