Periodic Trends. Essential Question How can the properties of an element be predicted using the periodic table?

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1 Periodic Trends Introduction The periodic table is often considered to be the best friend of chemists and chemistry students alike. It includes information about atomic masses and element symbols, but it can also be used to make predictions about atomic size, electronegativity, ionization energies, bonding, solubility, and reactivity. In this lesson, you will look at a few periodic trends that can help you make those predictions. Like most trends, they are not perfect, but useful just the same. Essential Question How can the properties of an element be predicted using the periodic table? Pre-Lesson Exercise Prior to beginning the lesson, answer the following questions: Many things in the real world exist in patterns or trends. What are some examples of patterns or trends in the real world? What does the term periodic mean? How vigorously will each of the following metals react with water: calcium, lithium, magnesium, potassium, and sodium? o Rank the metals in order of MOST reactive to LEAST reactive. o Explain why you ranked the metals in this order. Page 1 of 8

2 Part 1 Procedure In this part of the activity, you will work with compounds of Group 2, the alkaline earth family, to determine the variation and the pattern in the solubility and reactivity within the family or group. 1. Always wear goggles and aprons in the lab. Take care not to get solutions on skin. Wash immediately if you do. 2. Place a well plate on a piece of black paper (or template if provided). Arrange your bottles around the well plate to coincide with the data table. This will help prevent placing the wrong solutions in the wells. 3. Place exactly 10 drops of the Mg 2+ solution into the first three wells of the first row. Be sure to hold the dropper bottles at the same angle each time so that the drop sizes are uniform. Put 10 drops of Ca 2+ solution into the first three wells of the second row. Put 10 drops of Sr 2+ solution into the first three wells of the third row. Put 10 drops of Ba 2+ solution into the first three wells of the fourth row. 4. For steps 5-7, be sure to note the rate of the each reaction in your data table. 5. Add 5 drops of sodium sulfate (Na2SO4) solution to each well in the first column and record your observations. 6. Add 5 drops of sodium oxalate (Na2C2O4) solution to each well in second column and record your observations. 7. Add 5 drops of sodium carbonate (Na2CO3) solution to each well in third column and record your observations. 8. Gently agitate your well plate and observe all of the wells again. Record any additional observations. 9. Read the information below, then add the terms precipitate or soluble to your data table. A precipitate is a solid substance formed when two solutions are mixed together. This precipitate does not dissolve in water and often shows up as a cloudy substance. This means a precipitation reaction occurred. A soluble substance is a substance that will dissolve in water. When two solutions are mixed together and no precipitate appears, that means the resulting substances were soluble. This means no reaction occurred. Color changes do not always indicate a chemical change has occurred. It is important to note the color of the solutions before the reaction as well as after the reaction. 10. Wait for cleanup instructions from your teacher. Wash your hands after cleanup is complete. Page 2 of 8

3 Data Table Na2SO4 Na2C2O4 Na2CO3 Mg 2+ Ca 2+ Sr 2+ Ba 2+ Questions 1. Describe the pattern for the precipitation of the sulfate ions as you move down the data table. 2. Describe the pattern for the precipitation of the oxalate ions as you move down the data table. 3. Describe the pattern for the precipitation of the carbonate ions as you move down the data table. 4. Did all of the reactions occur at the same rate? Include evidence from your lab data to support your answer. 5. From your data and observations, determine which metal ion Mg 2+, Ca 2+, Sr 2+, or Ba 2+ is the most reactive. Include evidence from your lab data to support your answer. 6. From your data and observations, determine which metal ion Mg 2+, Ca 2+, Sr 2+, or Ba 2+ is the least reactive. Include evidence from your lab data to support your answer. 7. Write a general statement of your conclusions concerning the relative reactivity of metal ions as atomic number increases within a group, family, or column on the periodic table. Page 3 of 8

4 Part 2 Source: Periodic Trends. In POGIL: Activities for High School Chemistry, edited by Laura Trout. Batavia, IL: Flinn Scientific, Procedure In this part of the activity, you will work with your group members to complete the Periodic Trends POGIL activity. Through this activity, you will determine the variation and the pattern in atomic radius, ionization energy, and electronegativity for the main group elements on the periodic table. 1. Get the POGIL handout from your teacher. 2. All group members should work together (no one should be ahead and no one should be behind). 3. Allow your teacher to check your work at each stop sign before moving to the next section. Page 4 of 8

