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Spectroscopy Lab
Chemistry Materials from Ray Tedder

Measuring Electron Energy Changes

 

Every element is the universe has a unique combination of protons, neutrons, and electrons.  The arrangement of these subatomic particles gives each element its characteristics chemical and physical properties.  The electrons, which occupy regions called orbitals or energy levels around the nucleus, constantly interact with their surroundings.  Specific amounts of energy are absorbed when electrons move from low energy orbitals to higher energy orbitals.  When the electron falls back to its original orbital, a photon of light is emitted.  This light has an energy, wavelength, and frequency that are unique, since the arrangement of energy levels for each element is unique.  When the light is observed through a spectroscope or diffraction grating, a bright-line spectrum is produced.

 

In this laboratory activity, you will observe the color and location of the spectral lines of various metallic elements and will determine the frequency and energy of light emitted by each element.

 

OBJECTIVES:

Students will:

-Observe the characteristic spectra of three common elements.

-Explain how these spectra (lines of colored light) are related to the electron energy levels of an atom.

-Determine wavelength of light emitted by each element.

-Determine the energy associated with each energy level change.

 

MATERIALS:


        Apron                        

        Goggles            

        Laboratory burner (Bunsen burner)

        Diffraction grating (this looks like a photo slide)

        Mount for diffraction grating

        Flashlight

        Two meter sticks

        Black hardboard with slit

        White board with vertical black line

        Spatula

        3 salts with known formulas

        1 salt with a formula that you do not know

        Your lab notebook with a list of procedures and tables for compiling data

micro-spatula.jpg
Figure 1. This is what a spatula (actually a micro-spatula) looks like.

Click here to download speadsheet with tables for calculating wavelengths of the light bars (spectral lines) you saw.

LAB PROCEDURE:

You will work in groups of three or four.  You must be sure to switch roles (see step 7) as you carry out the procedure for three different samples.

1.      The flame spectroscopy apparatus should be set up for you when you arrive at the lab.  The apparatus should include a lab burner (Bunsen burner) set behind a black board or card with a slit cut into it.  On the side of the board that is opposite the burner you should have a 2 meter sticks arranged in a kind of “T” shape with the black card and lab Burner at the top of the “T”.  At the bottom of the “T” there will be a diffraction grating on a holder.  The diffraction grating looks like a kind of photo slide.  You will be looking through this slide and to the left of the slit in the black board for the spectral lines. 

Spectroscopy Lab
spectroscopy-set-up.jpg
Figure 2. Illustration shows the way the equipment (or apparatus) will be set up for the lab.

2.      Have the instructor light your lab burner (Bunsen burner).  Adjust the flame until it is totally blue.  Look through the diffraction grating at the slit in the board.  Have another team member move the lab burner so that you can see the brightest possible light from the burner in the slit.

3.      Obtain labeled bottles of salts and write the chemical formula in a table in your lab notebook.  Also obtain a labeled bottle of an unknown salt.  Recall that salts are combinations of metals and a non-metal, or a metal and a polyatomic anion.  Emission spectra that you will be looking through the diffraction grating for will look like bars of colored light.

4.      Now divide the work into three jobs.

A.      One student will look through the diffraction grating.

B.      Another will use the white card with the black line on it to mark the place where the student in A sees a spectral line (or light bar).

C.      The third student will take the salt and hold it into the flame. 

Be very careful not to spill the salt into the Bunsen burner.  If you do spill salt into the Bunsen burner, all of your observations that follow that accident will be affected.

5.      Once all the materials are assembled, the apparatus is set up and the burner has been lit, it’s time to turn out the lights.  Student A observes the burner flame through the diffraction grating and slit. Student C should take a spatula, dampen it slightly, and dip it into the small container of one of the salts so that a small amount clings to the tip.  Student C then eases the salt on the tip of the splint or spatula into the flame.  Be sure to ease the salt into the flame a few centimeters from the top of the burner and on the side of the flame that is closest to the slit in the black card.  The idea is to get the salt into the flame without allowing the spatula into the flame.  If the spatula gets too hot, it will emit spectra (light cars) that will interfere with your observations and measurements.  Once student C sees a color change in the flame, student C should hold the spatula steady and not move it further into the flame.  Be careful not to allow any of the salt to drop into the burner as this will contaminate the burner and ruin your results for other salts.   

6.      Student B will hold the white card with the vertical black line behind the meter stick that is forming the top of the “T”.  Student A will look through the diffraction grating, locate the colored spectral line, and will have student B move the white card until the line on the white card is directly in line with spectral line.  Student C can now remove the salt from the flame.  Then, while student B continues to hold the card in place, the team will use the flashlight to determine the distance from the slit to the spectral line.  This distance and the color of the spectral line should be recorded in a table in your lab notebook.  The name of the student performing each job should also be recorded. 

7.      Repeat steps 5-6 with the other salts but changing jobs so that no student has the same job a second time.  Every student in each group should do each job at least once. 

 

 

DATA, OBSERVATIONS, and CALCULATIONS: 

 

The tables in your lab notebook should look like the ones below.  If you group has only 3 members in your lab group, you should leave the last column out of table 1 (the column labeled “Student who recorded data for the group”).  Some of your salts may have more than one colored line.  Be sure to record all the colored lines that you see.  You may need more rows that are listed shown in the table below.

spectroscopy-table-1.jpg

spectroscopy-table-2.jpg

If you have any questions, contact Mr. Tedder.