Topic:
Emission Spectroscopy Lab
Lesson Objectives:
Students will be able to
1. use a spectroscope to examine continuous spectra from light sources
and the bright-line spectra of some gaseous elements (3.4.10.A,
3.1.10.C, 3.1.12.C, 3.1.12.D, 3.2.10.B, 3.2.12.B).
2. calculate the energies that correspond to the wavelengths of bright-line
spectra (3.4.10.A, 3.1.12.A, 3.1.10.B, 3.1.12.B, 3.1.10.D, 3.1.12.E).
Materials:
Lab Handouts
Gas Tubes Containing the following:
Spectroscopes (24)
Hydrogen
Argon
Colored Pencils (6 packs)
Helium
Krypton
Graph Paper
Neon
Xenon
Gas Tube Power Supply (6)
Procedures:
Do a pre-lab discussion about the purpose, objectives, and safety concerns.
Briefly go through a demonstration as to how to use the spectroscopes.
Evaluation:
Labs are evaluations of student understanding of content material.
EMISSION SPECTROSCOPY
Purpose:
The purpose of this laboratory exercise is to observe the light produced by certain
elements and calculate the amount of energy produced from the wavelength of this light.
Discussion:
All atoms give off electromagnetic radiation when their gases or ions are
energized by heating or by a high voltage electric discharge. If the light emitted by a gas
is passed through a spectroscope, a pattern of narrow lines of light is produced. Each
element produces its own distinct pattern that differs from the pattern of every other
element. The particular pattern of frequencies of light emitted by an atom is referred to
as its emission spectrum, or bright-line spectra. The emission spectrum of an element can
be used as a means of identification.
Objectives:
In this lab, you will...
1. use a spectroscope to examine bright-line spectra of several gaseous
elements.
2. calculate the frequencies and energies that correspond to the
wavelengths of bright-line spectra.
Materials:
Spectroscope
Colored Pencils
Graph Paper
Gas Tube Power Supply
Gas Tubes Containing the following:
Hydrogen
Argon
Helium
Krypton
Neon
Xenon
Safety:
The gas tube power supply carries a high voltage. Do not touch the glass tubes.
Procedure:
1. Your instructor will load the glass spectrum tubes into the power source.
2. Using the spectroscope, examine the spectrum emitted by the gas in the glass
tube.
3. Record your observations by drawing the spectrum using the colored pencils
provided. Remember that spectra include spectral lines, so only draw vertical
lines. For example…
Hydrogen
400 nm
500 nm
600 nm
700 nm
(draw vertical spectral lines here)
4. For hydrogen and helium, record the colors and wavelengths for the brightest
spectral lines. Hydrogen should have three and helium should have seven. Be
sure to record the wavelengths in nanometers to the nearest nanometer.
Data:
1. Create a data table for all six element spectra.
2. Create a second data table for hydrogen and helium’s wavelengths (in nm and
m), frequencies (calculated below) and energies (calculated below).
Analysis:
Show work for one example of calculation 1, 2 and 3.
1. In order for us to calculate frequency, wavelength must be in meters. Convert
all wavelength values to meters. 1 m = 109 nm
2. Using the mathematical relationship between wavelength (in meters) and
frequency, calculate the frequency for each spectral line.
c = 299792458 m/s
3. Using the mathematical relationship between energy and frequency, calculate
the amount of energy emitted for each spectral line.
h = 6.62606876 x 10-34 J·s
4. Analyze the relationship between wavelength (in meters) and corresponding
energy (in Data Table 2) by graphing the two variables on a piece of graph
paper. Use the ten wavelength and energy values for both hydrogen and
helium. Label your graph as Graph 1, and remember to provide a descriptive
title. Remember the requirements for graphing outlined by your instructor.
5. Analyze the relationship between frequency and corresponding energy (in
Data Table 2) by graphing the two variables on a second piece of graph paper.
Use the ten frequency and energy values for both hydrogen and helium. Label
your plot as Graph 2, and remember to provide a descriptive title.
Error Analysis:
1. Calculate the percent error between the experimental energy values and the
theoretical energy values for hydrogen. Use the given theoretical wavelengths
and frequencies to match up the experimental energy values and the
theoretical energy values.
Data Table 3: Theoretical energies for the hydrogen line emission spectrum.
Emission Line
Color
Blue-violet
Blue-green
Red
Wavelength
 (nm)
434.037
486.1327
656.279
Frequency
f (Hz)
6.90707 x 1014
6.166886 x 1014
4.56806 x 1014
Energy
E (J)
4.57668 x 10-19
4.086225 x 10-19
3.02683 x 10-19
2. Interpret the percent errors you calculated in terms of errors that occurred in
your lab experiment.
Conclusions:
1. Why do atoms of gases larger than helium have more spectral lines?
2. Why do the colors of the spectral lines go from mostly bright reds, oranges
and yellows for H, He, and Ne to dark greens, blues, and purples for Ar, Kr,
and Xe?
3. Compare the results of your graphs to each other, relating their relationships
to their mathematical relationships shown below. Explain whether the two
variables are directly or inversely proportional.
c = f, E = hf
4. How might the study of emission spectra be used in studying stars?
5. List and explain one other application of emission spectroscopy.