The Born-Mayer equation gives the total electrostatic interaction energy for a given structure in terms of the Madelung constant, A, for that structure. Something else that varies from structure to structure is the number of ions within the formula unit for a given structure.

Kapustinskii noted that the ratio of the Madelung constant, A, to the total number of ions per formula unit for a series of compounds with different structures deviate by less than 10% from 0.87, the value found for the NaCl structure.

By replacing A with 0.87ν, taking the average value of n, and making the approximation that the internuclear separation is the sum of the ionic radii, he derived a new expression for the total electrostatic interaction from the Born-Lande equation. This is known as the Kapustinskii equation.

The Kapustinskii Equation: (note that r+ and r- are measured in pm)

The Kapustinskii equation predicts lattice enthalpies to within 10% of the experimental values for most compounds.