Formulated by the German-born US physicist Felix Bloch (1905-83) in 1928.

A theorem relating to the quantum mechanics of crystals stating that the wave function y for an electron in a periodic potential has the form y(r) = exp (Ik-r)U(r), where k is the wave vector r is a positive vector and U(r) is a periodic function that satisfies U(r + R) = U(r), for all vectors R of the Bravais lattice of the crystal.

Block's theorem is interpreted to mean that the wave function for an electron in a periodic potential is a plane wave modulated by a periodic function.

This explains why a free-electron model has some success in describing the properties of certain metals although it is inadequate to give a quantitative description of the properties of the most metals.