Note for Teachers: Show the students several
examples of emission spectra before asking them to record
what they see. Seeing is not enough for most students since
they do not understand what they are looking at. An emission
spectrum is so far outside normal experience that without
preparation students may not know how to even record what
they do in fact see in using the spectroscope, diffraction grating, or CD.
Here is a Spectrum Data
Sheet for students to use to record their spectra. For Reference:
Atomic Spectra Applet: http://astro.u-strasbg.fr/~koppen/discharge/discharge.html
Objective
In this lab students will learn about atomic energy levels,
atomic emission spectroscopy, and the spectral
“fingerprints” of elements.
Overview
Students will draw their own continuous spectrum. Then they
will analyze the emission lines from a number of different atomic
emission light sources. These light sources are gas discharge
tubes filled with gaseous samples of various elements. They
will record the spectra they observe in such a way as to relate
them to the continuous spectrum they drew. They will then use
the spectra they drew to identify several unlabeled atomic
emission lamps. This is how scientists identify elements found
in distant stars.
Background
The electrons in an atom occupy different energy levels, as
you know. When all of the electrons are at the lowest possible
energy level they are said to be in the ground
state. Electrons do not always stay in the ground
state. Sometimes they can be promoted to a higher-energy
electron shell. This can happen in two ways. First, the
electron can absorb a photon of just the right amount of energy
to move it from one quantum shell to another. Second, when
atoms are heated or energized with electricity their electrons
can gain energy. This promotes them to the higher-energy shell.
When an electron is in a higher-energy shell it is said to be
in an excited state.
Electrons in excited states do not usually stay in them for
very long. When electrons lose their energy they do so by
emitting a photon of light. Photons are particles with energy
but no mass. Their energy is directly proportional to the
frequency of the light (remember: E = hf). The photons emitted
precisely match the quantum energy difference between the
excited state and the ground state.
For different elements the spacing between the ground state
and the higher energy levels is different. This gives rise to a
way to uniquely identify elements based on their spectrum. A
spectrum is the scientific name for a rainbow: light broken
into the different wavelengths that make it up. You can see
spectra using a spectroscope, a prism or a diffraction grating. A spectroscope is a device which uses a diffraction grating to create a visual spectrum in a way that places the spectrum on a scale. This enables the user to measure the wavelengths of light being observed. The back of an ordinary CD is a reflective diffraction grating. Atoms produce very sharp lines in a spectrum
when they are heated. You will look at these lines in this lab.
These lines show the energy differences between the excited
states and the ground state. The atomic spectrum
of hydrogen is shown below:
H Emission Spectrum
When you look at the hydrogen gas discharge tube you will
see a mixture of these four colors. To see the lines you have
to use a diffraction grating or a prism. Even so, the mixed
color alone can be enough to identify an element. Put
simply, each element produces a unique color spectrum when
energized sufficiently.
Because every element has a unique spectrum the spectrum of
an element can be used to identify it. Distant stars are too
far away for us to take a sample to analyze in a lab. Even so,
we can gather information about what they are made of by
looking at the spectrum of light they produce. By collecting
data here on Earth for every element we can record their
spectral “fingerprints”. These can be used to
identify them in far off stars and galaxies.
The atomic emission lamps use high voltage sources to
energize the atoms in the discharge tubes. These voltages are
5,000 volts or more. This voltage could be deadly:
never put a finger, pencil, or any other object into
the socket of the atomic emission lamps!
The discharge tubes are fragile and should be handled
with care. Do not remove them from the lamp fixtures and do
not move the fixtures to avoid breaking the tubes.
Always turn off the lamps when you are not using them to
save power.
Procedure
Remember to record your observations in your lab notebook or
on a piece of paper in your binder before you leave
class.
Near the top of the page (with the long side held
vertical) write the title: Atomic Emission
Spectra. Also write your name.
Your copy may not be in color. Using the chart at right,
color the reference spectrum at the top of the page so that
you will have a complete visible spectrum in color with which
to compare the line spectra of the elements you will
examine.
Obtain a diffraction grating or a spectroscope. Either of
these devices will split the light produced by the elements
in the tubes into a spectrum you can see. The spectroscope
will make it easier to assign numerical values to the
wavelengths of the light.
Observe the spectra of all available
elements and record the unique atomic emission
spectrum of each of the different elements in one of the
rectangles on your Atomic Emission Spectra page. Label each spectrum carefully! Reference spectra can be viewed online at http://astro.u-strasbg.fr/~koppen/discharge/discharge.html. Be as
careful as you can to place the lines of the spectrum as
close as possible to the correct numerical value for the
wavelength. This will be critical for identifying the
spectrum later. Use the color spectrum at the top of the page
to line up the lines as best you as can so that you can
estimate the wavelength of the lines you draw. The lines need
not be in color, unless you wish them to be.
Once everyone has had a chance to record all of the atomic emission spectra your teacher will remove
the labels and move the lamps to new locations. Your job is
to identify all of the numbered lamps by
comparing each spectrum to the spectra you recorded. Correct
identification is required to earn full credit for this part
of the lab.
page break
Grading
For this activity you must turn in the following items:
Your Atomic Emission Spectra
Your identifications of the numbered lamps.
Answers to the following questions
Questions
What is a spectroscope and what is
it for? Remember, you used a spectroscope in this lab.
You observed the spectral lines for
a variety of different elements. What is happening within an atom
that causes it to emit light in specific lines in a
spectrum?
Why did each of the different
elements have a different emission spectrum? Explain
your answer.
How can you use the emission
spectrum of an element to identify it?
What were the identities of the lamps your teacher set out for you to identify? Give
the letter and element for each lamp you identified.
Note: Water and hydrogen are hard to distinguish
Krypton and helium are also hard to distinguish
Carefully determine the wavelength
of each of the emission lines in the spectra of
two of the elements that you observed.
Calculate the frequency (c = λf) and the energy (E =
hf) for each of the lines for the two elements. Make a table
of your results with the following columns: 1 wavelength in
meters, 2 frequency in Hz, and 3 Energy in J. List the
results in order from the least energetic to the most
energetic photons.
Why do the elements in the tubes
have to have high voltage electricity run through them before
the colored light is emitted? In other words, why don't
the cold, un-energized tubes glow?
In your own words, write a short explanation (2 or more
paragraphs) of how an electron absorbs energy and re-emits it
as light and why different elements have different
spectra.
