Fred E. Stafford and James H. Wortman
Northwestern University Evanston, lllinois
1 I
Atomic Sp~trcl A Physical Chemistry Experrmenf
Fundamental to the ideas of chemical structure is the understanding of atomic energy levels and the periodic table. The study of a relatively simple atomic spectrum provides a tangible introducdion both to these and to elementary quantum mechanics as taught in the undergraduate courses. In this experiment, the student measures the features of a simple spectrum, that of sodium (potassium or hydrogen also will do). An empirical relation is deduced for the frequencies of the lines and the position of unobserved lines is predicted. Calculated results include the important Rydberg constant, the ionization potential of the atom, and an energy level diagram with which the general features of the periodic table are explained.
For best results, the wavelength scale of the instrument must be calibrated. For this purpose, we used a 4 watt mercury germicidal lamp which cost less than $10 complete. The observed mercury wavelengths were compared with known values (2) by means of a graph of (Aoba - Jmr.)versus Anha. Systematic deviations of about 10 A have been observed. Treatment of Experimental Data
A sodium spectrum as obtained by a student is shown in Figure 1. Under each line is indicated whether the line belongs to the sodium sharp (S),principal (P)
The Experiment
The light source is the General Electric NA-I sodium lamp, commonly used for polarhetry. Convenient sources are available also for potassium1 ( I ) and hydrogen.= Sodium offers the unparalleled advantaees of havine a snectrum in a verv easilv accessible region that may be interpreted in part in a very simple fashion (D series only), or more completely in a way to intrigue the brightest students. A Cary Model 11 spectrophotometer was used to record the spectra. The tungsten lamp of the Cary was pushed to the hydrogen lamp operating position and the sodium lamp without collimating lens, was placed in front of the slit by removing the six screws holding the back plate of the lamp housing. The spectrophotometer was then operated according to the manufacturer's instnictions for "Calibration," using the slowest scan speed and the fastest chart speed. The sodium and mercury emission spectra have been observed also with the Beckman DU and DK-2 spectrophotometers, but no reduction of these data was undertaken. It is important that the instrument be properly aligned mechanically and electronically and that the photomultiplier voltage be adequate. A simple test of the over-all sensitivity is to observe the slit width when used for absorption a t 5400 A with the tungsten lamp and a "slit control" setting of 20. If the gain is sufficient, the automatic slit control mechanism will close the slit down to 0.005 to 0.01 mm. Otherwise, some of the weaker sodium lines may not he observed.
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A wide variety of Osram spectral lamps are available commercially from the George W. Gates, Franklin Square, Long Island, New Yark; or the Ealing Corporation, Camhridge 38, Mass. ' Central Scientific Company, Chicago, Ill.
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Journal of Chemical Education
Figure 1. Spedrum of the Sodium NA-1 Polorimeter lamp. Under each line is indicated whether tho line belongs to the sodium thorp ISI, principal (PI or diffuse ID1 rerielor is due to a potmtsium impurity IKI.
or diffuse (D) series, or is due to a potassium impurity (I
