The last chapter left us with a planetary model of the atom that takes into account the existence of the nucleus, but which is inherently unstable. To recap, the model proposes that the centrifugal force of the revolving electron just exactly balances the attractive force of the nucleus. However, the issue is that the electrons—being electrically charged—must radiate energy as they move in a circular orbit. This is because Maxwell’s equations of electromagnetism predict that accelerating electric charges must emit electromagnetic waves. If the electrons radiate energy, they would lose velocity, thus spiraling into the nucleus.

Even if an electron’s orbit could somehow be stabilized, a related puzzle became a real quandary: it had been known since the late nineteenth century that excited atoms of a single element do not radiate a continuous spectrum. Instead, they produce a discontinuous spectrum of many lines. That is, as shown in Figure 77, instead of a smooth spectrum containing all colors, the light emitted by excited atoms consists of some number of discrete waves of different wavelengths. Each element has an individual, characteristic line spectrum, called its emission spectrum.

Figure 77 (a) The spectrum of light produced by white light is continuous, containing all colors of the visible spectrum. (b) The light produced by excited hydrogen is discontinuous, having just a few, well-defined spectral lines.

In 1862, Anders Ångström ...

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