5 Part 3 Procedure In this part of the lesson, you will look at graphs of periodic trends studied in Part 2 to determine where the properties do not fit the trend identified and why this occurs. NOTE: Skip ahead to the questions if you are using printed graphs instead of a lab quest device. 1. Pick a partner, get a Lab Quest or Lab Quest 2 from your teacher, along with your assigned property (atomic radius, electronegativity, or first ionization energy), and turn on the device. 2. Turn on the device, then click on the file menu, select open, select the periodic table data file, the click open. 3. Click on the picture of the graph at the top of the screen. 4. Click on the y-axis label and select your assigned property from the list provided (atomic radius, electronegativity, or first ionization energy). 5. On the x-axis, click the arrow on the right of the atomic number label until you see the symbol and atomic number for potassium appear (box in bottom right corner). 6. Place your stylus on the point for potassium on the graph and drag it across the screen until you see the atomic number and symbol for bromine appear. 7. Click on the graph menu then select zoom in. Use this graph to answer the questions below. Questions 1. What was the independent variable in your graph? What was the dependent variable in your graph? 2. What is the overall trend of the property according to your graph (from potassium to bromine)? What type of relationship exists between your dependent and independent variables? 3. There is at least one point near the end of your graph where the property does not fit this overall trend. Record this element s symbol, atomic number, and electron configuration. 4. Describe what happens to the assigned property at this point in the graph in comparison to the overall trend. 5. Propose an explanation for the exception you just identified. Use your knowledge of atomic structure and Coulombic attraction in your hypothesis. Also, refer to the property definitions from the POGIL. 6. Repeat the procedure and answer questions 1-5 for the two remaining trends (you may wish to use a separate sheet of paper). 7. If you used a lab quest device, go to system > shut down. One the device has shut down, return it to your teacher for charging. Page 5 of 8

6 Part 4 Source: C-E-O-SE. In Science Formative Assessment Volume 2-50 More Practical Strategies for Linking Assessment, Instruction, and Learning, by Page Keeley. Thousand Oaks, CA: Corwin, Commit-Explain-Observe-Scientific Explanation (C-E-O-SE) What is the question? How vigorously will each of the following metals react with water: calcium, lithium, magnesium, potassium, and sodium? Rank the metals in order of MOST reactive to LEAST reactive. C- Commit to a prediction or answer choice: Record your predicted metal ranking from the pre-lesson. E- Explain your thinking: Why did you rank the metals in this order? Record your response from the prelesson. O- Part 1: Describe how you will make observations to test your prediction. Or, describe reliable information sources you will use to obtain scientific information. Your teacher will show you a video of each metal reacting with water. You should describe what happens when the metal samples react with water. O-Part 2: Record your observations or the information you obtained. SE- Now that you have completed parts 1-3, re-rank the metals in order of reactivity (most to least) and write a scientific explanation to answer the question. Your explanation should include: 1) a scientific claim, 2) scientific evidence from your observations or obtained information, and 3) scientific reasoning. Your reasoning should describe how the evidence links to the claim using scientific concepts, principles, and vocabulary like protons, electrons, atomic radius, ionization energy, Coulombic Attraction, etc. Go back to #14 in your POGIL activity and add METAL REACTIVITY to the appropriate diagram. Page 6 of 8

7 Part 5 Source: Lab 7. Periodic Trends: Which Properties of Elements Follow a Periodic Trend? in Argument-Driven Inquiry in Chemistry: Lab Investigations for Grades 9-12 by Sampson, Carafano, Enderle, Fannin, Grooms, Southerland, Stallworth, and Williams. Arlington, VA: NSTA Press, Procedure In this part of the activity, you work with your group members to complete the Periodic Trends Argument Driven Inquiry (ADI). Through this activity, you will determine if other properties of elements are periodic trends or quasi-periodic trends. 1. Get the ADI handout and data set from your teacher. 2. Complete the eight stages of the ADI process: a. Identify the task and guiding question. b. Design a method and collect data. c. Analyze the data and develop a tentative argument. d. Participate in the argumentation session. e. Participate in the explicit and reflective discussion. f. Write an investigation report. g. Complete a double-blind peer review. h. Revise and submit your report. 3. Once you have completed the ADI process, answer the following questions: a. Describe the difference between a periodic and a quasi-periodic trend. b. Fluorine (F), boron (B), and lithium (Li) are all in the same period, but are in different groups. Fluorine has a higher electronegativity than boron, which has a higher electronegativity than lithium (F = 4.0, B = 2.0, Li = 1.0 on the Pauling scale). However, boron has a higher melting point than both fluorine and lithium, and lithium s melting point is higher than that of fluorine (F = 220 o C, B = 2300 o C, Li = 180 o C). Using your knowledge of periodic trends and the information above, describe the nature of the periodic trends for electronegativity and melting point. c. The element has a high ionization energy is an example of an observation. I agree with this statement I disagree with this statement Explain your answer, using an example from your investigation about periodic trends. d. Cultural values and expectations affect how scientists conduct an investigation. I agree with this statement I disagree with this statement Explain your answer, using an example from your investigation about periodic trends. Page 7 of 8

8 e. Scientists often look for and attempt to explain patterns in nature. Explain why patterns are important, using an example from your investigation about periodic trends. f. Scientists often need to look for proportional relationships. Explain why looking for a proportional relationship is often useful in science, using an example from your investigation about periodic trends. Reflection Revisit the pre-lesson questions for this activity and your answers. Based on what you learned in this activity: Why do we call atomic radius, ionization energy, and electronegativity periodic trends? Describe the relationship between the first ionization energy of a metal and its reactivity. Explain why this relationship exists based on what you learned in this lesson. Page 8 of 8