Atomic electrons and energy
An atom permits its electrons to have only certain energies. These allowed states are called energy levels.
The permitted state with the smallest energy is the ground state. If extra energy moves an electron into one of the higher allowed states, the atom is described as excited.
Excitation
Excitation is the promotion of a bound electron from its current level to a higher permitted energy level after energy is transferred to it.
The electron remains bound to the atom; only its energy state changes.
Ionisation
Ionisation leaves an atom with a net charge because its number of electrons has changed.
In electron-collision ionisation, an atomic electron is removed completely. An electron occupying any bound level can be removed if sufficient energy is supplied.
The ionisation energy is the smallest energy input capable of freeing an electron that initially occupies the ground state.
De-excitation
An electron in an excited level can later fall to a lower permitted level. This downward change is called de-excitation.
The decrease in the atom's energy is carried away by a photon, whose energy equals the separation between the initial and final levels.
| Process | What happens to the electron? | Energy change |
|---|---|---|
| Excitation | Moves to a higher allowed level | Energy is transferred to the electron |
| De-excitation | Falls to a lower allowed level | Energy leaves as a photon |
| Ionisation | Becomes unbound from the atom | Sufficient energy is supplied for escape |