Day 1: Introduction to Coulombic Attraction

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Objective

SWBAT explain the atomic model in terms of relative electrical forces between particles based on number and distance between particles.

Big Idea

As the distance between protons and electrons increases, the attractive forces decrease.

Introduction

This lesson addresses the NGSS 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".  The goal of the lesson is to provide evidence that electrons exist and they are located in different energy levels, in varying quantities.  This is aligned with the NGSS Disciplinary Core Idea (DCI) PS1.A (Structure and Properties of Matter): “each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons”. 

In this lesson students explore the structure of the atom using the NGSS Practice:  Developing and using models.  In this investigation my students use POGIL (Process Oriented Guided Inquiry Lesson) to investigate Coulombic attraction.  This process illustrates evidence that subatomic particles are present within an atom and further builds a foundation for exploration of HS-PS1-1.

The Cross Cutting Concept (CCC) that is illustrated in this lesson is Patterns.  Patterns are illustrated as students continue to see that as elements change so do the number of protons and electrons.   However, this concept is expanded when students see that electrostatic forces are present between subatomic particles.

Engage

10 minutes

I start class by showing my students a picture of three Bohr’s models of alkali metals and a picture of the attractive forces between a proton and electron.  These pictures are the same pictures they see in the POGIL activity later in the lesson. I instruct them to look at the pictures, and using their question stem list, write at least two questions about each picture in their science journals.

At this point in the unit my students should have a good understanding of the atomic structure of the 1st 18 elements including their Bohr’s models and recognize model 2. 

The purpose of having students engage in this questioning is to see if they can generate higher-order questions that require more than superficial knowledge of the atomic structure.  The goal is to get them to question at a deeper level and make connections between what they have learned and what they will be learning.  The number of questions, the quality of questions and the sophistication of their questions can also tell what knowledge they have about the unit up until this point.

I follow-up their questions by having each student answer another student's question (example of student work).  This engages them in the process of sharing their knowledge, and provides a layer of differentiation for the varying levels of understanding. 

After their brief discussion, we share out as a class for a few minutes.  This gives me an idea of some of the questions they have, such as:

  • What is a nm?
  • Why are the attractive forces between the nucleus and the outer electron stronger in lithium than sodium?
  • What happens to forces as you add equal number of electrons and protons to an atom?

Any question is perfectly acceptable, but ultimately I want students to make accurate observations about the diagrams and ask what causes the attraction between the + and - charge in the picture to change.  Or what would happen if more distance or attractive force was present?

After the brief discussion I hand out the POGIL activity and introduce them to the idea of Coulombic attraction.

Explore/Explanation

35 minutes

I can only show parts of the activity, and not show any answers, due to copyright laws. I have provided a POGIL Model 1&2 diagram that are used in this lesson and summarized the questions.  I highly recommend visiting the POGIL website and purchasing the Chemistry manual; it is an excellent resource that I used to teach a variety of topics, such as periodic trends and reactions.

With this being said, I like to make this activity as inquiry based as possible, and have students work in groups of two.  The concept of particle attraction can be difficult for some students so I break this activity into sections (which are labeled as “Model 1", "Model 2", and so on) so that we can discuss each one before moving on to the next.  This activity typically takes two 50 minute periods with checks for understanding, group discussion and end of day assessments.

Model 1

This model illustrates how force changes as distance of an electron gets further away from nucleus. I start out this activity by having each group (based on lab partners) read the “Why” introduction and answer the Model 1-Distance and Attractive Forces questions.  The most important part of this activity is make sure that students make good observations and read everything thoroughly.  if they don’t, they will be lost.  I find it important to not rush through the sections and provide ample time for discussion with their partners. 

The first section may take up to 10 minutes, but 5 should be enough.  To save time I explain to the class that a nm (nanometer) is a very small unit of measurement and scientific notation is used to show very small and large numbers.  A brief explanation is adequate because neither concept is the focal point of the lesson, but does have to be explained to provide clarity.

As they work on questions #1-5 I circulate the room and check for understanding about the main idea, that as distance between particles increases attractive force decreases. After Model 1 is completed I start a brief class discussion and check to see if any warm-up questions that were asked earlier relate to what was learned.  This gives ownership of knowledge to the students.

Summary of Model 1 questions:

      1.  What subatomic particles are represented by + and -?

     2.  Would you expect to observe attraction or repulsion?

     3.  What are the independent and dependent variables in the data?

     4.  Would the attractive forces be greater than or less than 0.26 x 10-8N if the particles were              0.50nm apart?

     5.   What would happen to the attractive force if two protons are 0.10 nm away from one                  electron?

      6.  Write a complete sentence stating the relationship between the independent and                          dependent variable.

Model 2

Model 2 shows three separate Alkali metals and how force of attraction decreases as the valence move further from the nucleus.  After the discussion, I instruct my students they will need a periodic table for this section and need to recall how to make a Bohr’s model from lesson 6.  I follow the same process here as I did in Model 1, except I check for understanding that as atoms get bigger the outermost electron has less attractive force to the nucleus.  After the questions are answered we have a group discussion.

 Summary of Model 2 questions:  

  1. What do the arrows represent?
  2. What does the thickness of the arrow represent?
  3. Are the elements in the same column or row?
  4. How does the distance between the outer most electron and nucleus change as you move down the group?
  5. How do the attractive forces change as you move down the group?
  6. As you move from the smallest atom to the largest atom in model 2, how does the attractive force between the valence electron  and nucleus change?
  7. Are the answers to 6 & 7 consistence with Model 1?

 

These two models take a good portion of the class.  I find it important to not rush through them and provide students with enough time to grasp what can be a difficult concept for many students.

The process of understanding attractive forces is an central theme in HS-PS1-1 and ties directly to HS-PS1-2, which addresses periodic trends and bonding.

Evaluate

5 minutes

To conclude the day I have students write in their journal 3 things you found out, 2 interesting things and 1 question you still have. 

The goal is that students understand that attractive forces increase as the number of protons increase, as distance increases force decreases (a qualitative rather than quantitative description) and write a higher-order question that builds on what they have learned. 

The other 2 models will be completed at the beginning of class, followed by a quiz (see day 2).