Lesson 6 of 7
Objective: Student will be able to describe how electrons are organized for the first 20 elements, and to identify and model valence electrons.
In the last lesson student began to grapple with electron configuration. In this lesson students work to cement their understanding of electron configuration through various activities, including the use of plastic models of the electron shell, drawing, and explaining.
This lesson aligns to the NGSS Disciplinary Core Idea PS 1: Structure and Properties of Matter because they learn how each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons. This is prerequisite knowledge as they build towards their ability to 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.
It aligns to the NGSS Practice of the Scientist #2, Modeling. The easiest way for students to get a conceptual understanding of the atom as a three dimensional structure is by using a model.
It aligns to the NGSS Crosscutting Concept Patterns by giving students the chance to study patterns at the scales of the atom. Later on this understanding of patterns on the periodic table will be used to provide evidence for causality in explanations of how atoms of different elements behave in bonding scenarios.
In terms of prior knowledge or skills, students should have a basic understanding of the structure of the atom. My students began to use the order of filling for the first twenty electrons. I choose to focus on the first 20 elements because this is the lower level chemistry class and by doing so I am able to avoid in-depth discussion of d-orbitals, a subject covered more extensively in my honors chemistry course
The materials needed for this lesson include the following:
plastic model for electron configuration
a periodic table
Do Now: Students start class by drawing the electron configuration for Al, C, and Ca without using any notes. If they cannot do it, but they could do it looking are their notes, I let them. I also ask that they read a section in the book similar to this material that describes what valence electrons are and why they are important.
I have chosen this approach because I want to gauge readiness for what I would like to teach today, which is how to identify valence electrons. However, I expect that students will be in various places in terms of knowing the last lesson that dealt with electron configuration. As I watch the complaints file in, I note that about a third of the class does not need notes because they have memorized the order of electron filling, which they were taught as 1s22s22p63s23p64s2. The good news from this brief check-in is that nearly every student recognized that the key to completing the electron configuration component of the Do Now is to know this order.
Activator: After I have alleviated any anxiety that resulted from the no-notes mandate and taken attendance, I ask students to share with the class what they got for the electron configurations.
They report out that the configurations are:
I have chosen this approach because I want to build confidence. Most students are able to do this, albeit many still need to look at their notes. However, by experiencing a little anxiety around the order of filling, I have made the point that this detail is quite important. I note that Ca, atomic number 20, contains the entire order of filling for the first 20 elements.
Mini-lesson: Now that we have focused on electron configuration, I am ready to start new material. I begin by talking about valence electrons as shown in this video about the importance of valence electrons. I have chosen this approach because I want to reinforce what students have already started to learn through reading.
However, I know anticipate that while students might understand why they are important, they have not really understood valence electrons because the wording in their book talks about valence electrons as being those electrons found in the outermost shell. I note that sometimes students write down information but the words they have copied do not make sense to them. I note that unless they actually understand what they are writing, or plan to figure it out and have in some way notated that in their notes that this needs to happen, then they are not really following the spirit of obtaining information.
With that said, I give this lecture video that explains energy levels in order to help students differentiate between filled and unfilled shells. Here are the slides for valence shell lecture slides I use during this portion of class.
During both of these mini-lectures I ask students to take notes on the content.
Guided Practice: Once I have explained the material, I teach students how to identify valence electrons. First, I ask the to look back at the three examples from the Do Now. I ask them to circle the electrons in the outermost shell. At first students are confused because they do not know whether a shell consists of 3s and 3p or just 3p. They think this because in our model the 3s electrons are in a line slightly separate from the 3p. I explain that they are in the same energy level because they both start with the number 3 and they are both in the same tier of the model.
Once students understand this, I explain that to identify the valence electrons, they simply add up the subscript numbers and that will tell them how many valence electrons they have. Finally, to symbolize these we use Lewis dots. I model this by explaining that you can put a dot at the 12, 3, 6, and 9 spots around the chemical symbol until you run out of valence electrons. If there are more valence electrons, repeat going around the symbol until there are no more.
I have chosen this approach because because I want students to relate the electron configuration to the valence electrons. I can show them how to use the periodic table after they show proficiency in the underlying reasons for why Group 1 elements have only 1 valence electron.
Student Activity: At this point in the lesson students are ready to begin practicing identifying valence electrons using the electron configurations for the elements. I distribute the Lewis Dot notation for the First 20 Elements for their practice. During this time I walk around and encourage students to use the plastic model if they are having difficulty in understanding how electron configuration works. I remind students that the have to fill from spots closest to the nucleus first, and then fill spots outwardly.
I am careful to look for students who are confused about what constitutes the outer shell. Some students still are circling only the p-orbitals, and so I re-teach them about what constitutes the outer shell.
The worksheet provides students the opportunity to apply what I have taught them, and to discuss the ideas with other classmates and me.
Catch and Release Opportunities: One exception to the rule of how to write Lewis dots comes with helium. It has a full outer shell with only 2 valence electrons and so has two dots written over the top of the chemical symbol. This is a good time to explain why Lewis dots are important. I explain that where we are heading is to show how atoms bond. Atoms bond when they have an unfilled outer shell.
I show the Lewis dot structures for oxygen and hydrogen, and point out that unpaired electrons are bonding sites. This is why helium is drawn the way it is, because we want to indicate that there are no bonding sites for helium because its outer shell is already full.
Once students have filled a lot of the worksheet I explain to them that they do not have to keep asking me if they are correct for each of their elements. They can use the periodic table to check the number of valence electrons. Groups 1A-8A are so labeled to indicate the number of valence electrons. Students get mad at me for not showing them this "trick" sooner, but I remind them that I want them to understand how valence electrons relate to electron configuration.
To wrap this lesson up I ask students to stand with me in a group. This is a tone-changer. Students tend to focus on the ending of class better in this structure, perhaps because it is informal and everyone has proximity to the teacher. I ask the group the following questions and receive the following answers:
How do you know how many electrons an atom has? (the atomic number)
Which are the valence electrons? (the electrons in the outer shell)
Why are they important? (they are responsible for bonding)
What is the procedure for writing valence electrons using Lewis dot structures? (write dots around the chemical symbol until you run out of electrons)
From looking at student work my sense is that students for the most are ready to discuss ions in the next lesson. In this valence electrons fluorine one student has already begun to think about this, as evidenced by his desire to give F one more electron. In this video a student shows what he knows about the electron configuration for lithium. He shows a solid understanding of today's lesson and he is typical in his understanding.
This student work sample confirms my read on the class. Almost every line has the proper number of valence electrons in the Lewis dot structure. Granted, some of them are stacked to look like a snowman, but this is a convention that I can teach to. Helium with its full outer shell, needs to be addressed, and provides a great opportunity for me to reinforce the duet rule as well as the idea of the full outer shell as the reason for atoms gaining, losing, or sharing electrons.