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.