Hydrogen Atom Spectrum Using an AA Spectrophotometer John Douglas and Ellak I. von Nagy-Felsobuki University of Newcastle, Newcastle, N.S.W. 2308, Australia The Bohr model of the hydrogen atom is often discussed in freshman chemistry courses (I) and in textbooks in physical chemistry (2) as an introduction to quantum chemistry and/or spectroscopy. Furthermore, the Bohr model has been reported in detail in this Journal (1,3-6). There is a lack of information in the literature of "Bohr atom" experiments that can be performed using equipment. readilv available in laboratories. For examnle. . . most chemistry departments in Austrulia possess atumic al)wrption rAAS) in order that studentscan identist~rctru~hurumeters f i anal5es in sample solutions. However, AAS can also be used to measure the Balmer series of the hydrogen atom spectrum. While this experiment must surely be known, the authors are unaware of any previous article reported in the literature that details such an experiment.
discovered a hvdroaen . - emission series. Figure 2 places the Balmer series in context of the other well--kno&hydrogen atom emissions (in terms of the first few members of each series). While operating conditions will vary from spectrophotometer to spectrophotometer, the AA-5 optimum conditions were the lamp intensity set t o nine, the amplifier gain set to high, and the fine- and coarse-gain set so that the Balmer emission 2 3 recorded a transmittance intensity of 100%. Figure 3 yields a plot of intensity versus wavelength, graphed a t intervals of 0.5 nm. The students were asked to complete the following tasks:
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Experimental Apparatus Descriptions of atomic absorption spectrophotometers are well reported in the literature (7, 8). Nevertheless, a brief resume highlighting the components needed for this experiment will be aiven below. An AA-5 atomic absorption spectrophotometer was used, althouah the same experiment has been oerformed on the more c~mmerciallyavailable AA-175. Thebasic design components of the spectrophotometer are given in Figure 1. - The background coriector lamp usedis a ~ a r i a hydrogen n hollow cathode lamp that was speciallv developed to measure the background absorption-in the 190-350-nm region (hydrogen continuum). The monochromator is an Ehert type, with the rule grating yielding a wavelength reproducibility of f O . l nm. The detection system is composed of a photon multiplier that is suitably amplified. Procedure, Results and Calculations The region of interest in this experiment is 660-370 nm since this was the region where J. J. Balmer (in 1885) first
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background corrector i m p
Figure 1. Components of an atomic abswption spectrophotometer used in the "Bohr atom" experiment.
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Figwe 2. Hydrogen atom spectral series
Journal of Chemical Education
Figure 3. Hydrogen atom spenrum obtained from atomic absorption spectraphotometer.
