Electrons: Where they Live and How they Act

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Objective

Students will be able to state the electron configuration for the first twenty elements on the periodic table, and they will be able to use the speed of light formula to calculate frequency when wavelength is known.

Big Idea

Electron configuration describes where electrons are located. The speed of light formula helps explain what happens when electrons go from the ground state to the excited state.

Introduction

In this lesson students take notes about electron configuration and they then practice naming electron configurations for the first 20 elements. They then take notes on electrons moving from ground state to excited state and back, and how to calculate frequency when wavelength of visible light is known.

This lesson aligns to the NGSS Disciplinary Core Idea of 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." Students are studying electron configuration.

It aligns to the NGSS Practice of the Scientist of  Using Mathematics and Computational Thinking. Students use algebraic thinking and analysis to figure out frequency from known wavelengths. It also aligns to the Practice of Developing and Using Models by giving students to model their understanding of electron configuration.

It aligns to the NGSS Crosscutting Concept of Energy and Matter: Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system. Electrons going from the ground state, to the excited state, and back, is an example of this.

In terms of prior knowledge or skills, students should have a basic understanding of the structure of the atom.

The materials needed for this lesson include the following:

  • atom modeling kits, or some way to show electron orbitals; this could be as simple as pinto beans and posters that students make
  • scientific calculators

Do Now/Activator

10 minutes

Do Now: Students begin class by reading about electron configuration in the text book. (Similar material can be found at this website.) I ask students to record in their notes what they think electron configuration means, and how it works, and to record 1-2 questions about electron configuration.

I have chosen this approach because I want students to start thinking about the material before I have even said a single word on the subject. I also want to expose students to some of the vocabulary we will be using, and to pictures that may also help them to start thinking about the subject.

Activator: After I have taken attendance and given students time to complete the Do Now, and after I have recorded a grade for how they started class, I begin by asking them to share some ideas that they have about electron configuration.

They understand that it has something to do with electrons, they think the notation is confusing, and they recognize that this has something to do with energy. A common question they have is, "Why do electrons behave like this?"

I have chosen this approach because it is a way for me to quickly measure where students are at. On this day students seemed ready to learn something new, but occasionally there may be something going on in their lives that will cloud their emotional space. I can tell from the start of class if I need to address something larger or if students are mentally prepared to begin rigorous material.

Mini-lesson and Guided Practice

15 minutes

Mini-lesson Part 1: I explain that we are going to look at where electrons live, and how they act. The first part of class will be about where the live. I begin by asking students to take notes on electron configuration and I review the electron location and behavior lecture. I have given them each a copy of the slides. I use the first slide simply to explain that while we will be looking at 2-dimensional models of the atom, students should understand that atoms have shapes, and the more electrons an atom has, the more complicated its shape becomes. 

In the following sections I use analogies like concert halls and cell phones. This is important--my goal is to connect abstract concepts to things students know and understand so that the abstract idea of the electron cloud becomes more tangible and thus understandable. 

I then invite students to look at our plastic model, and to look for the different letters. Once they find them I ask them to count how many 1s, 2s, and 2p locations there are. For the s orbitals I get "2" as a response, but for the p orbital I get different responses depending on how organized the student is in counting. Once we are all in agreement that s-orbitals have 2 locations and p-orbitals have 6, I move on to explain that if you could build an atom, then you would start by filling the inner most orbitals first, and then move out from the nucleus. I liken this process to general admission at a concert hall--the good seats fill up first. I then explain we can use the order of filling as a way to describe the electron configuration for an element, even when we do not have the model. 

Guided Practice, Part 1: I interpret the electron configuration for lithium, and then ask them to do the same for boron. A student shares their answer and we discuss how the student got that answer. I then release them to the Application Part 1 of the lesson.

I have chosen this approach because it provides different access points for different kinds of learners. Some students are quite comfortable looking at the model just one time, but some students benefit their learning by creating models for each of the elements. 

