Download Atomic Emission Spectrometry - San Diego Unified School District

Lab: Atomic Emission Spectroscopy
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.
Materials
1.
2.
3.
4.
atomic emission tubes with power supply
diffraction gratings, and/or spectroscopes
paper & colored pencils
Atomic Emission Spectra page
Safety
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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. Treat it accordingly.
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
Read all instructions completely before beginning your work in the lab. Remember to record your observations
before you leave class.
Observing Atomic Emission Spectra
1.
2.
3.
4.
5.
7.
Color
Representative Wavelength
Wavelength (nm) Region (nm)
Near the top of the next page write the title: Atomic Emission Spectra.
Violet
420
400 - 440
Using the chart at right, color the reference spectrum at the top of the page so
Blue
455
440 - 470
that you will have a complete visible spectrum in color with which to compare
Blue-green
480
470 - 490
the line spectra of the elements you will examine.
Green
525
490 - 560
Obtain a spectroscope. It 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 Yellow-green
565
560 - 570
numerical values to the wavelengths of the light.
Yellow
580
570 - 585
Observe the spectra of the available elements. The spectrum tubes a delicate
Orange
620
585 - 630
and must be used properly in order to ensure that they have as long a useful life
Red
660
630 - 700
as possible. To prolong their usefulness please operate them by cycling them on
and off for 30 seconds at a time.
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! 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.
Identify the unknown gas based on its spectral “fingerprint.”
http://astro.u-strasbg.fr/~koppen/discharge/
http://physics.nist.gov/PhysRefData/ASD/lines_form.html