Introduction to Ions
Lesson 7 of 7
Objective: Students will be able to model how atoms become ions.
In this lesson students will learn about what ions through reading about the atomic structure of ions. They will then build the electron configuration of atoms and record what happens to the valence electron shell when the atom becomes an ion.
This lesson aligns to the NGSS Disciplinary Core Idea of HS-PS1 Matter and Its Interactions because it provides scaffolding toward the idea that "the periodic table orders elements horizontally by the number of protons in the atom’s nucleus and places those with similar chemical properties in columns. The repeating patterns of this table reflect patterns of outer electron states." The ultimate goal of this unit is that students can understand the structure of the atom and how it relates to the organization of the periodic table and the behavior of different elements' atoms.
It aligns to the NGSS Practice of the Scientist of Modeling because students will use a model to show what happens to an atom's valence electrons when the atom becomes ionized. Ultimately they will be able to "Use a model to predict the relationships between systems or between components of a system." In this case the model is literal--a plastic model of the electron cloud, which can be used to predict the charge that will form for the atom.
It aligns to the NGSS Crosscutting Concept of Patterns by focusing attention on how the pattern of valence electrons causes the atoms in groups to behave in a certain manner, namely gaining or losing electrons.
In terms of prior knowledge or skills, students will need an understanding of electron configuration, which my students gained from this lesson.
The only materials needed for this lesson are plastic models of the electron cloud.
Do Now: Students begin this lesson by reading from their text and taking notes about the definition of ions. As shown in these student ion notes the point of this exercise is to remind students that charge occurs when there is a mismatch between the number of electrons and protons. Students have already had some exposure to this idea in this atom modeling lesson. I start class in this way because I want students to explicitly name what an ion is and I appreciate the visualization of mismatched numbers of protons and electrons in ions as opposed to equal numbers of protons and electrons in neutral atoms.
Activator: Once I have taken attendance I ask a student to show their notes from the Do Now, and I use the student notes as a teaching tool by projecting the notes with a document projector and asking the student to explain the conditions for a positive, negative, and neutral atom.
Mini-lesson: I begin by asking all students to construct a model of a lithium atom. I ask them to notice the outer shell, and ask them what they see. They notice that there is one electron in the outer shell. I ask them to record in their notes and important fact:
Atoms like to have a full outer shell, and one way they can get that is to gain or lose electrons.
I then draw their attention to the the two bullet points in the Ion Drawing Practice handout. They state that
- If there are less than 4 electrons in the outer shell, remove electrons so that the outer shell is full
- If there are more than 4 electrons in the outer shell, add electrons to fill it
I then ask them to decide what they will do fo the lithium atom. Is it more likely to gain or lose electrons? They respond with both responses, and so I push the class to explain why. A few students discuss the question, and they then reach consensus that it should lose an electron.
We then discuss the charge on this new ion that they have created. I ask them what the charge is and some say it is 1- and some say it is 1+. I ask one student who answered that the charge is negative to explain why, and he says it is because the atom lost an electron. However when I ask a student who answered that the charge is positive, the student answers that because the atom is losing a negatively charged particle, and because now that there are more protons than electrons, the charge is positive. She is correct. While this is not complicated math, it is confusing for many students because they are used to associating the loss of something with a negative numerical value. I have to really highlight that what is being lost is a negative value, thus making the charge more positive.
Guided Practice: I ask students to do the first row on the handout, and we discuss it. Seeing that most students understand what they are working on, I release them to work on modeling and drawing.
Student Activity: During this time students construct models, draw their models, and predict charges. I walk around and look for common mistakes that I can then use in catch and release moments.
I have chosen this approach because I want to give students time to meet the learning objective, to ask questions of me and their peers, and to practice this skill so that they can gain a better understanding of the relationship between valence electrons and the charges that different atoms form when they are ionized. They will need to know this in order to understand ionic compounds.
Catch and Release Opportunities: This ion drawing shows a common mistake in the formation of Aluminum. The student got rid of an electron in the 3-p orbital but left the 3-s electrons in place. This showed me that some students needed a refresher course on what constitutes valence electrons. I reteach this using the valence shell lecture slides from the previous class.
To wrap this lesson up I ask for a student to show their work. Here is completed student work that shows that the student understands how ions are formed. Students compared their work with this student's. A common mistake continues to be that students are confusing the sign of the charge.
Nonetheless, the conversations I have with students when I ask them about their progress indicate that they do understand the material. Here is a brief check-in I have with a student toward the end of class about modeling ion formation.
For homework I ask each student to redo this assignment for practice using six elements of their choice. I have chosen this approach because practicing this skill is going to increase the likelihood that students will remember this material later in the course. I let students pick their own elements because there is value in letting students choose which elements to do. I find that students own their work so much more when they have a choice, so I look for opportunities to give them choice. Bringing this work back to class affords the opportunity for students to have richer conversation. They are more eager to share answers, and find other students who did the same elements.