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 (CaF_{2}) |
8:4 |
2.51939 |
0.84 |

Rutile (TiO_{2}) |
6:3 |
2.408 |
0.80 |

Corundum (Al_{2}O_{3}) |
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.

(note that rThe Kapustinskii Equation:^{+} and r^{–} are measured in pm) |

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