Electrons were once thought to orbit a nucleus much as planets orbit the sun. That picture has since been obliterated by modern quantum mechanics.
An atom is best visualized as a tight, dense nucleus surrounded by buzzing, orbiting electrons. This picture immediately leads to a question: How do electrons keep whirling around the nucleus without ever slowing down?
Related: Weird 'gravitational molecules' could orbit black holes like electrons swirling around atoms A second, more subtle issue had to do with the nature of the radiation. Scientists have known that atoms emit radiation, but they do so at very discrete, specific frequencies. An orbiting electron, if it followed this solar system model, would instead emit all sorts of wavelengths, contrary to observations.
Planck's constant has the same units as angular momentum, or the momentum of an object moving in a circle. So Bohr imported this idea to electrons orbiting a nucleus, saying that the smallest possible orbit of an electron would equal the angular momentum of exactly one Planck constant. Higher orbits could have twice that value, or three times, or any other integer multiple of the Planck constant, but never any fraction of it .
Future developments in quantum mechanics would continue to refine this picture, but the basic point remains: An electron can't get any closer to a nucleus because its quantum mechanical nature won't let it take up any less space.
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