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Atomic Spectra and the Structure of the Atom

 

By Steve Humphrey

 

When last we checked in, I was saying that Rutherford’s planetary model of the atom was mistaken. Remember, this was the view that negatively charged electrons revolve around a positively charged nucleus. Well, traveling in a circle is a form of acceleration (i.e., deviation from constant straight line motion) and it was known by then (1910’s) that charged particles give up energy in the form of radiation when they are accelerated. An electron revolving around a nucleus would lose energy and spiral into the nucleus in a very short period of time. The planetary model thus implies that matter would be unstable. Atoms couldn’t exist for more than a fraction of a second. The presence of a stable Universe tells us that this view is incorrect. Also, remember from our discussion of the double slit experiment, it doesn’t seem as though electrons travel along determinate trajectories, so there couldn’t be orbital paths at all.

Since we can’t directly observe an atom, we have to infer its structure from certain experiments, and one of the most important is spectrographic. As we all know, when an object is heated sufficiently, it will begin to glow. An iron poker, for example, will go from black to red to white as it gets hotter and hotter. This glow is electromagnetic radiation (i.e., light) which can be run through a prism. If we look at something like the Sun, which radiates at all wavelengths, the spectrum is a continuous band of color, ranging from red to violet. But if we heat a gas of a single element, such as hydrogen, the spectrum is completely different, and consists of a black background with a few bright lines crossing it. And each element, such as oxygen, nitrogen, etc., has a different spectrum. These lines represent photons of different frequencies. (For radiation, frequency is the same as energy.) The puzzle then is what is it about the atom that can explain the presence of these spectral lines?

Visible light spectrum

In 1913, the Danish physicist Niels Bohr made a radical proposal. The electrons surrounding the nucleus don’t revolve around it; they reside in different “shells” at different distances from the nucleus, each shell corresponding to a particular energy. And this energy is discrete, as Max Planck and Albert Einstein had suggested earlier. That means that the energy of an electron is some integral multiple of a constant, h. So, an electron could have energy equal to 1h, or 2h, or 3h, but nothing in between. If this were true of macroscopic objects, it would be as if your car could travel at 20 mph, or 25 mph, but at no speeds in between. This is weird!

When atoms of hydrogen gas, say, are heated, or energized in some way, the electrons absorb that energy and jump to higher energy shells. Then, after an unpredictable period of time, each electron spontaneously drops back to a lower shell, giving off a photon with energy equal to the difference between the two shells. Each line in the spectrum represents photons with a particular frequency. So, if the electron in one atom jumps from the ground state, i.e., the lowest energy shell, to the next shell, and then decays back to the ground state, it will give off a photon with energy equal to the difference between those two shells. And if the electron in another atom jumps to a higher shell, and then decays back to the ground state, it will give off a photon will a larger energy. And it happens that an electron might jump to a still higher energy level, and then decay in stages, gradually going from one level to a lower one until reaching the ground state, and each of those transitions produce a photon with a particular frequency, which result in a particular line in the spectrogram.

Hydrogen emission spectrum

What makes this really counter-intuitive is that the electron cannot occupy any energy level between the various discrete shells. It is as though it disappears from one shell and reappears at another without visiting any points in between. If it could occupy intermediate points, it would radiate at intermediate frequencies, and all elements would have spectra consisting of identical continuous gray smudges, and spectrography would warrant barely a page in any physics text.

One further oddity. There appears to be no rhyme nor reason to the timing or extent of the “quantum leaps” from one orbital shell to another. They are totally random, allowing only probabilistic predictions, which bothered many at the time. Coming out of the Newtonian zeitgeist, with its deterministic, “clockwork” Universe, this was a hard pill to swallow, causing Einstein to utter his famous rebuttal “God does not play dice with the universe.”

Steve Humphrey has a Ph.D. in the history and philosophy of science, with a specialty in the philosophy of physics. Questions? Comments? Suggestions? Email him at Steve@thevoicelouisville.com