The experimental Rydberg formula gives positions of absorption and emission lines in the spectrum of atomic hydrogen. Classical physics cannot explain the spectrum of atomic hydrogen.

The Bohr model of hydrogen was the first model of atomic structure to explain the radiation spectra of atomic hydrogen correctly. It was preceded by the Rutherford nuclear model of the atom. In Rutherford’s model, an atom consists of a positively charged point-like nucleus that contains almost the entire mass of the atom and of negative electrons that are located far away from the nucleus.

Bohr’s model of the hydrogen atom is based on three postulates: (1) an electron moves around the nucleus in a circular orbit, (2) an electron’s angular momentum in orbit is quantized, and (3) the change in an electron’s energy as it makes a quantum jump from one orbit to another is always accompanied by the emission or absorption of a photon. Bohr’s model is semi-classical because it combines the classical concept of electron orbit (postulate 1) with the new concept of quantization (postulates 2 and 3).

Bohr’s model of the hydrogen atom explains the emission and absorption spectra of atomic hydrogen and hydrogen-like ions with low atomic numbers. It was the first model to introduce the concept of a quantum number to describe atomic states and to postulate the quantization of electron orbits in the atom. Bohr’s model is an important step in the development of quantum mechanics, which deals with many-electron atoms.