SCH4C                           Lab #2 - Element 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.
Problem:  How does the spectrum of an incandescent light differ from the spectra produced by gas discharge tubes?
Apparatus:  student spectroscope
                    incandescent light source
                    high-voltage power supply
                    gas discharge tubes (several varieties)
Procedure
1.    Listen carefully to the instructions on how to use the student spectroscope.
2.    Prepare your retort stand with a clamp to hold the student spectroscope steady.
3.    The room will be darkened and the incandescent light source will be turned on. 
       Using your spectroscope diagram on the data sheet the emission spectra that 
       is visible through your spectroscope.
4.    The teacher will periodically change the light source.  Draw the line spectra 
       of each element.
5.    Repeat Step 4 for each gas.
6.    Draw the line spectra for several unknowns that your teacher will also have.
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?
Data Table:
               Element or Source  Spectrum (wavelength in nm)
  700      650      600      550      500      450      400
 
  700      650      600      550      500      450      400
 
  700      650      600      550      500      450      400
 
  700      650      600      550      500      450      400
 
  700      650      600      550      500      450      400
 
  700      650      600      550      500      450      400
 
  700      650      600      550      500      450      400
 
  700      650      600      550      500      450      400
 
  700      650     600      550      500      450      400
 
  700      650      600      550      500      450      400
 
  700      650      600      550      500      450      400
 
  700      650      600      550      500      450      400
 
  700      650      600      550      500      450      400