The Danish physicist Niels Bohr, who worked in Rutherford’s lab, was the first to describe orbits of fixed size and energy in which electrons are free to travel without losing energy and falling toward the nucleus. According to this model, published in 1913, electrons can only occupy or jump between fixed energy levels and cannot reside in between these levels. In addition, once in their “ground state,” electrons maintain the energy they contain. This energy keeps them in perpetual motion, allowing them to resist the attractive force of the nucleus.

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