Western Carolina University

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1 CHEM 132 Lab 03 Chemistry 132 Lab 03 Flame Test and Electron Configuration Prelaboratory Exercise Go to Chem21Labs.com and complete the on-line prelab by answering the questions below. The prelab will be due the day before your lab section. Reading the lab procedure will help you find the answers. 1. What color of light does a low-pressure sodium street lamp give off? 2. What color of light does a true neon light give off? 3. What element on the periodic table has the electron configuration 1s 2 2s 2 2p 6 3s 2 3p 5? Introduction Flame Tests Atoms for each element on the periodic table have their own characteristic spectrum, which are colored lines separated by dark areas. An atomic spectrum is produced when light emitted from heated (high-energy) atoms is passed through a prism. When electrons in these hot atoms absorb energy, they can attain higher energy levels. In order to return to the lower, more stable energy levels, electrons release energy. If the energy released is the same frequency as visible light, the element produces a characteristic color, which can be used in identifying that element. An example of such a phenomenon is the difference in the light given off from a neon light and a low-pressure sodium street lamp. Think back to the street lights lining the roads of a city. These lamps tend to give off a warm yellowish glow. Now think about the lights declaring a local restaurant as OPEN, usually showing up as bright red or blue. The red light given off is a true neon light, since the atoms inside the glass tube are atoms of neon gas. Blue lights, while often called neon signs, don t actually have neon gas inside the tubes that light up. (A different element is used to produce blue light.) When an electric current is passed through the gas in one of these lights, the atoms in the tube absorb energy, and the electrons are raised to a higher energy level. When the electrons fall back to their original level they give off the extra energy, and light of a characteristic color is produced. Electron Configuration Electron configuration shows how electrons in an atom are arranged by shells and subshells, starting with the lowest energy level and counting up. The number of electrons in each subshell is written as a superscript. Think of the electron configuration of an element (or exact position on the periodic table an element holds) as if it were directions from your house to another city. When leaving home, you always start from your driveway and take the route that gets you to your proper destination. 1

2 CHEM 132 Lab 03 Writing an electron configuration of an element can be done by following these simple steps: 1. Locate the element on the periodic table. 2. Write the filled sublevels in order going across each period (left to right) starting at the lowest energy level and smallest subshell. Energy levels: 1 < 2 < 3 < 4 < 5 < 6 < 7 Subshells: s < p < d < f 3. Count the number of electrons in the sublevel for the element and complete the configuration. The following is an example of figuring out the electron configuration for chlorine (Cl), using the periodic table. Remember that the main shell is given by the period (row) number of the periodic table. Period Sublevel Sublevel Blocks Filled 1 1s sublevel (H He) 1s 2 2 2s sublevel (Li Be) 2s 2 2p sublevel (B Ne) 2p 6 3 3s sublevel (Na Mg) 3s 2 3p sublevel (Al Cl) 3p 5 Final electron configuration for chlorine: Cl : 1s 2 2s 2 2p 6 3s 2 3p 5 Abbreviated electron configuration for chlorine: Cl: [Ne] 3s 2 3p 5 The total number of electrons found in an atom of chlorine is 17, while the number of valence electrons for chlorine is 7. Valence electrons are the electrons in the s and p sublevels with the highest energy level (or all electrons after the previous noble gas in the main groups). Figure 1 - Electron Configuration and the Periodic Table In today s lab, flame tests will be used to see how the valence electrons of an atom are excited by the energy of the flame, and then release that energy by giving off a characteristic color of light. 2

3 Procedure Flame Tests CHEM 132 Lab 03 Materials: Bunsen burner, spot plate, flame-test (nichrome) wire, cork, 100mL beaker, 1 M HCl, 0.1 M solutions (dropper bottles) of: CaCl 2, KCl, BaCl 2, SrCl 2, CuCl 2, NaCl, and unknown solutions. CAUTION! 1 M HCl is corrosive! Use gloves and be careful when you handling it. Wash off any HCl spills on the skin with tap water immediately after they occur for 10 minutes. Then wash area with soap and water. Obtain a spot plate, flame-test wire, and cork stopper from your equipment bin. One end of the flame-test wire should be bent into a small loop, so the other end can be secured into a cork stopper. Pour a small amount (about ml) of 1 M HCl into the 100 ml beaker. Wash and then rinse the spot plate in distilled water. Place 6-8 drops of each test solution into separate wells of the spot plate. Label the spot plate diagram on your data sheet to match the solutions. Make sure the different solutions don t become cross contaminated! Spot plate diagram use data sheet Adjust the flame of the Bunsen burner until it has a blue cone flame. Clean the test wire by dipping the loop into the HCl beaker and then placing it into the flame of the Bunsen burner for a few seconds. After this, rinse with de-ionized water and then dry in the flame. If you see a strong color in the flame while heating the wire, repeat the steps above until only a slight coloration appears when the wire is in the flame. Observing Flame Colors for Known Solutions Allow the cleaned wire to cool and then dip it into one of the solutions on the spot plate. Make sure that a thin film of the solution adheres to the loop. Put the wire loop briefly into the top of the blue cone flame and record the color you observe in Table 1. For each solution, it is the first element (the metallic element) in the formula that is responsible for the color. Notes: Make sure you properly clean the wire loop between each color test! Record the color for each flame test in the blanks below. Don t heat the wire for more than 5-6 seconds. Overheating may cause the wire to decompose. The color of potassium in the KCl flame is short-lived. Be sure to observe the color of the flame from the KCl solution within the first few seconds of heating. Repeat each flame test until you can describe the color of the flame produced. 3

4 CHEM 132 Lab 03 Table I Flame Colors for Known Solutions Solution Element Color of Flame CaCl 2 Ca use data sheet KCl BaCl 2 SrCl 2 CuCl 2 NaCl K Ba Sr Cu Na Identifying Unknowns by Flame Color Follow the same procedure above for your three solutions containing unknown metallic elements, and record the data in Table 2 of your data sheet. Be sure to record the ID number for your unknown solutions. Table 2 Flame Colors for Known Solutions Unknown ID Color of Flame Element 4 Figure 2 Use this periodic table to write the electron configurations for the problems on the next page. 4

5 CHEM 132 Lab 03 Electron Configuration of Neutral Atoms answer on-line Atom Electron Configuration Abbreviated Electron Configuration # of Valence Electrons O Na Ca F C Li K Sr Br Cu S Electron Configuration for Ions Ion Electron Configuration O 2- Na + F - Li + Sr 2+ Abbreviated Electron Configuration # of Valence Electrons Questions: answer on-line at Chem21Labs 1. Why is the electron configuration for the oxygen atom different than the electron configuration for the oxygen ion? 2. How many valence electrons are found in an atom of chlorine? 3. How many valence electrons are found in a chloride ion (Cl 1 )? 5