The atom that Bohr proposed1 in July 1913 looked like a miniature Solar System, with electrons arranged in concentric orbits around a positively charged nucleus. In Bohr's model, electrons were point-like particles that were quantized, meaning that they could jump from one orbit to another but could not exist in between. The advent of quantum mechanics in the 1920s retained the concept of orbits but re-imagined electrons as spreading everywhere around the nucleus. The location of each electron can be described only in probabilities, in the form of a mathematical wavefunction.
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