Electrons in a metal behave differently than particles in a normal gas because electrons are fermions and they obey Fermi–Dirac statistics. This behaviour means that every k -state in the gas can only be occupied by two electrons with opposite spin. The occupied states fill a sphere in the band structure k -space, up to a fixed energy level, the so-called Fermi energy. The radius of the sphere in k -space, k F, is called the Fermi wave vector.