Electronic Transition
Definition and meaning of Electronic Transition in chemistry.
An electronic transition is the movement of an electron between two quantized energy levels (orbitals) in an atom or molecule, occurring when a photon is absorbed or emitted.
In more detail
Because electronic energy levels are quantized, a transition only occurs when the photon's energy exactly matches the gap between the initial and final states, following ΔE = hν = hc/λ. This is why atoms produce sharp line spectra and molecules produce characteristic absorption bands rather than continuous spectra. In molecules, common transitions include σ→σ*, n→σ*, π→π*, and n→π*, each requiring different photon energies depending on the orbitals involved. Selection rules based on spin and orbital symmetry determine which transitions are "allowed" (intense) versus "forbidden" (weak or very slow, as in phosphorescence).
Key facts
| Field | Physical Chemistry |
|---|---|
| Governing equation | ΔE = hν = hc/λ |
| Common molecular types | σ→σ*, n→σ*, π→π*, n→π* |
| Underlies | UV-Vis and atomic emission/absorption spectroscopy |
In the hydrogen atom, an electron falling from the n=3 to the n=2 energy level emits a photon of 656 nm, the red line of the Balmer series visible in hydrogen's atomic emission spectrum.
Frequently asked questions
What determines the energy needed for an electronic transition?
The energy gap between the initial and final electronic states of the atom or molecule; only photons matching that exact energy are absorbed or emitted.
Why are some electronic transitions called 'forbidden'?
Quantum mechanical selection rules based on spin and orbital symmetry make certain transitions highly improbable, so they appear weak or very slow (e.g., phosphorescence) rather than truly impossible.