- A possible state at which an electron can exist in an atom. The fact that any particular atom can house electrons only at specific energy levels gives rise to the lines in spectra, which exist as sharp features because they reflect the difference in energy between two energy levels. They are produced as bright features when electrons go from higher levels to lower, or as dark ones when electrons are raised from lower levels to higher, absorbing energy rather than emitting it.
- For a quantized system, one of the allowed energy values which may be adopted. Electrons in such systems can only change from one specific level to another, and in the process absorb or emit energy.
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