Niels Bohr

In 1913 Niels Bohr came to work in the laboratory of Ernest Rutherford. Rutherford, who had a few years earlier, discovered the planetary model of the atom asked Bohr to work on it. Soon after, he came up with the Bohr model of the atom. The Bohr model was based off Rutherford's model, but with he suggested that electrons assume only certain orbits around the nucleus. Each orbit was then stated to have an energy level associated with it. For example the orbit closest to the nucleus has an energy E1, the next closest E2 and so on. These energy levels are called excited states of the electrons, with E1 being the ground state.
Leon, N. De, Prof. "Bohr Model of the Atom." Bohr Model of the Atom. Indiana University, n.d. Web. 04 Oct. 2015. <http://www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/Bohr-model.html>.