Reactivity and the Periodic Table

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Objective

Students will be able to relate the position of an element on the periodic table to its relative reactivity.

Big Idea

Variation in the reactivity of different elements is due to variations in valence shell electron configurations. Group 1 and 2 elements are more reactive than transition metals.

Introduction

In the last lesson students focused on how atoms lose or gain valence electrons to complete their outer shell.  In this lesson students compare an element's position on the periodic table to its reactivity. They do this by watching a video showing what happens when the different Group 1 metals are placed in water, and they then conduct a mini-lab to see what happens when copper and magnesium are each placed in 0.5 M HCl.

This lesson aligns to the NGSS Disciplinary Core Idea of 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. Each of the learning activities is designed to reinforce the idea that elements are organized on the periodic table based on their electron configurations, and these configurations help to explain the observable behavior of the element.

It aligns to the NGSS Practice of the Scientist of Constructing explanations by giving the students the opportunity to explain what they observed on screen and in their test tubes to their understanding of the periodic table and electron configuration.

It aligns to the NGSS Crosscutting Concept of Cause and effect: Mechanism and explanation by linking what is known at the nanoscale about electron configuration to what we observe at the macroscale.

In terms of prior knowledge or skills, students should have and understanding of electron configuration in order to derive the meaning that this lesson is designed to address.

 

The materials needed for this lesson include the following:

  • a computer for viewing videos
  • two test tubes
  • 0.5 M HCl (20 ml per group)
  • 1 g of Cu, and 1 g of Mg
  • a test tube stand
  • a waste disposal container
  • thermometers (either standard or electronic)

 

 

Do Now/Activator

10 minutes

Do Now: I was at a conference yesterday and in my absence students worked on periodic table reading questions. Some of these questions asked students to color on blank periodic tables, which caused some confusion because students were unclear about which information was supposed to go on which periodic table. Students were able to use their text book to answer these questions. I read the book ahead of time to develop them. A similar text can be found at this ChemWiki website.

I begin class today, as I do every day, with a Do Now posted on the board. It reads "Please take out your reading questions from yesterday. Discuss answers with your classmates, and identify which questions you feel we should go over." I have chosen this approach because I want to get students talking about the material that our mini-lab is grounded in, and I want to help them to identify things that were confusing. 

Activator: Once I have taken attendance and given a few minutes of conversation about the Do Now, I ask students to identify areas for confusion. One such area is having two periodic tables in the assignment. As this student's answers to the periodic table reading questions show, students often either filled in one table or the other. I explain that one periodic table is a more general classification of metals, nonmetals, and metalloids, while the other periodic table identifies groups within those groups that have shared characteristics for their electron configurations. The other level of confusion students had was with question 14, in which they had to compare the valence shell with the reactivity. I explain that elements that are only 1 or 2 electrons away from having a full outer shell are typically more reactive than ones that are not, and this is a nice segue into the lab.

I have chosen this approach because it follows my scaffolding for this unit. We have studied the differences among atoms for different  elements, and now we are nicely set up to discuss how the periodic table is organized according to these differences. We are also nicely set up to see why these differences matter at the macroscopic level.

Mini-lesson

15 minutes

Mini-lesson: I start the lesson by linking electron configurations to an elements place on the periodic table. I remind students that Group 1A metals have 1 valence electron, and that Group 2A metals have 2. I explain that the implications for this structure are that the atoms for these elements are eager to get rid of their outer shell electrons. I also note that they do this by reacting with other elements and compounds around them. 

I then review the Reactivity Mini-lab Directions. The lab consists of two parts--an online part consisting of videos and a lab bench component. I explain that we will all do the online portion together, and the partners will conduct the test tube portion of the lab.

We also review the Procedure for Reactivity Lab that they read in their book. I modified it to use 0.5M HCl because it produces a more vigorous reaction in the amount of time we have.

I have chosen this approach because I want students to see that the organization of the periodic table, and its relationship to electron configuration, is not just an academic exercise. Elements act a very specific way and the periodic table captures these differences in its organization.

I like how I sequence this lab too. Having students see the less vigorous reactions of Li and Na in water sets them up for what they can expect to see when an element is reacting in their test tubes.

 

Application

25 minutes

Student Activity: I found it easier to have everyone watch the alkali metals in water video at the same time because it meant only having to set up one computer. We watched the video one time for entertainment purposes--it was fun for students to see the violent reactions of Group 1 elements. The photo accompanying that lesson is from that video. It shows cesium blowing up the glass conainer of water it was placed in!

I explain to them that we do not do these in the classroom because they are dangerous. After students watch the video one time, we re-watch the video this time to record data. Some students have already done this, but some were so engaged by the video that they forgot to record data.

Students then put on safety goggles and follow the procedure for testing the reactivity of copper and magnesium:

  1. Fill two test tubes each with 10 ml of 0.5M HCl.
  2. Record the temperature using a thermometer.
  3. Place 1 gram of copper in one of the test tubes. Record any changes you observe or any temperature changes that occur.
  4. Record your data in the table for that element.
  5. Repeat steps 1-4 for magnesium.

Catch and Release Opportunities:

During this lab I walk around and observe what students are doing. I ensure that everyone is safe. I am also intent on helping students link what they are seeing at the macroscopic level in their test tubes to conditions at the nanoscale. This reactivity lab video is a good example of how I can use questioning to help students make this connection. The video also shows the lab setup. Notice how the student was very clear about why magnesium was reactive while the copper was not, but he was less clear about why. Judging from the student answers, the questioning helped to tighten this connection.

Debrief

10 minutes

To wrap this lesson up I begin by asking students to describe what they observed in class today by using the reactivity lab data table. I begin with the empty table, and I ask students to share their data, which I then enter. At the end of sharing the data table looks like this completed reactivity lab data table. I then ask students to name the electron configurations for the different elements we looked at today. I cold call students. What I hear from them is that they have not fully learned the connection between the electron configuration and the reactivity of the element. Subsequent analysis of student work such as this student lab report further reinforces this impression. In the future I could tighten this connection by having students write the valence electrons for each element tested right on the data table.

These observations inform my next move. I plan to re-teach the atom from this perspective. Students will understand ionic compounds and ion formation much better if they understand the structure of valence shell. This lesson is what I teach next.