Excited State
Definition and meaning of Excited State in chemistry.
An excited state is any quantum state of an atom, molecule, or ion that has higher energy than its ground state, produced when an electron absorbs energy and is promoted to a higher-energy orbital.
In more detail
Excitation occurs when a species absorbs a discrete packet of energy, commonly a photon, heat, or energy from a collision, equal to the gap between two allowed energy levels. Excited states are generally unstable and short-lived, decaying back toward the ground state either radiatively, by emitting a photon (fluorescence or phosphorescence), or non-radiatively, by releasing the energy as heat to surrounding molecules. Because the allowed energy gaps are unique to each substance, the wavelengths absorbed or emitted during these transitions form characteristic spectra, the basis of atomic emission and absorption spectroscopy.
Key facts
| Field | Physical Chemistry |
|---|---|
| Energy relation | E(excited) > E(ground) |
| Notation | Often marked with an asterisk, e.g., X* |
| Typical lifetime | Nanoseconds (fluorescence) to seconds (phosphorescence) |
When solid sodium chloride is heated in a flame, a valence electron in the sodium atom is promoted from the 3s to the 3p orbital. As it relaxes back to the 3s ground state, it emits a photon at 589 nm, giving the flame its characteristic yellow-orange color (the sodium D-line).
Frequently asked questions
How does a species return to the ground state?
It relaxes by emitting a photon (radiative decay, seen as fluorescence or phosphorescence) or by transferring the excess energy to its surroundings as heat (non-radiative decay, e.g., through molecular collisions).
Can a species have more than one excited state?
Yes. Atoms and molecules have a ladder of allowed energy levels, so there are typically many distinct excited states (first excited state, second excited state, and so on) above the ground state.