Alkali Metals: Valance electrons in action!

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Objective

TSWBAT Summarize the patterns of reactivity at the macroscopic level using the periodic table. e.g. patterns across periods and down groups/families of the main group elements.

Big Idea

Kids love this alkali metals demo day...they are amazed when water catches on fire!

Introduction

This lesson introduces the final trend that will be covered in this unit, metal reactivity.  Students will use the Science and Engineering Practices (SEP), Constructing explanations, to understand reactivity of metals.  During this lessons students will make observations based on physical properties and reactivity of four Alkali and Alkaline Earth Metals.  This information will be used to help students create an understanding of how reactivity of metals increases moving down the periodic table and decrease moving from right to left across a period.

This lesson is aligned with NGSS PS1-2, Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.  The periodic table orders elements horizontally by the number of protons in the atom’s nucleus and places those with similar chemical properties in columns. The repeating patterns of this table reflect patterns of outer electron states.

This lesson continues to engage students in the Crosscutting Concept Patterns as they see the trends in metal reactivity as they appear on the periodic table.  This lesson also provides a brief introduction to chemical reactions.

There is a list of materials that are needed for this lesson that are listed in the Explore/Explain portion of the lesson.

Engage

10 minutes

This class starts with a bell ringer that has students answer the following questions about periodic trends:

  1. Rank the following elements in order of increasing energy to attract an electron.  C, Li, Ne, F
  2. Rank the following elements in order of decreasing atomic size. Al, Mg, Cl, Rb, Sr
  3. Rank the following elements in order of increasing ability to lose an electron. Na, Li, Ca, Mg

Normally I have them answer bell ringers in their journals, but I want to give them immediate feedback by grading it in class so I have them answer on a sheet of notebook paper.  I chose this form of a formative assessment because students struggle with periodic trends and the more immediate the feedback, the better they can assess there own learning.

After a couple minutes, I instruct students to pass their sheet to the person next to them with the grader write their name on the bottom.  Having them sign the bottom is meant to discourage students grading their own.

I then provide the answers to the questions, each being worth 2 points, and have them pass it back to the owner when grading is completed. After they are done looking at their problems I collect them to put in the grade book. 

  1. Ne, F, C, Li
  2. Rb, Sr, Mg, Al, Cl
  3. Mg, Ca, Li, Na

This type of assessment offers students quick feedback and gives me a snapshot of understanding before moving to another topic.

The class overall did well on question one because the answers were located in one row.  Questions 2 and 3 were difficult because the answers required them to assess the trend both across a period and in a family.

I chose more difficult questions for 2 and 3 because it correlates with what they will be assessed on during the activity.  In the activity students will have to decide what Alkali Metal and Alkaline Earth Metals are most reactive both in the family and across a period.


Explore/Explain

30 minutes

While students were working on their problems I handed out the Period trends demo sheet. I let them know that I will be conducting a series of demonstrations to illustrate our last trend which is reactivity of metals.

Student instructions:

  • I ask them what types of metals are Li, Na, Mg and Ca? 
    • Student responses:  Alkali (or group 1) metals and Alkaline Earth (or group 2) metals.
  • I then ask which element do you predict will be the most reactive?
    • Student responses vary because some still have difficulty processing the concept of electrostatic attraction is a driving force in chemistry
  • I then ask them to write a scientific question based on the four metals that will be tested for their reactivity
    • Sample questions: What metal will be most reactive?  Based on electronegativity will the alkali metals be most reactive?
  • After they write a question I let them know they will be making observations based on physical properties, such as shiny, soft, hard, dull. Theses should be put in each box on their worksheet.
  • After making observations I tell them they need to describe the reaction the metals have with water, such as lots of bubbles, gas or nothing.
  • After the demonstration is completed they will have to answer the conclusion and critical thing questions

Demonstration Materials:

  • sodium, lithium, calcium and magnesium ribbon.
  • four 600 ml beakers filled with 300 ml of water
  • 4 pieces of copy paper labeled with the metal
  • forceps and a scalpel
  • Petri Dish
  • phenolphthalein
  • Hover cam (optional)
  • googles

Procedures:

  • Label 4 beakers and glass plates with each metal.
  • Position a document cam above demonstration area (if possible).
  • Keep students far away for safety.  They alkali metals can pop with increased reactivity.
  • A shield, or fume hood can be used for the demonstration, but makes it difficult for students to see.
  • Wear goggles and keep your distance after metals are placed into the beakers of water.
  • Start with Na because it is most reactive and provides a good starting point for a comparison.
    • Cut a small piece with an oiled scalpel and show the luster after cutting.
    • Ask if it was soft or hard.  Most students will say soft.
  • Place a couple of drops of phenolphthalein to show a reaction is occurring.
  • Place the metal into the beaker and stand back.  Let students make observations about reaction.
    • Ask students what they saw. 
    • Why did this happen? Guide conversation towards electron removal.
  • Repeat with Li, Ca and Mg in this order.
    • Students should be able to make comparisons about reactivity in this order.

 

 

 

After completing the demonstration I instruct students to answer conclusion questions 1-4 before we move on.  These questions summarize the reactivity of metals and should be discussed briefly for understanding.  The key objective is to make sure that all students understand that reactivity increase moving down a group and decreases moving across a period.

Elaborate

10 minutes

This is where I have students summarize what they have learned and predict what will happen when potassium is added to water.

  • Introduce potassium and ask what they believe the reactivity of it will be.
    • Student response will vary because they are still processing reactivity.
  • Have potassium demonstration video ready to go.
  • Show the video and have students pay close attention to reactivity of potassium.
  • Explain why it is not safe to perform in the classroom.

After completing this students have a better understanding of reactivity because they now have seen three Alkali metals react.  The potassium video demonstration will help them answer the analysis questions on the activity sheet.  They should be able to see that Alkali metals become more reactive going moving down the family.

Evaluate

5 minutes

The final part of this activity is to answer the Critical Thinking Question 1-5.  I count this as a lab, even though I did it as a demo, so I tell students to make sure that it's thoroughly completed.  It will be due the following day.


As class is ending I tell students that they will be quizzed on all the trends, the Periodic table and bonding.  This quiz comes a couple of days before the test which acts as a practice test for the students.

Sometimes I will give students a shorter version of the test the day before a test.  This allows students to see what they know and what they don’t know.  It is a very effective strategy in motivating students to study for a test.