A simple definition of Bohr’s atomic model is: electrons orbit the nucleus at set distances. When an electron changes orbits, it does so in a sudden quantum leap. The energy difference between the initial and final orbit is emitted by the atom in bundles of electromagnetic radiation called photons. This model was proposed in 1913 by Niels Bohr and was really an expansion on the Rutherford model of 1911. The Rutherford model had several flaws that the Bohr model overcame.
The Bohr model was based on his observations of the atomic emissions spectrum of the hydrogen atom. When white light is diffracted with a prism, all the colors of the visible spectrum can be seen. Each color corresponds to a specific amount of energy; however when the light given off by the hydrogen atom was passed through a prism, only certain colors of light could be seen. This led Bohr to theorize that electrons only have certain energies in an atom and they had to be in energy levels. Bohr found the energy of the colors of light that the hydrogen atom released. He used these energies to find the energies that the single electron in the hydrogen atom could have. Bohr said that the electron had to release energy to change its energy so the differences between the energies of light seen in the atomic spectrum should correspond to the differences in energies of the energy levels.
The Bohr model of the atom proposes that electrons orbit a nucleus at a predetermined distance. Bohr believed that each orbit was defined by a certain energy, so he said that the electron was in an energy level. In the stable atom, the electron is in the lowest energy level but when energy is added to the atom, then the electron jumps to a higher energy level because it now has more energy. When the electron returns to its groundstate (or most stable state, the lowest energy), it has to give off energy, and it does so in the form of light.
Bohr’s model worked great for the hydrogen atom, but failed when it was applied to other, more complex atoms. Quantum mechanics needed a better, all inclusive model to work from. That is where the Bohr-Summerfield model look up the slack. This revised model suggested that electrons travel in elliptical orbits around a nucleus instead of the Bohr atom model’s circular orbits. This model supplemented the quantized angular momentum condition of the Bohr model with an additional radial quantization condition. This model was eventually found to be inadequate, also. There were several more atomic models proposed before the modern quantum mechanics model was developed.
This link will take you to a very technical outline of Bohr’s atomic model. Here on Universe Today we have a great article about another atomic model, the Dalton model. Astronomy Cast offers a good episode about energy levels and spectra.