excited state
Summary
An excited state is an arrangement of an atom's or molecule's electrons with more energy than the ground state, usually because an electron absorbed energy and moved to a higher level. Testable point: a photon is absorbed when its energy matches the gap, and the electron returns by emitting energy equal to its drop.
Detail
Meaning: Electrons normally occupy the lowest-energy arrangement available, the ground state. When an electron gains energy from a photon, a collision, or heat, it can move to a higher-energy orbital, giving an excited state. Energy levels are quantized, so a photon is absorbed when its energy equals the gap between two levels. The excited state is short-lived: the electron drops back and emits energy equal to its drop, in one step or several. This produces line spectra.
Equation: ΔE = Ehigh − Elow = hf = hc/λ, where ΔE is the energy gap (J), h is Planck's constant, 6.63 × 10⁻³⁴ J·s, f is photon frequency (Hz), c = 3.00 × 10⁸ m/s, and λ is wavelength (m).
On the MCAT: Expect a hydrogen-like energy-level diagram or an electron configuration. A larger gap means a higher-frequency, shorter-wavelength photon. Moving to higher n is absorption; moving to lower n is emission. An excited configuration keeps the same electron count, such as sodium 1s²2s²2p⁶3p¹ versus the ground state 1s²2s²2p⁶3s¹.
Common confusion: An excited atom is not an ion. The electron stays bound and the charge is unchanged; ionization removes the electron entirely. Also, a stepwise return emits several lower-energy photons, so they need not match the one absorbed.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.