Symbol.  A factor of proportionality in the expression for the energy levels of electrons of atoms in an external magnetic field. The Landé g factor was proposed empirically by Alfred Landé (1888–1975) in 1921 in his analysis of the Zeeman effect in terms of quantum theory. Elucidating the significance of the factor was an important problem for atomic theorists in the first half of the 1920s. Its physical significance became apparent after the concept of electron spin was proposed in 1925. The expression for g can be derived from group theory or, equivalently, the quantum theory of angular momentum.