SCH3U – Chemistry 11
Laboratory – Atomic Emission Spectrum
Name:
Date:
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 higherenergy 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. 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.
Procedure
Remember to record your observations in your lab notebook or on a piece of paper in your binder before you
leave class.
Observing Atomic Emission Spectra
Representative Wavelength
Color
Wavelength (nm) Region (nm)
1. Obtain a copy of the atomic emission spectra worksheet.
Violet
420
400 - 440
You will use this to record your lab data.
2. Near the top of the page (with the long side held vertical)
Blue
455
440 - 470
write the title: Atomic Emission Spectra. Also write your
Blue-green
480
470 - 490
name.
3. Your copy may not be in color. Using the chart at right,
Green
525
490 - 560
color the reference spectrum at the top of the page so
Yellow-green
565
560 - 570
that you will have a complete visible spectrum in color
with which to compare the line spectra of the elements
Yellow
580
570 - 585
you will examine.
Orange
620
585 - 630
4. Obtain a diffraction grating or a spectroscope. Either of
these devices will split the light produced by the elements
Red
660
630 - 700
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.
5. 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.ustrasbg.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.
6. 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.
Questions
1. What is a spectroscope and what is it for? Remember, you used a spectroscope in this lab.
2. 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?
3. Why did each of the different elements have a different emission spectrum? Explain your answer.
4. How can you use the emission spectrum of an element to identify it?
5. 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.
6. 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?