EMISSION SPECTRA AND ENERGY LEVELS
Pre-Lab Discussion
One convenient method of exciting the atoms of an element is to pass an electric
current through a sample of the element in the vapor (gaseous) phase. This is the
principle behind the spectrum tubes you will use in this investigation (see Figure 15-1). A
spectrum tube contains a small sample of an element in the gaseious phase. An electric
discharge through the tube will cause the vapor to glow brightly. The glow is prudced
when excited electrons emit radiant energy as they return to their original levels.
*Diagram*
Figure 15-1
When visible radiant energy from a spectrum tube is passed through a diffraction
grating, an emission spectrum (or bright-line spectrum) is produced. Each element has
its own unique emission spectrum by which it can be identified. Such a spectrum consists
of a series of bright lines of definite wavelength. Each wavelength can be mathematically
related to a definite quantity of energy produced by the movement of an electron from
one discrete energy level to another. Thus, emission spectra are experimental proof that
electrons exist at definite, distinctive energy levels in an atom.
In this experiment you will study the emission spectra of two elements—
hydrogen and mercury. You will calculate the wavelengths of some of the spectral lines
of these elements and compare your experimental values with known wavelengths of
hydrogen and mercury spectra.
Purpose
Determine the wavelengths associated with specific spectral lines of hydrogen and
of mercury.
Equipment
Spectrum tubes, hydrogen and mercury
Transformer, high-voltage
Diffraction grating
Cardboard screen (with vertical slit)
Pencils, colored (red, blue, green, violet)
*Diagram*
meter stick supports (2)
meter sticks (2)
screen supports (2)
safety glasses
Figure 15-2
Procedure
1.
2.
3.
4.
5.
6.
7.
Set up the apparatus as shown in Figure 15-2. The cardboard screen should be
placed on the 50-cm mark of one meter stick. The transformer (with the hydrogen
spectrum tube in place) should be placed directly behind the screen so that the
glow from the tube is clearly visible through the slit. The diffraction grating
should be placed on the second meter stick 100 cm from the tube.
One lab partner will view the emission spectrum of hydrogen by looking through
the diffraction grating at the slit in the cardboard screen. The other partner will
stand behind the transformer and move a pencil slowly along the meter stick. The
student viewing the spectrum should indicate when the pencil is at the point
where the image of the spectral line closest to the spectrum tube appears to be.
Measure the distance, in centimeters, between the tube and the image of the
spectral line. Record this distance (x) in your data table.
Repeat step 2 for two more spectral lines of hydrogen.
Disconnect the transformer. Carefully remove the hydrogen spectrum tube from
the transformer and insert the mercury spectrum tube. CAUTION: Handle the
spectrum tubes with care. They are easily broken. Reconnect the transformer.
Following the procedure outlined in step 2, locate and measure two spectral lines
of mercury.
Disconnect the transformer. Return the transformer and spectrum tubes to your
teacher.
Using colored pencils, make qualitative sketches on the spectra of hydrogen and
mercury in the space provided under Observations and Data.
Observations and Data
Qualitative drawings of spectra:
hydrogen
mercury
*Diagram*
*Diagram*
DATA TABLE
Element
hydrogen
Color of line
red
blue-green
blue
X (cm)
Y (cm)
100
100
100
violet
green
violet
mercury
100
100
100
Calculations
Calculate the following and fill in the following table:
1.
Find the distance (z) from the diffraction grating to the image of the spectral line:
z = √x2 + y2
2.
Find the sine of angle θ: sin θ = x/z
3.
Find the wavelength in cm: wavelength = d x sin θ, where d = 1.9 x 10-4 cm
4.
Find the wavelength in angstroms: wavelength (in angstroms) = cm x 108Å/cm
Element
color of line
hydrogen
red
blue-green
blue
violet
green
z
sin θ
wavelength
cm
mercury
Å
violet
Conclusions and Questions
1.
Compare your experimental results to the known wavelengths listed:
hydrogen
mercury
red 6563 Å
green 5460 Å
blue-green 4861 Å
violet 4359 Å
blue 4342 Å
violet 4359 Å
2.
3.
4.
How might emission spectra be used in studying stars?
Relate spectral lines to energy levels in an atom and to the term quanta.
Look up and discuss the relationship between the wavelength of a spectral line
and the quantity of energy (E) it represents.