Lab #4 Emission Spectra
Introduction
If the light from an incandescent bulb is passed through a prism or diffraction grating, it is separated into a rainbow of colours ranging from red to violet.  When the light produced by passing an electric current through a gas is viewed in a similar manner, however, the result is quite different.  Each gas is observed to have its own spectrum of bright lines.  In attempting to explain why gases emit line spectra, Neils Bohr took the concept of the nuclear atom proposed by Rutherford and developed his "solar system" model.

According to Bohr's model, electrons can travel only in certain fixed orbits.   In each orbit, an electron can have only a certain definite energy.  Electrons normally occupy the lowest energy levels (innermost orbits)  possible.  If an electron absorbs energy from an external source, it can gain sufficient energy to jump to a higher energy level.  The "excited" electron then gives off its newly acquired energy as it falls back to it original energy level.  Because the electrons can have only certain fixed amounts of energy, only certain energy changes are possible.  Each energy change corresponds to a  specific line in the line spectrum.

Purpose: To observe and calculate the energies of the emission line spectra of several elements.
Safety Notes:  Do not touch the high voltage energy source.
Apparatus:  Incandescent light bulb, high voltage energy source, spectroscope, Geissler spectrum tubes, retort stand with test tube clamp.
Procedure
1.  . Set up the incandescent light source.  Support the student spectroscope with the test tube clamp on the retort stand.
2. Draw the visible continuous spectrum that you see along with the scale readings in the spectroscope.
3.   Prepare an observation table with the following headings.
 
 Material in the 
 Spectrum Tube 
Overall
Colour
Spectral Lines
4.   Your teacher will place one of the spectrum tubes into the high voltage energy source.  Record the name of the element being observed, and carefully describe the colour in the tube.
5.  Observe the Geissler tube through the spectroscope, and record exactly what you see.
6.  Repeat steps 4 and 5 for each spectrum tube
7.  Observe the "unknown" Geissler spectrum tube. Record its colour and spectral lines.  Identify it based on your previous samples and records.
8.  Repeat step 7 for any other "unknowns".
Calculations:
For each of the spectral lines in the samples calculate the frequencies.  Show one calculation in detail and just answers for each successive calculation.

Questions:
1.     Is the spectrum for the incandescent light a continuous spectrum or a line spectrum?
2. Why do the line spectra of the various gases differ?
3. Suggest an explanation for the differences in the brightness of the lines in the line spectra.
4. What are the unknown gases that were shown to you?
5. How were you able to use the spectrum of each unknown to establish its identity?
6. Why did each gas only show a colour when electricity was passed through it?
7. What do the different substances show different spectra?
8. How does the Bohr model of the hydrogen atom explain the spectrum of hydrogen?
9. What is one practical application of passing an electrical charge through a gas?
10. The spectra observed in this lab are emission spectra.  What are emission spectra?  How would they differ from absorption spectra?