Electrons and Flame Tests

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Objective

Students will be able to explain how spectral lines are a result of the transition of electrons between energy levels through taking notes, performing a lab, and answering practice questions.

Big Idea

Spectral lines, which we can see as colored lights, are the result of the transition of electrons between energy levels.

Introduction

In this lesson students learn about the behavior of electrons and emission spectra. They do this through taking notes based on a Powerpoint, performing a lab, and doing practice questions.

  • This lesson aligns with NGSS Physical Science Performance Expectation HS-PS1-1: "Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms".
  • This lesson highlights NGSS Science and Engineering Practice 3: Planning and Carrying out Investigations because students perform a flame test lab.
  • This lesson also highlights NGSS Science and Engineering Practice 5: Using Mathematics and Computational Thinking because students use math to quantify energy.

In this lesson students perform a flame test lab.  For the lab each group requires one Bunsen burner, 7 test tubes with solutions, metal tweezers or tongs, wood splints soaked in the solutions, and spectroscopes.  It is helpful to soak the splints in the solutions overnight.  I use Barium chloride, Calcium chloride, Copper II chloride, Lithium chloride, Potassium chloride, and Strontium chloride.  There are many options of salts that you can use for the lab so use what is available in your lab.  The major color that will be seen is the cation so nitrates are also an option.

Explain

40 minutes

To begin the notes I introduce electromagnetic radiation and waves including wavelength, amplitude, frequency.  I also teach students how to solve problems using the speed of light and planck's constant equations. The first portion of the notes are on slides 2-8 of the Powerpoint.  

When performing the practice questions in the notes I make sure to review the "plug and chug" technique of problem solving by underlining what students know, circling what they want, and figuring out how to get there (which equation).  My reflection on Problem Solving in Chemistry for Unit 1 lecture 5 temperature goes over this technique in detail.

I then introduce emission spectra and how electrons emit different colored lights when they fall back to their ground state on slides 9-15.

This is a copy of the filled in notes with worked examples.

Students have the most trouble with determining the proper equation to use when solving the math problems and using scientific notation when plugging into their calculators.   To help with this I go over the problems with them and make sure to reiterate how to plug into their calculators properly.

Elaborate

40 minutes

For this section students perform a Flame Test Lab to reinforce the idea that each element produces different colors when they are excited and fall back to their ground state.

  • When I introduce the lab I pass out the Flame Test Lab paper and have students get out a paper to write on.  I then give students a few minutes to read over the lab and start to answer the two prelab questions. 
  • I then present the flame test Powerpoint to review the concepts, safety, and what the students will be doing in the lab.  This Powerpoint has some details similar to the Powerpoint in the explanation part of the lesson because I do not have my students in my General Chemistry classes take notes about the topic.  As part of my explanation I show students a video of flame tests as well as some old student i-movies which have flame tests in them.

  • A lot of students have a hard time using the spectroscopes so I have them practice using them when the lights are on, pointed to the fluorescent lights in the classroom.
  • For lab safety when I refer to the "google song" This is a poster of the song that I have students sing if I catch them without their safety goggles.  It goes to the same tune as I'm a little teapot.  I write down names of students who I catch and have them sing together at the end of class. Usually, as was the case this year, the threat is enough to make sure that students keep their goggles on:)
  • I then have students perform the lab with their partners. I break students into cooperative groups of four with specific roles. 
  1. I have one student be in charge of holding the splint in the flame and looking for qualitative color
  2. One student is in charge of looking for qualitative color and recording the data
  3. Two students are in charge of using the spectroscopes to record the wavelengths.
  4. I tell them that for this lab they should switch roles after each test.  For more details on forming cooperative lab groups see my reflection in Unit 1 Lesson 8.
  • The lab works much better when the lights are turned off and the doors and blinds are closed.  I make sure to air out the classroom between active lab times with doors open and air on.
  • Because this is the first time students are using the Bunsen Burners I walk around with the Sparker and talk students through how to light the burners group by group.
  • As students complete the lab I have them clean up their stations, wash their hands, and work on the Analysis and Conclusion of their lab.
  • Some student complete the lab in class and turn in but I allow students to finish and turn in the next day if they need extra time.  
  • I grade the lab using the rubric on the second page of the Flame Tests Lab instructions paper.  This is an exemplary example of a graded lab with the rubric.
  • Some of the common mistakes for students include:
  1.  In prelab question number 2, not being specific of how we test qualitative vs. quantitative (color of flames and wavelengths). 
  2.  In post lab discussion/Analysis for question number one not being specific in terms of the data from the lab (different colors and different wavelengths for each element).
  3. In post lab discussion/Analysis for questions number two not being specific with what data shows the identity of the unknown (I did potassium so should have also had a lilac color and wavelengths of 4 and 6)
  4. In the conclusion not being specific about sources of error, how the error effects results and how to change.  Some examples could be...
  • The flame of the bunsen burner was too high.  This would have caused the stick to burn quickly which would cause the color to be orange and wavelength to be 6.  To circumvent this error you could turn down the flame.
  • The spectroscope was difficult to read.  This would effect the results by having incorrect wavelengths.  To prevent this you could get new spectroscopes or better understand how to read the wavelengths on the spectroscopes or have multiple people use the spectroscopes to compare answers. 
  • Not enough chemical was absorbed onto the stick. This would make it hard to see the colors.  Next time you could make sure to soak the sticks longer or dissolve more of the compound into the solution.

Evaluate

20 minutes

To check for understanding concerning the concepts in the lesson I have students perform the practice homework which is on the last page of the notes.  I have students begin the homework practice as they get done with the lab and complete for homework.  I stamp the homework for completion the next day and go over answers using the answer key.  The most common mistakes students make are using the incorrect equations, or plugging the values into the calculator without using parentheses to ensure correct order of operations, thus getting the wrong answer.