© Daniel Kleppner.
Niels Bohr's model of hydrogen depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity. The Bohr model has become a logo for nuclear physics, with a few elliptical orbits artfully displayed. However, Bohr himself did not take it literally, and it has been totally superseded by quantum mechanics. Bohr's model is essentially mechanistic with quantum ideas imposed on the allowed energies. In his model, elliptical as well as circular orbits are possible. (Unit: 5)