Chem 81 - Lab 1

EMISSION SPECTRUM OF SODIUM

In the experimental portion of this lab, each pair of students will record the emission spectrum of Na atoms from a sodium discharge lamp using a computer-controlled monochromator. The spectrum will be calibrated against the emission spectrum of atomic Hg.

The sodium atom behaves approximately like a one-electron atom, and hence its spectrum has series in it like those found in the H atom spectrum. You will assign as many lines as possible to such series, and then show that quantitative agreement with formulas derived for one-electron atoms is not obtained. The corrections which must be made can be qualitatively explained by the effects of electron repulsion and nuclear charge shielding. Such experiments are the source of much of our knowledge of the structure of polyelectronic atoms. You will also calculate the Rydberg constant and the ionization energy for Na.

There are two handouts for this lab. One covers the experimental method itself (bring this to lab!), and the other covers the theory, data analysis, and lab writeup. Click here for a (color) pdf version of the experimental methodology handout "Running the Sodium Emission Experiment" (about 60k in size).

Shown below are color versions of figures in the lab theory handout. Click here for a (color) pdf version of the handout (about 170k in size).

Click here for an interactive spectrum simulator that will help you assign your spectrum.

Front view of monochromator

Back view of the monochromator showing the entrance window and the photomultiplier.

Top view of the head end of the monochromator showing the grating and the light path.

View of the tail end of the monochromator with the cover removed. The left folding mirror directs the incoming light back to the grating, and the right mirror directs the dispersed light back to the exit slit and detector.

Schematic top view of the monochromator. The undispersed light is focused on the entrance slit in such a way that it just “fills” the grating after reflection from the lower folding mirror. Light of a particular wavelength then leaves the grating on just the right path so that on reflection from the second folding mirror, it is focused on the exit slit and detected by the photomultiplier.

Copyright 1999, 2000 John S. Winn. All rights reserved. Last updated September 10, 2000.