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.
| Structure | Coordination Number | Madelung constant, A | A/ν |
| ν is the number of ions in the formula unit | |||
| Sodium Chloride (NaCl) | 6:6 | 1.74756 | 0.88 |
| Cesium Chloride (CsCl) | 8:8 | 1.76267 | 0.87 |
| Zinc Blende (ZnS) | 4:4 | 1.638 | 0.82 |
| Wurtzite (ZnS) | 4:4 | 1.64132 | 0.82 |
| Fluorite (CaF2) | 8:4 | 2.51939 | 0.84 |
| Rutile (TiO2) | 6:3 | 2.408 | 0.80 |
| Corundum (Al2O3) | 6:4 | 4.1719 | 0.83 |
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.