Which physicist proposed the first quantum theory of the atom in 1913?

Niels Bohr proposed the first quantum theory of the atom in 1913.

Bohr’s model placed electrons in specific allowed orbits around the nucleus rather than permitting every classical orbit. An electron could occupy a stationary state without continuously radiating energy, and light was emitted or absorbed when the electron moved between allowed states.

The model explained the hydrogen spectrum, whose lines had already been described by the Balmer formula. Bohr connected the observed frequencies to energy differences between quantized atomic states. His proposal built on Ernest Rutherford’s nuclear atom and Max Planck’s quantum idea.

The Bohr model is historically important but is not the complete modern description of atoms. It works especially well for hydrogen-like atoms with one electron. Full quantum mechanics replaced fixed electron orbits with wavefunctions and orbitals, which describe probabilities rather than miniature planetary paths.