Atomic Spectra

Description

A quantitative analysis of the hydrogen spectrum is made and interpreted. Other spectra are examined and discussed qualitatively.

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• An important physical property of an element is its emission spectrum. An emission spectrum can be produced by passing an electric discharge through the vapor of an element. Another method for producing emission spectra is by introducing a volatile salt of an element into a Bunsen burner flame.
• Lithium imparts a red color to a flame, while sodium gives a bright yellow color. These colors are intense enough to serve as qualitative tests for the elements. Because the violet color produced by potassium is not as intense or bright as the yellow of the sodium, it can easily be masked by any sodium present. Viewing the flame through blue-colored glass -- called cobalt glass -- filters out the yellow and allows the violet potassium flame to be seen.
• Color in flames and emission tubes can be studied. Light from these sources may be diffracted using prisms or gratings. The result is the appearance of lines of colored light. These emission lines constitute the visible emission spectrum of the element.

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Several of the salt solutions are toxic. Handling electrical apparatus presents a potential hazard. Some ultraviolet radiation may be emitted from emission tubes. Handling hot spectrum tubes immediately after the power is turned off can lead to burns.

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Use caution when working with sources of high voltage. Touch only the power switch during an experiment. Do not continue to use the equipment if a vibrating or tingling sensation is noticed; this indicates an electrical hazard. Do not ingest the solutions. Do not touch spectrum tubes while power is on. Allow spectrum tubes to cool after turning power off.

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1. Obtain a set of solutions to be tested and a diffraction grating. Each solution should have a correspondingly labeled cork-wire-tube set.
2. Select a solution and its wire. To clean the wire, heat the wire loop in the hottest part of a hot burner flame. Use the cork as a handle. Heat until the wire is orange-hot. Dipping the wire into a few drops of clean, concentrated HCl facilitates cleaning of the wire. (Chlorides are more volatile than oxides.)
3. Dip the wire into the corresponding solution so that a drop of solution is retained in the wire loop. Heat the loop in the hottest part of a burner flame. Note and record the color of the flame.
4. Observe the flame through a cobalt blue glass.
5. Observe the flame through a diffraction grating.
6. Set up the apparatus shown.
7. Place the gas emission tube (and power supply) against a dark background in a darkened room. Place a meter stick in front of the tube, and a support stand at the end of the meter stick. Clamp a diffraction grating onto the stand using an appropriate clamp. Adjust the apparatus so that the position of the diffraction grating is directly above the end of the meter stick and the distance between the emission tube and grating can be measured. Tape the meter stick and support stand in place.
8. Place a second meter stick at the emission tube source. The second meter stick must make an angle of 90° (a right angle) with the first meter stick, and the end of the stick must be at the emission tube. Tape this stick in place.
9. Place a gas tube in the emission apparatus, and turn on the power. Observe the emission directly. Observe the emission through a diffraction grating.
10. This can be made into an elegant quantitative experiment. Measure a, the distance between the emission source and the diffraction grating.
11. The observer looking through a diffraction grating sees images of the emission tube to the left and right of that tube. While one student observes through the grating, assign a second student to help identify the position of the image. A marker -- a linear object such as a ruler or rod -- is held vertically above the second meter stick. The observer will see both the image and the marker. The marker is moved along the meter stick until the observer decides that the position of the marker coincides exactly with that of a spectral image. Record this position of the marker for each spectral line. Note the color of the line, and its position.
12. The distance a corresponds to the distance between the grating and the source. This distance is fixed for the apparatus once the teacher has set it up. The distance b, found by the cooperative effort of the observer and another student, is the distance at right angles between the emission source and the apparent position of the image of a spectral line.
13. Compute d, the spacing on the grating. This is the reciprocal of the number of lines/cm on the grating used, and is provided by the manufacturer of the grating material. When the grating has 13,400 lines/inch, d = 1.9 x 10^-4 cm.
14. The wavelength of the line observed can be calculated using the relationship,

    Wavelength = d sin θ

    Sin θ = b/c = b/(square root(b² + a²))

    Wavelength = d b/(square root(b² + a²))

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• The spectral lines that are observed provide evidence of the energy given off when an electron falls from a higher energy level to a lower one. Each element has a unique spectrum.
• Rydberg was able to calculate the wavelength of the light from hydrogen atoms. He was able to do this without any knowledge of energy levels using the formula:
• 1/wavelength = 1.1x10⁵cm^-1[(1/nf²)-(1/ni²)]
• Where nf and ni are whole numbers. These calculations fit only for the element hydrogen.

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Name _____________________________ Class _______

Teacher______________________________

DoChem 027 Atomic Spectra

Colors of Flames of Salt Solutions

Salt color to eye with blue glass with grating

(principal line)

________________________________________________

Colors of Emission Tubes

Gas color to eye color with grating

________________________________________________

Wavelengths Calculated for Hydrogen

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Name _____________________________ Class _______

Teacher______________________________

DoChem 027 Atomic Spectra

Watch the movie and record the color of the emission tubes and the lines.

