quantum number

In physics, one of a set of four numbers that uniquely characterize an electron and its state in an atom. The principal quantum number n defines the electron's main energy level. The orbital quantum number l relates to its angular momentum. The magnetic quantum number m describes the energies of electrons in a magnetic field. The spin quantum number m_s gives the spin direction of the electron.

The principal quantum number, defining the electron's energy level, corresponds to shells (energy levels) also known by their spectroscopic designations K, L, M, and so on. The orbital quantum number gives rise to a series of subshells designated s, p, d, f, and so on, of slightly different energy levels. The magnetic quantum number allows further subdivision of the subshells (making three subdivisions p_x, p_y, and p_z in the p subshell, for example, of the same energy level). No two electrons in an atom can have the same set of quantum numbers (the Pauli exclusion principle).