<P> If an atom, ion, or molecule is at the lowest possible energy level, it and its electrons are said to be in the ground state . If it is at a higher energy level, it is said to be excited, or any electrons that have higher energy than the ground state are excited . If more than one quantum mechanical state is at the same energy, the energy levels are "degenerate". They are then called degenerate energy levels . </P> <P> Quantized energy levels result from the relation between a particle's energy and its wavelength . For a confined particle such as an electron in an atom, the wave function has the form of standing waves . Only stationary states with energies corresponding to integral numbers of wavelengths can exist; for other states the waves interfere destructively, resulting in zero probability density . Elementary examples that show mathematically how energy levels come about are the particle in a box and the quantum harmonic oscillator . </P> <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> <P> In the formulas for energy of electrons at various levels given below in an atom, the zero point for energy is set when the electron in question has completely left the atom, i.e. when the electron's principal quantum number n = ∞ . When the electron is bound to the atom in any closer value of n, the electron's energy is lower and is considered negative . </P>

Who said electrons occupy specific energy levels or shells