The main question that remained after Rutherford's experiment, which Bohr helped to answer, was: What prevents the negatively charged electrons from spiraling into the positively charged nucleus due to electromagnetic attraction, leading to the collapse of the atom?
Rutherford's gold foil experiment demonstrated the existence of a small, dense, positively charged nucleus at the center of the atom, surrounded by electrons. However, classical physics could not explain the stability of the atom, as it would predict that the electrons, being attracted to the nucleus, should lose energy and spiral into it.
Bohr introduced the idea of quantized energy levels for electrons. He proposed that electrons occupy specific energy levels or orbits around the nucleus and that they can only gain or lose energy in discrete amounts (quanta) when transitioning between these levels. This quantization effectively prevents electrons from spiraling into the nucleus, thereby providing a stable structure for the atom.