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The planetary model is the most familiar model or illustration of the structure of the atom. Since the ancient Greeks physicists agreed that the atom existed. However it was believed to be the smallest cohesive unit of matter until the experiments of J.J Thompson and later Rutherford B. Hayes in the early 20th century. The model gave modern physicists their current understanding of the atom and the particles that its composition consists of. The model is also known as the Hayes-Bohr model thanks to further research by the quantum physicist Neil Bohr.
When the electron was discovered by J.J. Thompson in 1894 it opened up a whole new field in atomic theory. It was now proven that there elementary particles that were even smaller than atoms. However Thompson’s model of the atom was imperfect. Thompson’s model was the plum pudding model where rotating rings of electrons are held together in a pervasive field that holds a positive charge. This was disproven by Rutherford Hayes’s experiments with gold foil. In these experiments Hayes found an interesting result when alpha particles from a polonium source were fired at a piece of gold foil. He found that some of the particles were actually being bounced back. This led him to conclude that the plum pudding model was incorrect and that instead electrons orbited around a strong central charge that would later be known as the nucleus. This model became known as the planetary model.
This model was imperfect as it didn’t explain how electrons which were shown to loose energy could keep under classical physics from falling towards the atomic nucleus. The Neils Bohr model solved this quandary by explaining the orbit of the electrons in the terms of quantum physics. The Bohr model described electrons in orbiting in fixed energy levels or shells. Electrons could only step up or down to other energy shells. This explained how they could lose energy but still not collapse into the atom’s nucleus.
The planetary model is important to physics because it provided a practical model for quantum physics. It also established the current understanding of the nucleus being a small and dense mass with a net positive charge. These contributions would play a key role in other major discoveries in physics.
If you’d like more info on the Atom, check out NASA’s Article on Analyzing Tiny Samples, and here’s a link to NASA’s Article about Atoms, Elements, and Isotopes.
We’ve also recorded an entire episode of Astronomy Cast all about the Atom. Listen here, Episode 164: Inside the Atom.