In 1913, the Danish scientist Niels Bohr put forward his theory of energy levels in an atom. He based his theory on the analysis on the emission spectra he obtained through excited hydrogen atoms in a discharge tube. Electrons can only occupy certain areas within the atom, called energy levels. An energy level is the fixed energy value of an electron in an atom. The ground state of an electron is when it occupies the lowest energy level available to it. (in a stable state) When an electron absorbs energy through heat or electricity, it jumps from a lower energy level to a higher energy level. The electron is not in an excited state (unstable. The electron tends to quickly drop back down to the ground state. when the electron drops from the excited to the ground state, it released a definite amount of energy in the form of a photon of light. The definite amount of energy released when an electron drops from higher to lower states of energy is equal to the difference in energy between the two energy levels. This is given ad E2 - E1 = hf E1=energy value of higher level. E2=energy level of lower level h=Planck's constant f= frequency of the light emitted The energy levels are given in the number (n) called the principal quantum number.