In 1913 he published his explanation of how atoms, with electrons orbiting a central nucleus, achieve stability by assuming that their angular momentum is quantized (see Bohr theory). Movement of electrons from one orbit to another is accompanied by the absorption or emission of energy in the form of light, thus accounting for the series of lines in the emission spectrum of hydrogen. In addition to this work, for which he was awarded the 1922 Nobel Prize for physics, Bohr made further contributions to the quantum theory of atoms, invented the idea of complementarity, and made important contributions to nuclear theory.