Question

what are energy levels?

Answer 1

Energy levels in atoms also called shells or orbits in which electrons revolve around atoms. The energy levels are fixed in atoms and are at constant distance from centre of aroms.

Answer 2

the K,L,M,N shells are known as energy levels. this is commonly used for energy levels electrons in atoms,ions,or molecules which are bound with the electric field nucleus but can also referred to energy level of nuclei are vibrational or rotational energy levels in molecules.
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