<P> Long - lived excited states are often called metastable . Long - lived nuclear isomers and singlet oxygen are two examples of this . </P> <P> A simple example of this concept comes by considering the hydrogen atom . </P> <P> The ground state of the hydrogen atom corresponds to having the atom's single electron in the lowest possible orbit (that is, the spherically symmetric "1s" wavefunction, which, so far, has demonstrated to have the lowest possible quantum numbers). By giving the atom additional energy (for example, by the absorption of a photon of an appropriate energy), the electron is able to move into an excited state (one with one or more quantum numbers greater than the minimum possible). If the photon has too much energy, the electron will cease to be bound to the atom, and the atom will become ionized . </P> <P> After excitation the atom may return to the ground state or a lower excited state, by emitting a photon with a characteristic energy . Emission of photons from atoms in various excited states leads to an electromagnetic spectrum showing a series of characteristic emission lines (including, in the case of the hydrogen atom, the Lyman, Balmer, Paschen and Brackett series .) </P>

When can we say that an electron is in an excited state