A constant arising in calculations of the cohesion of ionic crystals. The electrostatic interaction per ion pair, U, is given by U(r)=−αe2/r, where α is the Madelung constant and e 2/r is the Coulomb interaction between the ions, with r being the lattice constant. The value of α depends on the type of lattice. For the sodium chloride lattice, α has a value of about 1.75. To calculate cohesion, short-range repulsions with an inverse power law are included, i.e.
where C and n are constants. The value of α can be used in calculations to determine C and n. A more realistic calculation uses exponential repulsion rather than power law repulsion. The constant was first introduced by the German physicist Erwin Madelung (1881–1972) in 1918.