Name: _________________________________
Spectrum of Light Lab
Introduction: The tubes used in this experiment are filled with gases that we are interested in studying. An
electric current will run through the gas, providing the valence electrons in the gas atoms to move to a higher
energy level, called the excited state. When the electrons fall back down to their lowest energy levels, known as
the ground state, the electrons release energy in the form of radiant energy. The radiant energy can be seen
using spectroscopic glasses, which diffract the light. The spectrum appears as a series of colored vertical lines
that are unique to each element.
Purpose: To understand the process by which the gases give off colored light and be able to identify an
unknown gas by its spectra.
Pre-lab:
1. Define the following:
Valence electrons- ___________________________________________________________________
excited state- ________________________________________________________________________
ground state- _________________________________________________________________________
2. Draw the Bohr diagrams for Hydrogen and Neon.
3.
Based on the Bohr diagrams, which element do you think will have more lines in its spectra? Why?
Procedure:
1. Obtain and wear a pair of spectroscopic glasses.
2. As each gas is demonstrated, write the name of the gas, its visible color (without your glasses) and the
spectrum colors (with your glasses). During the lab use crayons or colored pencils to record the color of
the light you see with your glasses on. If you see multiple lines of the same color, show approximately
how many.
Gas
Visible Color
Spectra Code
Colors
Group Questions:
1. Identify the unknown gas shown in the experiment and give evidence to support your answer.
2. Why do you think spectral lines often referred to as a fingerprint for gases?
3. Explain how some of the noble gases were most likely discovered.
Spectra Regents Questions:
1. State the total number of valence electrons in a cadmium atom in the ground state.
2. Identify all the elements in the mixture shown above.
The Balmer series refers to the visible bright lines in the spectrum produced by hydrogen atoms. The
color and wavelength of each line in this series are given in the table below.
3. Explain, in terms of both subatomic particles and energy states, how the Balmer series is produced.
4. Recreate diagram above and draw four vertical lines to represent the Balmer series.
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5. Write an appropriate number of electrons in each shell to represent a Mg-26 atom in an excited state.
6. What is the total number of valence electrons in an atom of Mg-26 in the ground state?
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Many advertising signs depend on the production of light emissions from gas-filled glass tubes that are
subjected to a high-voltage source. When light emissions are passed through a spectroscope, bright-line
spectra are produced.
7. Explain the production of an emission spectrum in terms of the energy states of an electron.
8. Identify the two gases in the unknown mixture.
Name: ____________________________________________
Date: ______________
Spectra Lab Summary
Directions: Read the following passage related to the bright line spectra and the study of stars. Then answer the
following questions based on the passage and your knowledge of chemistry.
Spectra and What Scientists Can Learn From Them
Each element in the periodic table can appear in gaseous form and will produce a series of bright lines
unique to that element. Hydrogen will not look like helium which will not look like carbon which will
not look like iron... and so on. Thus, astronomers can identify what kinds of stuff are in stars from the
lines they find in the star's spectrum. This type of study is called spectroscopy. The science of
spectroscopy is quite sophisticated. From spectral lines astronomers can determine not only the
element, but the temperature and density of that element in the star. The spectral line also can tell us
about any magnetic field of the star. The width of the line can tell us how fast the material is moving.
We can learn about winds in stars from this. If the lines shift back and forth we can learn that the star
may be orbiting another star. We can estimate the mass and size of the star from this. If the lines grow
and fade in strength we can learn about the physical changes in the star. Spectral information can also
tell us about material around stars. Spectroscopy is one of the fundamental tools which scientists use
to study the Universe.
Excerpt from NASA.gov
1. How might emission spectra be used when studying stars?
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2. Determine what gas(es) are present in the spectra of star A. Give evidence to support your answer.
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3. Identify errors inherent in the spectra lab. Suggest possible changes to the lab that could be made to make
the experimental data more accurate.
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