The most known of these is the electron spin g-factor (more often called simply the electron g-factor), g e, defined by μ μ s = g e μ μ B ℏ ℏ S where μ s is the magnetic moment resulting from the spin of an electron, S is its spin angular momentum, and μ B = eħ /2 m e is the Bohr magneton. In atomic physics, the electron spin g -factor is often defined as the absolute value of g e : g s = | g e | = − − g e .