5: ATOMIC STRUCTURE AND FLAME TESTS
- Page ID
- 548891
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- To observe the light emitted from different metals.
PURPOSE
The purpose of this experiment is to
· Perform flame tests on solutions containing various elements, and examine the colors produced.
· Explore the emission spectrum of hydrogen.
INTRODUCTION
During the lecture, you learned about two models of atomic structure: the Rutherford and Bohr models. The Rutherford model states that the majority of the atom’s mass comes from the nucleus, which is positively charged and resides at the center of the atom. In this model, electrons randomly orbit the nucleus in circular paths. The Bohr model, proposed after hydrogen’s structure was examined in more detail, explains that protons and neutrons are located inside the atom’s nucleus, whereas electrons exist in specific energy levels outside the nucleus.
When solutions containing various elements are heated, electrons transition from a lower to a higher energy level. Electrons eventually will return to their original energy levels, and when they do so, they often emit energy in the form of visible light. Evidence of the Bohr model can be seen by the light produced as materials containing certain elements are heated. Because electrons exist in specific energy levels and are not randomly placed outside the nucleus, each element has its characteristic pattern of energy levels and, as a result, emits its characteristic pattern of light.
As you will see in this experiment, flame tests produce a continuous, characteristic color for given elements. When viewed through a diffraction grating (a component that divides light into its constituent colors), the spectrum is discontinuous, and the colors appear as thin, vertical lines. Each vertical line corresponds to specific energy transitions for electrons in an atom of a heated element. This produces a characteristic fingerprint for the element and can be used as an identifying feature. In this lab, you will examine the emission line spectrum for hydrogen using a spectroscope containing a diffraction grating.
1) Wear chemical splash goggles throughout the experiment.
2) Gloves are provided if you wish to wear them.
3) Make sure that no materials (book bags, purses, notebooks, etc.) are placed near open flames.
4) Turn off the gas immediately after working with the Bunsen burner.
5) Place all materials in the appropriate waste container and clean your work area when finished.
PROCEDURE
Part I: Flame Tests
1) Add 10 drops of solutions containing Li, Na, K, Ca, Sr, Ba, and Cu+2 to individual wells in a well plate.
2) Using a 50.0 mL beaker, obtain 10.0 mL of 1 M HCl.
3) Obtain a Bunsen burner with a hose and nichrome wire.
4) Bring all equipment (listed in 1-3) to your lab bench. Once you are ready to begin, carefully turn on/light the burner, as guided by your instructor.
5) Dip the nichrome wire into the HCl solution and hold the tip of the wire into the flame until it produces no color. This amounts to cleaning the wire.
6) Dip the tip of the wire into the lithium solution and hold it in the flame until a specific color is produced. You may have to repeat this procedure a few times to saturate the wire with the lithium solution.
7) Once you observe the color lithium produces in the flame, repeat steps 5-6 for the remaining elements.
8) When you are finished with this part of the experiment, be sure to thoroughly clean the nichrome wire and turn off the gas.
Part II. Hydrogen Emission Spectrum
Using a spectroscope, observe the hydrogen emission spectrum (which will be set up by your instructor) and record the thin, vertical lines you see in the data/observations section of this lab.
PRE-LAB QUESTIONS
Name ____________________________________
1) Identify the location, mass, charge, and role of the proton, neutron, and electron.
2) Differentiate between the Rutherford and Bohr models of atomic structure.
3) How can the Bohr model be applied to the colors observed from fireworks?
DATA AND OBSERVATIONS
Part I: Flame tests
|
Element |
Color |
|---|---|
|
Lithium |
|
|
Sodium |
|
|
Potassium |
|
|
Calcium |
|
|
Strontium |
|
|
Barium |
|
|
Copper |
Part II: Hydrogen emission spectrum
Replicate the observed lines in the hydrogen emission spectrum below (draw a vertical line by the indicated colors)
Red Orange Yellow Green Blue Violet
POST-LAB QUESTIONS
1) What was the biggest takeaway from performing this experiment?
2) Many streetlights are orange/yellow. Based on the results from part I, what element may be present in streetlights?
3) Based on the results from part I, which element(s) may be present in colors produced by the following fireworks?
Part A: Violet
Part B: Crimson red
4) How many vertical lines did you observe in the hydrogen emission spectrum? What do these vertical lines indicate?
5) How might the hydrogen emission spectrum differ from that of another element, such as neon?