Rutherford came up with a new model of the atom that accounted for his scattering results: he proposed that the positive charge and its associated mass is concentrated in the nucleus, and that the negatively charged electrons are orbiting around it, like planets. In his 1911 paper discussing the alpha-particle scattering results,20 Rutherford mentioned the planetary atomic model that had been proposed back in 1904 by Japanese physicist Hantaro Nagaoka.
Nagaoka was inspired by the structure of Saturn, and imagined the atom could have a very massive nucleus, analogous to the massive nature of Saturn (Figure 75). Electrons would revolve around the nucleus bound by electrostatic forces, just like Saturn’s rings revolve around the planet bound by gravitational forces.
Figure 75 The planetary model of the atom. (a) In 1904, Hantaro Nagaoka proposed a model of the atom based on an analogy with the rings of Saturn. He proposed the existence of a very massive nucleus (analogous to a very massive planet), and electrons revolving around the nucleus, bound by electrostatic forces (analogous to the rings revolving around Saturn, bound by gravitational forces). This model was mentioned by Rutherford in his 1911 paper, in which he announced the discovery of the nucleus. (b) Despite its inaccuracy, the Rutherford model caught the imagination of the public, and is still used as a symbol for atoms and atomic energy.
Nagaoka believed that such a system would be stable, analogous to Saturn’s rings, which remain steady because they orbit a very, very massive planet. However, the problem with this assumption is that electrons, unlike Saturn’s rings, are electrically charged.
The issue is that the electrons in the planetary model move in circles. You may remember from basic physics that if a body is moving around a circle, even if it is moving at a constant speed it is accelerating. This is because it is changing direction (it isn’t moving in a straight line). Now, Maxwell’s equations of electromagnetism predict that accelerating electric charges should emit electromagnetic waves. As such, the electrons in the planetary model should lose energy by radiating electromagnetic waves. Then, like a satellite losing energy because of friction with Earth’s upper atmosphere, the electrons should quickly spiral to their death into the nucleus (Figure 76). All atoms in the universe would have disappeared just an instant after their creation!
Figure 76 The planetary model of the atom is inherently unstable. This is because an orbiting electron is an electric charge under acceleration, and must therefore radiate electromagnetic energy at the cost of centrifugal force. The electron in this model would thus spiral to its death in a very small fraction of a second.
We will see how quantum physics comes to the rescue by explaining how electrons may nevertheless remain in stable orbits around the nucleus.
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