Lewis Structures & Ions

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Objective

Students will be able to identify valence electrons, determine ions formed, and explain the relative sizes of ions compared to parent elements through taking notes, performing an activity, and practice.

Big Idea

Atoms will gain or lose electrons to become stable ions.

Introduction

In this lesson students learn about Lewis Structures and ions. They do this through taking notes based on a Powerpoint, watching a video, performing an activity, 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 2: Developing and using models because students use Lewis Dot diagrams to understand valence electrons and use models to help students understand what happens with electrons to form anions and cations.

In this lesson students perform an activity where they are able to visualize what happens when electrons are added or taken away from atoms.  To do this activity each student should have a bag of 10 positive marked papers and 10 negative marked papers.

Engage

5 minutes

To engage students in this lesson I have them answer two questions:

  1. How do you determine the number of valence electrons for an atom?
  2. How many valence electrons do each of the following atoms contain? Potassium, Sulfur, Silicon, and Calcium

The goal of these questions are for students to recall what they learned about Valence electrons in Unit 2 lesson 6.  For the most part students remember how to calculate valence electrons but some students need the reminder so this is helpful.

I present the two questions on the first slide of the Powerpoint and students answer the questions at the top of their Notes Graphic Organizer.

If students are stuck I have them refer back to their electron configuration part I notes that are found in Unit 2 Lesson 6.

This is a copy of a student's notes with the first section answers.

Explain

15 minutes

In this portion of the notes I present the first part of the Powerpoint (slides 2-7) while students fill in the first page of their notes graphic organizer.  This copy of a student's notes has the explanation section filled in for page 1.

I first explain to students how to perform Lewis Dot Structures for atoms using what they already know about valence electrons on Slide 2 and have students practice what they learned on Slide 3. 

I then introduce students to the idea of ions in terms of the octet rule and becoming isoelectronic with the noble gases on slides 4-7.  

A lot of students get confused with the idea that when an atom GAINS an electron it become negative and when it LOSES and electron it becomes positive.  Therefore I make sure to stress that electrons are negative.  Also on slides 6 and 7 I go through in detail how each ion is formed.

  • I show how Fluorine gains an electron so the electron is on the left side of the arrow and becomes negative because it now has more electrons than protons. 
  • I explain how Sodium loses an electron so the electron is on the right side of the arrow and sodium becomes positive because it has more protons than electrons.

Explore

20 minutes

To help students understand how ions are formed in terms of gaining or losing electrons I have them perform an activity.

I lead them through the activity on slides 8-10 of the PowerPoint and the they perform the activity on the second page of their Notes Graphic Organizer.

To do the activity each students receives a bag of 10 + sign squares and 10 - sign squares.

The goal of the activity is for students to visualize what happens to the atoms when electrons are lost or gained.  I lead them through two examples using the charged squares and then have students try 5 examples on their own without the manipulative.

This is a copy of a student's notes.  Page two has answers to this activity.

Explain

20 minutes

After students have performed their activity I continue with the notes on the third page and lead students in learning more about ions on slides 11-19 of the PowerPoint.

As I present information about cations and anions I have students fill in information on a table.  This helps students visualize the difference between the two.  After students fill in the second row that cations lose electrons and anions gain electrons I show them slide 13 and the charges that groups tend to make to their periodic tables (see attached picture).  I stress to them that this is not exact for all elements, but overall it tends to work for charges.

After students write down how the size of the atom changes when an atom becomes a cation or anion then I show this video to reinforce the idea and give a visual to students.

This is a copy of the filled in notes page 3 has the information for this section.

Evaluate

30 minutes

For the last portion of the lesson students work on the last page of their notes with the practice questions regarding ions and ion formation. 

From previous years I know that this is a section where students often get confused on what I am asking for the homework so I go over the first example for each section with students as a group (see attached picture).

If students do not have time to complete the practice then they are expected to finish it at home for homework. 

The next class I stamp the homework for completion and go over the answers with students.  This is a copy of the answer key.

For the most part students do well on the homework because of my reviewing an example of each prior to their trying on their own.   The most common mistakes are students' writing of the equations showing the formation of the ions with where to put the electrons and how many to use.