Day 2: Ionic Compound Formulas

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SWBAT determine molecular formulas for ionic compounds using a periodic table and a list of polyatomic ions.

Big Idea

Ionic compounds have an overall neutral charge which determines their molecular formulas.

Why This Lesson?

This lesson helps students understand that the molecular formula for an ionic compound is electrically neutral and can be determined by finding the ratio of cations to anions that results in an overall zero charge.  Students need to understand this concept as a precursor to studying ionic chemical reactions and acid-base chemistry.

This lesson addresses the NGSS Performance Expectations of HS-PS1-1 and HS-PS1-2 because students are using position on the periodic table to determine ionic charges (PS1-1) and then will be using their understanding of ionic compounds to predict results of chemical reactions (PS1-2).  Students have already investigated dissolution of ionic compounds and know that the ions in solution conduct electricity.  Later, we will investigate the strength of acids and bases based on level of conductivity, because strong acids/bases dissociate fully into ions.  A basic understanding of ionic compounds and their dissociation will help students understand acids and bases later.

Students will be using simple paper models to demonstrate charge ratio (SP 2) and then explain their reasoning for their determined molecular formula of the assigned ionic compounds (SP 6), addressing two of the Science and Engineering Practices.

This lesson was originally going to be a one-day lesson, however, during the student activity portion of Day 1: Ionic Compound Formulas, it became apparent to me that another day was needed to help solidify student understanding and to address misconceptions that were occurring for a large majority of my students.


5 minutes

While I take attendance, students do a warm-up activity in their composition Warm-Up/Reflection books.  I use warm-ups to either probe for students' prior knowledge about the day's upcoming lesson or to have them bring to mind and review what they should have learned previously.  (To read more about Warm Up and Reflection Books, please see the attached resource.) 

Today's Warm-Up: "How do we know that the chemical formula for an ionic compound made from ionic magnesium and ionic chlorine is MgCl2?"

In this case, the warm-up is asking students to recall the previous day's lesson in which they worked to determine ionic compound chemical formulas.  More importantly, it is asking students to be able to explain why the given formula is correct using what they learned the day before about balancing charges.

If time permits, I walk around with a self-inking stamp to stamp the completed warm-ups indicating participation, but not necessarily accuracy.  On days when there is too much business keeping, I do not stamp.  Students have been told that warm-ups are occasionally immediately checked and other times not.  At the end of each unit, Warm-Up/Reflection Books are collected and spot-checked.  

Intro to Activity

10 minutes

In the previous day's lesson, we focused on determining ionic compound chemical formulas.  At this point, students should have an understanding that in order to balance a chemical formula, the overall charge has to be neutral, or zero.  My honors level students understood the assignment well and needed a higher level challenge, but my general level students were struggling with the assignment.  In order to accommodate the different levels of ability in my average of 45 students classroom, I added a layer of difficulty for the honors level but kept the level of difficulty the same for the general students, allowing them more time to master this particular content.

I pass out the handouts entitled Ionic Bonds - 2 so that each student has one.  At the whiteboard, I remind students how to determine what ions are formed by each element as predicted by the periodic table.  I compare the elements' total number of electrons to the nearest noble gas' number of electrons to reinforce the idea that elements only gain or lose electrons to be more stable, which the noble gases already are and the other elements want to be.  I then quickly repeat the same example I had used the previous day, but pause less and allow for more direction from the students in telling me what comes next.

I model how the first line in the table would be figured out and explain that the cations are listed first (and remind students that cations are positive) while the anions are listed second (also reminding that anions are negative).  I explain that the "# cations" and "# anions" titles are not hashtags (!) and that they are also not the charges, but rather the number of ions necessary to create an electrically neutral compound.

Using MgCl2 as the example, I explain in this way: 

Then, I move to the side of the class that has my honors level students and explain further the idea of polyatomic ions and how we would balance a chemical formula for ionic compounds made with them.  Here is a short explanation of the key points I make sure my students get:





Partner Activity

30 minutes

Students work in pairs that are pre-determined by their Partners Sheet (see the Reflection entitled "Compass Pairs" in my Common Ionization States lesson).  Today, I tell students to work with a different direction partner (if yesterday was South, I might tell students to work with their West partner).  I do this because I want students to have the opportunity to engage in discussion about the same topic with someone else.  This helps the pairs that had two struggling people before potentially get help from their new partner.  It also might pair up two struggling students who perhaps did not realize how much they were relying upon their previous partners the day before, which will challenge them to work towards understanding the material independently.  

I tell students to re-use their cations and anions from the previous day, but if students need more, I do have extra ANIONS sheets and CATIONS sheets on hand.  Students should already know how to use them from the previous day's lesson in which I explained that the pieces with  “+” correspond to an ion with +1 charge, the “+ +” pieces correspond to ions with a +2 charge, and the “+ + +” pieces correspond to ions with a +3 charge.  Likewise for the anions (one “-“ equals -1 charge, etc.).

I have designed the pieces so that their size corresponds to how many positive or negative charges it carries, so that when students are trying to balance charges, they can do it by matching the sizes of the charge papers.  Once they have balanced the charges in a compound, they can count up cations and anions to determine what the subscripts are in the chemical formula by following the scaffolded steps in the table on the handout.

Two different compounds are pictured here.  The one on the left is composed of cations charged +1 and anions charged -2.  By matching charges until all charges are balanced between positive and negative, the students can see that there are 2 cations needed and 1 anion, so the chemical formula would be Cation(subscript=2) Anion(subscript=1).  Likewise, the compound on the right is composed of cations with charge +3 and anions with charge -2.  Here we see that 2 cations and 3 anions are needed for all 6 positive and negative charges to be balanced which means the chemical formula would be Cation(subscript=2) Anion(subscript=3).

A key for the handout can be found here: Ionic Bonds - 2 KEY

Samples of student work follows below.  Today, on our second go-round with this topic, students were definitely more successful.

Student Reflection

5 minutes

In student's Warm-Up/Reflection Books, students should spend about 3-5 minutes writing a response to the day's reflection prompt.  Prompts are designed to either help students focus on key learning goals from the day's lesson or to prompt deeper thinking.  The responses also allow me to see if there are any students who are missing the mark in terms of understanding.  The collection of responses in the composition books can also show a progression (or lack thereof) for individual students.  

Today's Reflection Prompt:  "How many ions would form if one molecule of magnesium chloride dissolved in water?  Explain how you know this."

Desired student responses should include:

  • Magnesium chloride has a chemical formula of MgCl2
  • There is one cation for every 2 anions in magnesium chloride
  • If one molecule dissolved, one cation (Mg2+) and two anions (Cl-) would form
  • 3 ions form if one molecule of magnesium chloride dissolved in water