<P> The first evidence of quantization in atoms was the observation of spectral lines in light from the sun in the early 1800s by Joseph von Fraunhofer and William Hyde Wollaston . The notion of energy levels was proposed in 1913 by Danish physicist Niels Bohr in the Bohr theory of the atom . The modern quantum mechanical theory giving an explanation of these energy levels in terms of the Schrödinger equation was advanced by Erwin Schrödinger and Werner Heisenberg in 1926 . </P>
who said in an atom electrons are located in energy levels
Niels Bohr