Explain why lattice energy of lithium fluoride (LiF) is higher than that of sodium fluoride (NaF)

LiF and NaF have the same charge product, as lithium and sodium are both 1+ and fluorine is 1-. The fluorine radius will be the same in both compounds. However, lithium is smaller than sodium, so LiF will be have a greater lattice enthalpy than NaF because of shorter intermolecular distance leading to stronger attraction.