In the discussion of Planck’s work, I suggested that we could understand quantization as being like money - everything is a multiple of the penny. Let’s use that analogy to understand Bohr’s atom.
If an electron is a very short distance away from the nucleus, we could say that it had one cent of energy.
Of course, attraction between positive and negative don’t depend on direction, so that same one cent worth of potential energy could be anywhere on a circle around the nucleus. (Actually, it’s the surface of a sphere, although it is almost never shown as one).
If energy is quantized, then if an electron has more energy, it must be at a distance of “two cents” or of “three cents” etc. This gives us a diagram that looks like this:
Assuming that we are looking at a hydrogen atom, there is only one electron. It makes sense that the electron will be as close to the nucleus as possible, which means it will be on the first ring with the smallest amount (1 cent) of potential energy.
If we give the electron some energy (say 2 more cents), then it will have 3 cents of energy. In our picture of the atom, that means that it would need to be on the third ring. Bohr suggested that the electron would absorb the energy by “jumping” from the first level to the third. (By the way, this is called a quantum leap.)
Since the electron is still attracted to the nucleus it “wants” to go back down. More correctly, the electron would be more stable in the lower ring, so the electron will lose its excess energy and drop back down to the more stable, lower energy level. That “lost” energy is given out in the form of light.
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