Colors of Emission Tubes

Wavelengths Calculated for Hydrogen

a (cm)	b (cm)	d (cm)	Wavelength (cm)	Freq (Hz)	Energy (J)
98.5	36.9	0.00019
98.5	25.9	0.00019
98.5	23.7	0.00019
98.5	22.2	0.00019

1. Cite evidence that the color observed is due to the metallic element and not due to chlorine.
2. Explain the use of flame tests to identify unknowns.
3. Given the known wavelengths for hydrogen, compare our experimental answer by % difference:

   red	6.563 x 10-5 cm
   blue-green	4.861 x 10-5 cm
   blue	4.340 x 10-5 cm
   violet	4.102 x 10-5 cm (very hard to see)

4. The visible hydrogen transitions correspond to hydrogen electrons falling from higher levels down to the second level. Using Rydberg's formula, determine the value of ni for the transition responsible for the spectral lines you see.

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Purpose

To observe and study the characteristic visible light emitted by certain excited atoms.

To calculate the wavelength and corresponding energy of spectral lines.

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(for 10 students in groups of 2)

• 5 diffraction gratings
• 5 cobalt glass plates
• 35 nichrome wires fitted into corks and with protective tubes
• gas spectrum tubes (He, Ar, Ne, Hg, H₂, etc.)
• 5 spectrum tube power supplies
• 5 Bunsen burners, igniters
• 1 support stand, 1 single buret clamp
• 2 meter sticks
• 1 ruler or rod
• tape (masking tape)
• 5 sets of bottles. Each set should contain bottles filled with 0.1 M solutions of salts: LiCl, CaCl₂, KCl, CuCl₂, Sr(NO₃)₂, NaCl, and BaCl₂. (Chloride salts are preferred, but nitrate salts with added HCl may be used.)
• 5 mL concentrated HCl

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• Diffraction gratings may be purchased through various science supply houses. (They may be available in the school's physics department.)
• Nichrome, platinum or iron wires are inserted into corks. Label the corks according to the solution they are to test. Do not mix the corks or their tubes. If the corks are properly labeled and always used with the same solution, they do not have to be cleaned after each test.
• Gas spectrum tubes and spectrum tube power supplies may be purchased through a scientific supply house. (They may be available in the school's physics department.) It is unlikely that more than one or two power supplies will be available, so some sort of scheduling or data sharing system is needed.
• Recording observations will be difficult unless as much light as possible has been eliminated from the room.
• The wires on the corks will be hot. Warn the students to avoid touching them.
• Instruct the students on how to look through a diffraction grating.
• Set up the apparatus so that the meter stick used to measure distance b is at or near eye level.

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Teacher preparation: 30-40 minutes

Class Time: 45-50 minutes

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• Save the materials for reuse. It is best not to store the wires in their solutions.
• Each wire should be removed and heated to dryness in a hot flame. The flame-dried wire can be stored in a protective test tube.
• Be certain to label the tube and to store the wire and tube near the original solution. Store a separate wire for each solution used.

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Closure Questions:

1. Cite evidence that the color observed is due to the metallic element and not due to chlorine.
2. Explain the use of flame tests to identify unknowns.
3. Given the known wavelengths for hydrogen, compare our experimental answer by % difference:

   red	6.563 x 10-5 cm
   blue-green	4.861 x 10-5 cm
   blue	4.340 x 10-5 cm
   violet	4.102 x 10-5 cm (very hard to see)

4. The visible hydrogen transitions correspond to hydrogen electrons falling from higher levels down to the second level. Using Rydberg's formula, determine the value of ni for the transition responsible for the spectral lines you see.

Answers to Questions:

1. Since the nonmetallic salt ion was chloride in each case and the spectra were different, the color was most likely due to the metallic ion.
2. To determine the identity of an unknown, one needs to match the color and/or spectra of the unknown with the known colors and/or spectra. The data in Table I could be used, for example, to help identify an unknown.
3. Assume from Table II, the red line wavelength

   % difference=[(true value - experimental value)/(true value)] x 100%

   Accepted	Observed	100% x ((Acc - Obs)/Acc)
   6.563 x 10-5	6.667 x 10-5	0.67
   4.861 x 10-5	4.786 x 10-5	1.54
   4.340 x 10-5	4.402 x 10-5	1.44
   4.102 x 10-5	4.137 x 10-5	0.86

4. Consider the red line, the lowest energy line (6.607 x 10^-5 cm)

   1/l = 1.1 x 105 cm^-1 (1/m² - 1/n²)

   1/l = 1.1 x 10⁵ cm^-1 (1/2² - 1/n²)

   1 = 6.500 x 10^-5 cm x 1.1 x 10⁵ cm^-1 (1/2² -1/n²)

   1 = 7.268 (1/4 - 1/n²)

   1/7.268 = 0.250 - 1/n² = 0.140

   1/n² = 0.250 - 0.140 = 0.112

   n²= (1/.112) = 9

   n = 3

   The red line corresponds to the transition from the 3rd energy level to the 2nd energy level.

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• Sodium vapor lamps are gas discharge tubes containing sodium vapor as the gas. They emit a bright yellow light. They are more economical to operate then incandescent lamps. Mercury vapor lamps also are in widespread use.
• The element helium was discovered on the sun before it was discovered on earth as the result of the examination of spectral lines from sunlight. Many elemental and molecular species have been identified in outer space as the result of examining spectral light.

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An excellent way to analyze the experimental data is to use a computer spreadsheet applications program. Students may input formulas to see the effect that a change has upon the calculated answers. See EXPT 134 for suggestions.

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See sample data for colors, hydrogen frequencies, & wavelengths. See Closure? answers

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• spectrum
• emission spectrum
• line
• color
• energy
• energy level
• wavelength
• frequency

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