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By the end of this section, you will be able to:
  • Explain the difference between the absorption spectrum and the emission spectrum of radiation emitted by atoms
  • Describe the Rutherford gold foil experiment and the discovery of the atomic nucleus
  • Explain the atomic structure of hydrogen
  • Describe the postulates of the early quantum theory for the hydrogen atom
  • Summarize how Bohr’s quantum model of the hydrogen atom explains the radiation spectrum of atomic hydrogen

Historically, Bohr’s model of the hydrogen atom is the very first model of atomic structure that correctly explained the radiation spectra of atomic hydrogen. The model has a special place in the history of physics because it introduced an early quantum theory, which brought about new developments in scientific thought and later culminated in the development of quantum mechanics. To understand the specifics of Bohr’s model, we must first review the nineteenth-century discoveries that prompted its formulation.

When we use a prism to analyze white light coming from the sun, several dark lines in the solar spectrum are observed ( [link] ). Solar absorption lines are called Fraunhofer lines    after Joseph von Fraunhofer , who accurately measured their wavelengths. During 1854–1861, Gustav Kirchhoff and Robert Bunsen discovered that for the various chemical elements, the line emission spectrum    of an element exactly matches its line absorption spectrum    . The difference between the absorption spectrum and the emission spectrum is explained in [link] . An absorption spectrum is observed when light passes through a gas. This spectrum appears as black lines that occur only at certain wavelengths on the background of the continuous spectrum of white light ( [link] ). The missing wavelengths tell us which wavelengths of the radiation are absorbed by the gas. The emission spectrum is observed when light is emitted by a gas. This spectrum is seen as colorful lines on the black background (see [link] and [link] ). Positions of the emission lines tell us which wavelengths of the radiation are emitted by the gas. Each chemical element has its own characteristic emission spectrum. For each element, the positions of its emission lines are exactly the same as the positions of its absorption lines. This means that atoms of a specific element absorb radiation only at specific wavelengths and radiation that does not have these wavelengths is not absorbed by the element at all. This also means that the radiation emitted by atoms of each element has exactly the same wavelengths as the radiation they absorb.

Figure depicts the solar emission spectrum in the visible range from the deep blue end of the spectrum measured at 380 nm, to the deep red part of the spectrum measured at 710 nm. Fraunhofer lines are observed as vertical black lines at specific spectral positions in the continuous spectrum.
In the solar emission spectrum in the visible range from 380 nm to 710 nm, Fraunhofer lines are observed as vertical black lines at specific spectral positions in the continuous spectrum. Highly sensitive modern instruments observe thousands of such lines.
Figures A and B show the schematics of an experimental setup to observe absorption lines. In Figure A, white light passes through the prism and gets separated into the wavelengths. In the spectrum of the passed light, some wavelengths are missing, which are seen as black absorption lines in the continuous spectrum on the viewing screen. In Figure B, light emitted by the gas in the discharge tube passes through the prism and gets separated into the wavelengths. In the spectrum of the passed light, only specific wavelengths are present, which are seen as colorful emission lines on the screen.
Observation of line spectra: (a) setup to observe absorption lines; (b) setup to observe emission lines. (a) White light passes through a cold gas that is contained in a glass flask. A prism is used to separate wavelengths of the passed light. In the spectrum of the passed light, some wavelengths are missing, which are seen as black absorption lines in the continuous spectrum on the viewing screen. (b) A gas is contained in a glass discharge tube that has electrodes at its ends. At a high potential difference between the electrodes, the gas glows and the light emitted from the gas passes through the prism that separates its wavelengths. In the spectrum of the emitted light, only specific wavelengths are present, which are seen as colorful emission lines on the screen.

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Source:  OpenStax, University physics volume 3. OpenStax CNX. Nov 04, 2016 Download for free at http://cnx.org/content/col12067/1.4
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