Lesson 4 of 8
Objective: Students will be able to model how ionic compounds form and to name them.
This lesson aligns to the NGSS Disciplinary Core Idea of PS1.A: Structure and Properties of Matter. This idea is about how attraction and repulsion between electric charges at the atomic scale explain the structure, properties, and transformations of matter, as well as the contact forces between material objects. In this lesson students focus on the attraction of oppositely charged ions that is responsible for the creation of ionic compounds.
It aligns to the NGSS Practice of the Scientist of Modeling. Due to the incredibly small size of the atoms involved in ionic compound formation, modeling is an appropriate activity for examining ionic compounds because it allows students to add and remove electrons to create monatomic ions.
As such it also aligns to the NGSS Crosscutting Concept of Scale Proportion and Quantity. One crosscutting concept is that “some systems can only be studied indirectly as they are too small, too large, too fast, or too slow to observe directly.” Certainly when looking at electron behavior in the high school classroom this is the case.
In terms of prior knowledge or skills, students should have a clear understanding of how atoms gain or lose electrons to become ions. My students learned this material using Introduction to Ions lesson and this Ion Formation Review lesson.
There are no materials needed for this lesson besides the materials that are included in this lesson.
Do Now: Class begins with students reading about charge balance in ionic compounds. The reading is in their textbook and is akin to the introduction and Example 1 material found at this website. I then ask students to explain why they think there are 2 chloride ions for every one calcium ion.
I have chosen this approach because I want students to begin thinking about balancing the charge in ionic compounds as that is the main focus of today's class. Some students will intuit how balancing charge works from their reading, and my mini-lesson on the subject will simply reinforce what they already know. Other students will partially understand the concept and they may get stronger from the lesson. A few students will need me to link back to previous material in order to make sense of the new material.
Activator: I ask a student if the reading makes sense to them. A number of students raise their hand, and I ask one to explain to the class why there are 2 chloride ions for every one calcium ion.
My thinking behind this strategy is that I want to signal to students that some students have taught themselves the material simply by studying the material in the book. Facility with the text book and with self-teaching are both college-bound skills I want students to have. This video in which a student explains the bonding of calcium chloride shows that this is possible.
Mini-lesson: While the student has done a fine job of explaining what she read in the textbook, I do spend a little time reteaching this. Some students like to hear it from the teacher. I begin by passing around some calcium chloride so students can see that what we are talking about is a real substance. I explain that chlorine is a poisonous gas, and calcium is a silver metal, but when the two are combined we get a white solid.
I then link back to previous material about ions, reminding students that Group 2 metals lose 2 electrons and that halogens gain 1 electron. This happens in both cases to satisfy the octet rule. Once the atom is more stable, however, they both have a charge, and for this reason they are still reactive.
I note that positive ions are attracted to negative ions, and that in order to form a compound the amount of negative charge has to be balanced by the positive charge. I say that is why the student included 2 chloride ions for one magnesium ion.
I present the content in this way because I want to connect prior knowledge to today's lesson. Students should have a fairly decent understanding at this point about how and why atoms gain and lose electrons, but linking that concept to the ionic compounds is critical to student understanding of ionic compounds--you cannot have ionic compounds without ions!
Guided Practice: I then ask students to model the first practice problem--magnesium chloride--in the Ionic Bonding and Naming Practice practice problems. The student already discussed this at the beginning of class, so now I want to see how clearly students understand the idea.
A consistent mistake that students make is not indicating the 2 chloride ions in the Lewis dot structure. I explain that this is how a student can model the balancing of charge. I also explain that the naming rule is as follows: the positive ion gets its element name, but the negative ion, which comes second in the name, replaces its ending with "-ide."
Student Activity: During this section of class students practice drawing Lewis dot structures for the various compounds in the practice set. I supervise students to check for understanding. I remind students of the importance of balancing the charge; this is the one thing that students are not finding intuitive.
I have chosen this approach because students learn best by practicing the skill they are expected to learn. When students have a chance to practice, they reveal misunderstandings they have about the subject. I walk around the room and discuss errors I see or reassure students when they are providing evidence of understanding. An example of one such conversation is found in this video of a student explaining how his ionic bonding of calcium iodide.
Catch and Release Opportunities:
Once most students have modeled most of the Lewis dot structures, I explain to them that ultimately I would like them to be able to write the chemical formula for the different ionic compounds by looking at the ions that are involved and write the chemical formula for those compounds. I note that on page 3 of the packet, students are now being asked to determine chemical formulas. I note that the packet shows an easy way to balance the charge, namely by crossing the charges. I also note that in the event that the numeral is the same on the positive and negative ion, then the charge is already balanced.
To wrap this lesson up I remind students that atoms can gain or lose electrons to satisfy the octet rule. They then become ions, and can get attracted to the opposite charge. Today's lesson has been about balancing the charge for ionic compounds and modeling compound formation using Lewis dot structures.
I then ask a student to show their work using a document projector and explain to the class how he or she completed the work. This student work is typical of what I see in today's class. Students need a lot of work still on balancing charge in the Lewis dot structures problems. However, as class goes on and students get more practice and one-on-one instruction from myself and peers, students gain more facility in writing the formulas. This is encouraging. For homework I ask students to go back and look at the Lewis dot structures and find places where they have failed to balance the charge.
This instructional choice reflects my desire to solidify the point of this class, like I do every day. I also want to give students a yardstick by which they can measure their mastery of today's learning objective. By comparing their work to a classmate's work, they can see how they are doing.