Mini-lesson Part 2: After students have had a chance to work with electron configuration, I explain that the second part of class will be about how electrons act. Going back to the slides, I explain that electrons that they have been working with are in the ground state--they are not excited. one way to get them excited is to heat them up, such as putting them in a fire. Electrons don't stay in the ground state for long--they come back down to the ground state. When this happens, the electrons give off colors depending on what atom they are a part of. 

I explain that light travels in waves at different speeds. I draw a wave with my finger and explain that their cell phones use electromagnetic energy to comunicate with cell towers which then communicate with satellites. All electromagnetic energy can be described in 2 different ways--by the length of its wave, and its frequency. I show them how different colors have different wavelengths. I also explain that the speed of light formula is wavelength x frequency, and the speed of light is always 3.00 x 108 m/s.

I explain that where we are going with all of this can be explained by looking at the flame test problem. If you can discern the color from a flame test, you can determine its wavelength and therefore calculate its frequency. At first, I give a brief explanation of how to find frequency, as depicted in this how to calculate frequency video. However, after doing this I could tell that many students had no idea what I was talking about. I then further review the problem in the slides by writing out the calculating frequency notes to give a more step-by-step explanation than what is written in the slides. 

Guided Practice, Part 2: I ask students to work the problem with me and make sure that they can get the same result as I did.

I have chosen this approach because what I have learned is that student's math ability varies considerably. For a few students, I am going much too slowly. Once they have demonstrated proficiency with these calculations, I use them to help other students. Students really wrestle with a couple of issues at this point in the lesson. First, they do not know how to work with scientific notation on their calculators. This was the major sticking point, and it is complicated by the fact that different calculators require different inputs in order to work. For example, on my calculator I have an EE button, which means "times ten to the ___ exponent" and so I type "1 EE 9" for a billion. Other calculators have buttonssuch as 10x, which also gets at the same math, and some have others, such as the "^" sign. So, I encourage students to help one another and to read their manuals for their calculators.

Application

25 minutes

Student Activity:

Part 1: Students work with the model and the Electron Configuration Practice sheet to name the electron configurations for the first 20 elements. 

I have chosen the approach of giving the students the elements in alphabetical order because I want students to wrestle with each element by using the model or the order of filling, rather than have them just figure out that one more electron is being added each time. My ultimate objective is that I can ask them what the electron configuration is for any of the first elements are and they can answer simply by knowing the order of filling. 

Part 2: Students practice finding frequency when wavelength is known by working with the frequency problems

Catch and Release Opportunities:

Part 1: I walk around the room answering questions. The most common question I get is "Is this right?" I answer with "What do the small numbers add up to? Is your answer the atomic number for the element? Did you follow the order of filling, being careful to only put 2 electrons in s-orbitals, and 6 electrons in the p-orbitals? If they answer yes to all of these questions, then I tell them it is correct." I have already seen if it is correct or not, but I want to build agency in them to be able to assess their own work and decide if it is correct or not.

Part 2: I walk around and answer questions. The big questions are:

"How do I use my calculator?"

"Is this right?"

"How come my classmate and I got different answers? Who is correct"

At this point in the lesson there not enough time to answer everyone's questions individually, as it is time to move into the debrief. I was planning on having a student show their answers, but instead I decide to give students a new access point for the assignment. 

Debrief

10 minutes

To wrap this lesson up I first go back to a place where most students found success--electron configuration.  A volunteer shows student work for the electron configuration practice problems. I remind students that the order of filling is listed in one of their slides, and the little numbers should add up to the atomic number for the element.

For calculating frequency, I encourage students to own a calculator, and to use the manual to understand how to enter scientific notation. I also suggest that students learn how to do calculations on their calculator that they already know the answer to, such as such as 4 x 103. If they get 4,000 then they are using scientific notation correctly, and if not they will need to experiment with their calculators in order to understand how scientific notation needs to be entered.

I explain that in the next class we will begin by reviewing answers for these problems, and answering questions about them. I want students to grapple more with this material before I provide more leadership on this issue. When students have spent time grappling with material, they will then be more ready to be ask questions and understand re-teaching.