jan 1, 1913 - Niels Bohr (1913)
In 1913, Neils Bohr, a student of Rutherford's, developed a new model of the atom. He proposed that electrons are arranged in concentric circular orbits around the nucleus. This model is patterned on the solar system and is known as the planetary model. The Bohr model can be summarized by the following four principles:
Electrons occupy only certain orbits around the nucleus. Those orbits are stable and are called "stationary" orbits.
Each orbit has an energy associated with it. The orbit nearest the nucleus has an energy of E1, the next orbit E2, etc.
Energy is absorbed when an electron jumps from a lower orbit to a higher one and energy is emitted when an electron falls from a higher orbit to a lower orbit.
The energy and frequency of light emitted or absorbed can be calculated by using the difference between the two orbital energies.
Added to timeline:
History of the Atomic Theory