Excited State
Definition and meaning of Excited State in chemistry.
An excited state is any quantum state of a particle that has higher energy than its ground state. This state is produced when an electron absorbs energy and jumps to a higher-energy orbital. The particle could be an individual atom, a molecule, or a charged ion.
In more detail
Excitation happens when a chemical species absorbs a specific discrete packet of energy. This energy usually comes from a photon of light or a physical molecular collision. The absorbed energy must exactly match the gap between two allowed energy levels.
In chemistry, electrons normally rest in their lowest possible energy arrangement. This stable lowest energy arrangement is known as the atom's ground state. When an electron absorbs enough energy, it moves outward to a higher energy level.
Excited states are generally very unstable and only last for a short period. The promoted electron will quickly fall back down toward the original ground state. It can release this excess energy by emitting a new photon of visible light.
This glowing emission is what we see as fluorescence or phosphorescence in certain materials. The particle can also release the energy as heat to surrounding molecules without glowing. The allowed energy gaps are unique to every specific chemical substance in the universe.
Therefore, the specific wavelengths of light absorbed or emitted create a unique chemical spectrum. Chemists use these unique spectra in atomic emission spectroscopy to identify unknown chemical elements. Students often forget that molecules can have many different excited states, not just one.
Key facts
| Field | Physical Chemistry |
|---|---|
| Energy relation | E(excited) > E(ground) |
| Notation | Marked with an asterisk (X*) |
| Typical lifetime | Nanoseconds to seconds |
| Decay methods | Radiative (light) or non-radiative (heat) |
| Typical cause | Absorption of a photon or thermal energy |
We can see excited states in action when solid sodium chloride is heated in a flame. A valence electron in the sodium atom jumps from the 3s orbital to the 3p orbital. As the electron relaxes back to the 3s ground state, it emits a specific photon. This photon has a wavelength of 589 nm, which falls in the visible light spectrum. This specific emission gives the flame its bright and characteristic yellow-orange color.
Frequently asked questions
How does a species return to the ground state from an excited state?
It relaxes by emitting a photon of light, which we see as fluorescence. It can also transfer the excess energy to its surroundings as heat through collisions.
Can an atom or molecule have more than one excited state?
Yes, atoms and molecules have a whole ladder of allowed energy levels. This means there are many distinct excited states sitting above the normal ground state.
Why do different elements emit different colors of light when heated?
Each element has a unique arrangement of electron energy levels in its atoms. This causes them to emit specific wavelengths of light when their excited electrons fall back down.