Electrons were pictured as orbiting a nucleus only at certain fixed distances

In 1913, Niels Bohr proposed that an atom's electrons orbit its nucleus only at specific fixed distances, each with its own fixed energy, and that an electron jumping between orbits emits or absorbs light at one exact frequency. The model explained why hydrogen gas glows at only a few sharp colors instead of a smooth rainbow, matching a pattern spectroscopists had measured decades earlier. It was soon superseded by quantum mechanics, but was the first model to bring quantum ideas into the atom.

— Niels Bohr, On the Constitution of Atoms and Molecules — Philosophical Magazine, 1913