Niels Bohr
Niel Bohr realized that Planck’s idea of quantization could be applied to this model to explain the line spectra of elements. He labeled each energy level and consequently each orbit by a quantum number (n) for the lowest energy level or ground state (n=1). Energy levels correspond to the orbit closest to the nucleus. Excited state is where the electron absorbs appropriate amount of energy then it ‘jumps’ to the level of higher energy: quantum numbers (n=2, n=3, n=4). Excited states represent larger orbits with electrons farther from the nucleus. When radiation is absorbed the electrons jump from ground state to an excited state. Energy of absorbed or emitted radiation equals the difference between the two energy levels involved.
In 1913 Bohr stated that an electron around an atom could only exist in definite energy levels, which possessed definite amounts of energy. These energy levels (shells/orbits) can only be found at certain distances from the nucleus. Bohr created the Stair case model to help explain his spectral line for elements.