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Section 7: The Spectral Lines of Hydrogen
Hydrogen, which consists of one proton and one electron, is the simplest atom. Conveniently, it also has the simplest emission spectrum of any element. In the late 1800s and early 1900s, the spectrum of hydrogen was closely studied to try to find a mathematical relationship between the wavelengths of the emission lines in order to make a better model of the atom. Early workers in spectroscopy looked at the visible emission of hydrogen and noticed that there is always one purple line, two blue lines, and one red line.
Figure 3-14. Spectral Lines of Hydrogen
The fact that hydrogen atoms emit or absorb radiation at a limited number of frequencies implies that these atoms can only absorb radiation with certain energies. This suggests that there are only a limited number of energy levels within the hydrogen atom. These energy levels are countable. The energy levels of the hydrogen atom are quantized.
© Wikimedia Commons, Public Domain. Authors: Merikanto, Adrignola, 10 September 2011.
The Swedish physicist, Anders Jonas Ångström (1814–1874), carefully measured these lines, and created a catalog of their locations. In 1885, the Swiss mathematician, Johann Balmer (1825–1898), developed an equation that predicts where the four lines would fall in the visible spectrum by their wavelengths, but he was unable to explain why hydrogen always produces the same distinctive pattern.
When elements are heated or subjected to a high voltage in a gas discharge tube, their emission line spectra can extend into the infrared or ultraviolet as well. Theodore Lyman (1874–1954), an American scientist, studied the hydrogen emission spectra in the ultraviolet range. He discovered the same pattern of lines in the ultraviolet that had been found by Balmer and Ångström in the visible range. Friedrich Paschen (1865–1947), a German physicist, studied the emission of hydrogen in the infrared range, and he also found the pattern. Each of these scientists (Lyman, Balmer, Paschen) has given his name to a series of lines in the hydrogen spectrum.
In 1888, the Swedish physicist Johannes Rydberg (1854−1919) presented an equation that unified the series of lines described by Balmer, Ångström, and the others by placing integers (n = 1, 2, 3, etc.) into the equation for the lines. Each series corresponded to a different n. However, Rydberg, like the others, did not have a physical explanation for the pattern that he had documented.
Other elements also produced predictable patterns, but were different from hydrogen. For example, helium produces multiple blue and purple lines, one green, one yellow, and one red; unfortunately, no simple equation, like the Rydberg equation, could model what was going on in helium, the second most simple element in the universe. The reasons behind the hydrogen emission spectra remained a mystery for more than 30 years until Niels Bohr (1885–1962), a Danish physicist, solved the puzzle of the emission spectra by developing a new model of the atom. But before we examine that, let us turn to another line of evidence that was perplexing physicists at the turn of the century: the photoelectric effect.