Secondly, the electron orbital g-factor, g L, is defined by μ μ L = − − g L μ μ B ℏ ℏ L , where μ L is the magnetic moment resulting from the orbital angular momentum of an electron, L is its orbital angular momentum, and μ B is the Bohr magneton. For an infinite-mass nucleus, the value of g L is exactly equal to one, by a quantum-mechanical argument analogous to the derivation of the classical magnetogyric ratio. For an electron in an orbital with a magnetic quantum number m l, the z -component of the orbital angular momentum is μ μ z = − − g L μ μ B m l which, since g L = 1, is − μ B m l