Star Crossed Charges pt 1

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Objective

SWBAT interpret models in terms of how the number of charges and distance between opposite charges affects attractive forces

Big Idea

Paper models help students understand Coulombic attraction between subatomic particles.

Introduction

Today's lesson is grounded in 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.  Patterns of valence electrons are based upon two things: the number of electrons, and their attraction to the nucleus.

To learn how electrons are attracted to the nucleus, I use an activity from the Process Oriented Guided Inquiry Learning (POGIL) Chemistry Book on Coulombic Attraction.  POGIL is a resource where students engage topics via diagrams and data sets, with guided questions to focus them and help them understand the concept.  For my students, it can need some revision due to the reading level, but the book comes with a CD of PDFs, and Word 2013 allows editing of the PDFs as Word files.

Since my students make the progression of Biology -> Chemistry -> Physics, this will be their introduction to Coulombic attraction.  If I were in a physics first district, this would be organized as a much more independent activity.

This activity incorporates three Science and Engineering Practices and one Cross Cutting Concept

  • Science and Engineering Practice 2: Develop and use models.  The use of models here are restricted to 2D pictures in the POGIL packet.
  • Science and Engineering Practice 4: Analyzing and interpreting data.   Data are provided about distance, number of charges and force of attraction for the students to determine the relationships between the variables.
  • Science and Engineering Practice 5: Mathematical and computational thinking.  Students need to engage the data as almost purely numbers.  This activity will be their first experience with scientific notation this school year, which can also intimidate students.
  • Cross Cutting Concept 6: Structure and function.  Students will begin to understand how the attraction between particles leads to differences in the structure of atoms, and how different atoms will behave based upon this structure.

 

Under the permission from Flinn Scientific, the entire POGIL packet and examples of student work cannot be posted to this site.  I strongly recommend it to teachers of chemistry at any level who are looking to incorporate more student thinking and inquiry activities into their classroom.

Whole Class Introduction

25 minutes

I ask for one person from each table to come to the front of the room to get the POGIL 11: Coulombic Attraction packets for each person at their table.  Since this is the first time we have worked with POGIL, I explain the set-up: students will engage with various visual models that have questions about each model to help determine what we are seeing.  I make it clear to the students that they need to use the information from each visual to answer the questions, that this is the instruction for the day.

I point out the Visual Prompts attached to each model, including the picture of the key for key ideas and the stop sign so students know this is a point to check in with the teacher.

We then begin by looking at Model 1 together and going through the questions as a whole class.  From the title of the model, I ask them to predict what relationship we are exploring.  Then students identify which particle is represented by each symbol.  Based on our prior experiences with subatomic particles, they should have little trouble.

The key question in this section surrounds identifying the independent and dependent variable.  Students have difficulty with this concept year in and year out.  In this case, I refer back to the title of the model- can we manipulate the force on the particles or the distance between them?  Since the trial manipulated the distance, we expect that the force depends on the distance, making the force the dependent variable.

Students also panicked at the force representation in scientific notation.  Since all the exponents are the same, we crossed them off as a class.  I stopped instruction to demonstrate why we use scientific notation, writing the numbers in decimal form underneath, and showing how the exponent represents where the decimal place is.  This extended the lesson beyond my expectations.  Depending on your student familiarity with variables and scientific notation, you may be able to complete this much quicker.

We complete page one together as a model for student work.  I then have them turn to model 2.

Guided Practice -- Tables

20 minutes

I tell students that they are to do Model 2- The Alkali Metals as a table (students are still seated in the groups they chose on day 1 at this point), using their help cards if they need my assistance and I am not nearby. Since we have not discussed groups of elements or the periodic table yet, I wait to see if students ask about it.  Knowing the identity of the alkali metals is not crucial yet, but I don't want the terms to interfere with their learning about the attractions.

While students are working, I circulate the room and listen to their discussion and evidence from the model in supporting their statements. Students should be indicating that the size of the atom is increasing with the additional valence shells, which decreases the attractive force from the nucleus on the outer electrons.

If their evidence doesn't support their assertions, I attempt to probe and ask questions regarding their relationships. Many students have difficulty with this section simply because they do not go in order, or read the instructions carefully.  For example, students will say that the force is increasing down the group, because they are looking at the length of the force vector instead of the thickness.  I reference them back to the understanding they developed in Model 1, and the key for Model 2, to show the connections between the different parts of the POGIL packet and help them develop appropriate evidence. 

I remind students to check in with me when they finish the page.

Closing

5 minutes

To close this lesson, I have students return to their binder page from the bell ringer activity.  Below it I have them respond to the question: "How can the attractive force between oppositely charged particles be decreased?"

I have them put their POGIL Activity directly behind this page before turning in the binders at the end of the period.  I will check their responses outside of class.  I am expecting to see:

  • Increase the distance between particles

Students retained this in the short term, but it has not translated as well long-term as I would hope.  I think my students need more concrete modeling than the POGIL provided in this case.  Next year I would consider creating an experiment where students use spring scales and neodymium magnets to investigate this relationship.