energies
in which C is a constant that depends on the type of crystal structure; Z+ and Z are the charges on the ions; and Ro is the interionic distance the sum of the radii of the positive and negative ions . Thus, the lattice energy of an ionic crystal increases rapidly as the charges of the ions increase and the sizes of the ions decrease. When all other parameters are kept constant, doubling the charge of both the cation and anion quadruples the lattice energy. For example, the lattice energy of LiF Z+ and Z = 1 is 1023 kJ/mol, whereas that of MgO Z+ and Z = 2 is 3900 kJ/ mol Ro is nearly the sameabout 200 pm for both compounds . Different interatomic distances produce different lattice energies. For example, we can compare the lattice energy of MgF2 2957 kJ/mol to that of MgI2 2327 kJ/mol to observe the effect on lattice energy of the smaller ionic size of F as compared to I.