We next include a spin–orbit interaction. First, we have to describe the system by the quantum numbers j, m j and parity instead of ℓ, m l and m s, as in the hydrogen–like atom. Since every even level includes only even values of ℓ, it includes only states of even (positive) parity. Similarly, every odd level includes only states of odd (negative) parity. Thus we can ignore parity in counting states. The first six shells, described by the new quantum numbers, are