Lab 4-2: Atomic Emission Spectroscopy
For Reference:
Atomic Spectra Applet: PhET Java Applet Neon Lights & Other Discharge Lamps.
Objective
In this lab students will learn about atomic energy levels, atomic emission spectroscopy, and the
spectral “fingerprints” of elements.
Overview
You will observe emission spectra and analyze the emission lines from a number of different light
sources. These light sources are gas discharge tubes filled with gaseous samples of various
elements. You will use the spectra 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. 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 quantum of light called a photon. 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. 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.
Element gas emission tubes with power supplies
diffraction gratings, and/or spectroscopes
paper & colored pencils
Spectrum Data Sheet
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: 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
Read all instructions completely before beginning your work in the lab. Turn in the Lab Report
Sheet with spectrograph drawings attached.
Observing Atomic Emission Spectra
Representative Wavelength
Color
Wavelength (nm) Region (nm)
1. Obtain a copy of the atomic emission spectra
Violet
420
400 - 440
worksheet. You will use this to record your lab
Blue
455
440 - 470
data.
2. Near the top of the page (with the long side
Blue-green
480
470 - 490
held vertical) write the title: Atomic Emission
Green
525
490 - 560
Spectrograms. Also write your name.
Yellow-green
565
560 - 570
3. Your copy is not in color. Using the chart at
right, color the reference spectrum at the top of
Yellow
580
570 - 585
the page so that you will have a complete
Orange
620
585 - 630
visible spectrum in color with which to
Red
660
630 - 700
compare the line spectra of the elements you
will examine.
4. Obtain a spectroscope. This device will split the light produced by the elements in the tubes
into a spectrum you can see. You will see a small number scale inside. This will make it easier
to assign numerical values to the wavelengths of the light.
5. Observe the spectra of all available elements. To prolong their usefulness please operate
them by cycling them on and off for 30 seconds at a time. Turn the lamps on for 30 seconds
and off for 30 seconds.
6. Record the emission spectrum of each element on the rectangles of your Atomic Emission
Spectra page. Label each spectrum carefully! Reference spectra can be viewed online at
http://astro.u-strasbg.fr/~koppen/discharge/. Be careful to place the lines of the spectrum as
close as possible to the correct numerical value for the wavelength. Use your colored 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.
7. Once everyone has had a chance to record all of the atomic emission spectra, you will
observe, compare and identify the unknown element.
Lab 4-2: Atomic Emission Spectroscopy Lab Report Sheet
Name: ______________________
Per _____
Using color pencils to color in the visible spectrum.
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. What was the identity of the unknown lamp?
5. How can you use the emission spectrum of an element to identify it?
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?
7. Carefully determine the wavelength of each of the emission lines in the spectra of the element
hydrogen. Calculate the frequency (c = λf) and the energy (E = hf) for each of the lines for the two
elements. List the results in order from the least energetic to the most energetic photons. Show
your calculations.
frequency
energy
Calculations
Wavelength
Color of emission line
Wavelength (m)
Frequency (Hz)
Energy (J)
8. 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.