Moving along atomic orbits without emission
From the standpoint of classical electrodynamics, electrons in the planetary model of atoms would be continuously emitting and then, having lost their energy, fall into the nucleus. In 1913, Niels Bohr suggested that an electron in an atom can only move in orbits with radii proportional to integer numbers of Planck’s constant divided by the momentum of the electron. While in orbit, electrons do not radiate, and emission or absorption occurs only at the moment of transition from one orbit to another, which explains the spectrum of the hydrogen atom. Bohr’s approach immediately attracted the attention of physicists and stimulated rapid development of quantum concepts. In 1949, Albert Einstein was writing about his impressions on acquaintance with the Bohr’s theory: All my attempts to adapt the theoretical foundation of physics to these results failed completely. It was as loosing the ground under your feet and not seeing the solid ground on which to build. It always seemed a miracle that this fluctuating and contradicting the principles basis was sufficient to allow Bohr to find the basic laws of the spectral lines of atoms and electron shells, including their implications for chemistry.