Chemical Bonding and Molecular Structure Part 1

Electrostatic force of attraction between cation and anion is called Lattice energy.

Lattice energy (F)=K‌

q1q2

r2

q1 and q2 are the charges of cations and anion. So Lattice energy depends on the charges of ions.
r= distance between center of ions
And, r=r++r−where r+=radius of cation and r−=radius of anion
So, more the size of ions less the value of Lattice energy.