The adjacent figure shows quantized energy levels for the hydrogen atom. These levels are labeled by an integer n that is called a quantum number. The lowest energy state is generally termed the ground state. The states with successively more energy than the ground state are called the first excited state, the second excited state, and so on.

Beyond an energy called the ionization potential the single electron of the hydrogen atom is no longer bound to the atom and the energy levels form a continuum. For hydrogen, this continuum starts at 13.6 eV above the ground state ("eV" stands for "electron-Volt", a common unit of energy in atomic physics).