Complex sentence: While the Bohr model worked well for hydrogen, it could not explain more complex elements, and by 1919, Bohr was moving away from the idea that electrons orbited the nucleus, and he developed heuristics to describe them.
Split into simpler sentences: While the Bohr model worked well for hydrogen, it could not explain more complex elements. By 1919, Bohr was moving away from the idea that electrons orbited the nucleus, and he developed heuristics to describe them.