The Rutherford Planetary and Bohr models of the atom have certain very satisfying qualities to them. It's just a shame they're incorrect...
Hey everyone, I'm back with another fun physics video. This time, I wanted to shed some light on a couple of models of the atom that existed around the early days of quantum physics. In other words, these are not the very first models of the atom - many models have preceded the Rutherford and Bohr models. But these two models kickstarted the field of quantum mechanics.
The first of these models to be devised was the Rutherford Planetary Model. Rutherford realised (based on an experiment he was leading, run by Geiger and Marsden) that atoms must have small but densely charged positive regions in their centre, and these regions must be surrounded by negatively charged electrons. This reminded Rutherford of the solar system. He hypothesised that atoms must look very similar to the solar system, except on a much smaller scale. The densely charged nucleus would substitute for the Sun, and the electrons for the planets (in that the electrons would orbit the nucleus). This model certainly agreed with the experimental evidence from Geiger and Marsen's Gold Foil experiment.
But not only was there a satisfying resemblance between the structure of the atom and the solar system in this model, there was a wonderful mathematical similarity too. When we consider the classical gravitational force exerted between two massive bodies, this force can be calculated using Newton's Law of Gravitation. This force is attractive, it is proportional to the masses of the two bodies, and it is inversely proportional to the square of the distance between the two masses. In a wonderful parallel, if we consider the electric force between two oppositely charged objects, THAT force is attractive, it is proportional to the charges of the two objects, and it is inversely proportional to the distance between the two objects. Hence, there was a lovely mathematical similarity between the forces seen inside the planetary atom, and the forces seen in the solar system.
Sadly however, the universe is not always pretty. It's often a lot more complicated than we'd like it to be. Rutherford's Planetary Model had a problem. Classical physics tells us that accelerating charges emit electromagnetic radiation - they lose energy this way. And electrons in orbit around the nucleus are indeed accelerating. Even if the electrons orbit at the same speed around the nucleus, they are constantly changing direction as they move along a circular path. This means that their velocity (vector quantity) is constantly changing, and hence the electrons are constantly accelerating. This would result in electrons constantly emitting EM radiation and spiralling towards the nucleus as they lose energy.
Clearly this doesn't happen in real life. Rutherford's Planetary Model is not a good description of what we observe in our universe. And this where Niels Bohr came in. He decided that the Planetary Model could be improved by placing some restrictions on it. He stated that electrons could only exist in a stable configuration at very specific distances from the nucleus. In other words, they could only have very specific energies. They could not exist in between energy levels - they would collapse down to the nearest available (lower) energy level. In other words, he avoided the problem of electrons radiating energy away and spiralling into the nucleus, by simply saying that they weren't allowed to do this. There was no explanation of why at the time (though nowadays we have come a lot further and we have a much better understanding). But Bohr's model was wonderful because it also explained another observed phenomenon: the fact that each element has a very specific emission (and absorption) spectrum, and it only emits specific wavelengths of electromagnetic radiation. This had to do with the fact that electrons emit this energy by falling to a lower energy level (if it is available), and the energy lost is always the same in that transition, hence a specific frequency of light is emitted.
Guys, I hope you enjoyed this video. If you want to read about emission spectra, check out the wikipedia page - it's quite handy: en.wikipedia.o...
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