Ionic vs. Molecular Compounds

7 teachers like this lesson
Print Lesson

Objective

Students will be able to discern whether a compound is molecular or ionic based on whether it contains a metal and then name or draw the Lewis dot structure for the compound.

Big Idea

Ionic compounds are made of a metal and a nonmetal, while molecular compounds combine nonmetals. Each of these two types of compounds has its own naming system and Lewis dot structure.

Introduction

In this lesson students will synthesize the material they have learned in this unit. In past lessons they have learned how to form ions as well as name and model ionic compounds and molecular compounds. In this lesson students are taught how to differentiate between the compound types and in a list of compounds and they must then either name or model the compounds based on their ionic or molecular designation. 

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. Students are only working with monatomic ions and can use the periodic table to discern bonding type.

It aligns to the NGSS Practice of the Scientist of Developing and using models because with these small parts students must model the compounds to understand what makes them ionic or molecular.

It aligns to the NGSS Crosscutting Concept of Patterns because the behaviors of these compounds can be explained by the very specific patterns that are revealed about them when examining them at the nanoscale.  

In terms of prior knowledge or skills, students should have a fairly strong understanding of how to name and draw Lewis dot structures for ionic and molecular compounds.

For materials students will need access to a periodic table for this lesson.

 

 

Do Now/Activator

10 minutes

Do Now: I ask students to name and draw the Lewis dot structure for H2O and CaO. 

I reason that this is a good way to start class because it gives students an assignment that they can lead themselves while I take attendance, and it gets to the heart of the work for today's lesson.

 

Activator: I ask a student to show their work. In different classes, different mistakes are made. Some students use prefixes for naming the ionic compound, some draw ionic compounds by sharing electrons, and some even bond two hydrogen atoms together. 

Choosing this approach fits perfectly into what I hope to highlight for students today--that you need to know whether you have an ionic or molecular compound before you can name it or understand its structure.

Mini-lesson and Guided Practice

15 minutes

Mini-lesson: I teach students and they record in their notebooks the difference between ionic and molecular compounds. I explain that ionic compounds typically form between a metallic atom and a nonmetallic atom, while molecular compounds typically form between 2 nonmetallic atoms. I review with students how the naming and Lewis dot structures are different for each type of compound using the compounds review notes.

This instructional choice reflects the fact that we are near the end of the unit. I want to give students a chance to practice naming and modeling  each compound type, and I want to drive home the point that each type has their own rules.

Guided Practice: Students are asked to do the first 3 problems on the naming molecular and ionic compounds practice problems and the modeling molecular and ionic compounds practice problems.

I chose this particular focus so that students would have the opportunity to name and model a molecular and an ionic compound. After students do this, we discuss the answers. Many students used the wrong system for at least one of the six problems. At this point, I reemphasize the need to ask whether the compound is molecular or ionic before attempting to name or model the compound.

Application

25 minutes

Student Activity: During this time students work on naming and modeling ionic compounds. I work the room, looking at student work, offering critique, and answering questions. 

This is a great time for differentiation. For students who are behind in their work, I can re-teach concepts like molecular and ionic bonding. Some students are not behind, but forgot ionic compounds after focusing on molecular. Some students are not following the notes, and this is a good time to teach about the importance of listening to a procedure and following it.

Catch and Release Opportunities:

There comes a point when I have interacted with enough students where I can recognize patterns for things I am saying. While I do not always anticipate catch and release moments, I know them when I see them.  

After talking with a number of students, the one thing that I notice is that many students begin to forget that the first thing they need to do is determine if the compound is ionic or molecular. They get so eager to write something down that they forget to think first. 

Stopping class to discuss this is important because otherwise students will not experience success with today's objective.

On another occasion I stop class because I notice that a lot of students are not balancing their charges when writing chemical formulas and Lewis dot structures for ionic compounds. For example, in this student's naming answers notice that I circled a number of incorrect answers. None of them have a balanced charge. I review with students how to balance the charge by reminding them that a charge is balanced when the total amount of positive and negative charge, when added together, equals 0.

Students already learned how to balance the charge. This revised student work shows that with a reminder, the student is able to do this important task. In this video students explain how they revised their modeling work to show a balanced charge.

Debrief

10 minutes

To wrap this lesson up I reemphasize the challenges students faced with today's work:

  • They need to balance the charge for ionic compounds
  • They need to know whether a compound is ionic or molecular before they try to name it or model it because ionic and molecular compounds have different naming and structuring rules.

I then cold call students to emphasize the points made on the review notes:

How to you model an ionic compound? (Positive ions have no dots around their symbol, negative ions have 8 dots. Both are in brackets with the charge written on the outside. If the charge is 1+ or  1-, the 1 is omitted. Balance the charge by adding a coefficient in front of elements that have more than one atom.) 

How do you model a molecular compound? (All elements have dots around them. Atoms share 1, 2, or 3 pairs of electrons so that they can satisfy the octet (or duet) rule.) 

What is the biggest difference between naming ionic and molecular compounds? (Molecular compounds use prefixes, ionic compounds do not)

What is a similarity in naming ionic and molecular compounds? (the second element ends in -ide)

Ending class this way allows me to get at the key learning objectives of the class. Then, depending on how much students completed, I show some of the answers from the Naming molecular and ionic compounds key and the Modeling compounds answer key. so that students can get a sense of how they stand with these skills.