Energy level is a particular value of the total energy that an atomic particle or group of particles can acquire, according to quantum theory. In this theory the energy levels for matter and radiation must fall into a discrete, quantized set.

Based on observations of absorption and emission spectra, a number of scientists pointed out in the latter half of the 19th century that the elements each have a set of characteristic wavelengths of electromagnetic radiation which they absorb when cool and emit when hot. An element only absorbs or emits radiation of its characteristic wavelengths. Any useful model of the atom must account for this observation.

J. J. Thomson (1856-1940) proved the existence of the electron in 1897. In 1911 Ernest Rutherford (1871-1937) proposed the nuclear model of the atom with a heavy nucleus at the center and much lighter electrons moving in orbits around the nucleus...