Lewis Dot Diagrams
Lesson 10 of 13
Objective: SWBAT use the periodic table to create Lewis dot diagrams of neutral atoms, and then to use those diagrams to predict ionic charges of elements.
This lesson addresses the following NGSS standards:
- HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties. By determining how atoms form ions to obey the octet rule, students can predict how substances might react with each other.
- Science and Engineering Practice 2: Developing and Using Models. Lewis dot diagrams are useful models for predicting the behavior of an element's atoms.
- Patterns Cross Cutting Concept: Graphs, charts and images can be used to identify patterns in data. Representing all the Lewis dot diagrams together enables students to recognize the structural valence electron patterns in the main group elements of the periodic table.
Ensuring student success with understanding valence electron structure and behavior is so critical to further work throughout the year that I place a strong emphasis on it. Therefore, this lesson is fairly direct instruction with multiple whole class check-ins. I don't want students leaving the classroom uncertain how or why we represent valence electrons.
This is the fifth time we have done Lewis dot diagrams, so the students should be very comfortable using and interpreting them, but there can be some issues with student confidence. This lesson comes after a scheduled interruption to the curriculum. Due to the demographics of my school, I stopped to honor Hispanic Heritage Month and did a Latino Latina Scientist Project.
This project took multiple class days of research and writing, so returning to the periodic table needed additional scaffolding to refresh students on concepts they hadn't worked with for nearly a calendar week.
This lesson runs in my shortened schedule day, and is designed to be completed in 42 minutes. It is possible to speed up or slow down this lesson, based on the needs and abilities of your students.
Opener -- Returned Work
As I pass work back, I remind students that the scientist papers will be late beginning the next day, and offer them the chance to hole punch their sheets and put them into their binders.
I remind students we have left the Periodic Table alone for about a week, and that it time to dig back in. This is met by a chorus of groans. I let the students get it out of their system, smile sweetly, and ask "Did you really think we were done with it? After a break, we should be rested and ready to finish with it." While students aren't thrilled with the idea of getting back to work, they appreciate the honesty and settle in.
Next, I pass out the day's work.
Due to the length of time since we engaged this material, I use my document camera to project the Valence Electrons and Ions Review Notes. I point out to the students that this is both a review, and their notes for the day to encourage them to work through it with us.
I walk the class through the opening fill-in section, asking for students to complete the blanks. I do this as a whole class for those students who don't recall the information to let their classmates help jog their memory.
"Valence electrons are found in the (pause) outer shell around the nucleus. To determine the number of valence electrons, you need to check the (pause ) last digit of the group number. Lewis dot diagrams represent the valence electrons possessed by an atom."
Now students realize they need their periodic tables and get it out. Then we go through the refresher of how to create Lewis Dot Diagrams.
Then I give students time to finish their chart. When they finish, I ask what patterns they see. hoping that they will discuss the extra valence electron as they go across the period, and that the groups are the same.
We then focus on the bottom half of the page, and complete the fill-in as follows:
" Atoms either (pause) gain or lose valence electrons to get a complete outer shell. When this happens, they either become positive (pause- might have to prompt what is a positive ion called?) cations or negative anions."
I then ask which elements will lose electrons to become positive cations. Students may remember it is elements with 1, 2, or 3 valence electrons. We then go through examples of how to complete the ion diagrams like this:
When we finish, I ask them to examine the charges on the ions for patterns. They may struggle, but someone sees the pyramid effect of +1, +2, +3, +4, -4, -3, -2, -1. I have students record this at the bottom of the page.
The completed work can be seen here.
Now I seize upon the pauses and difficulties of remembering the information for our Valence review and give the students the PT Basics Review. I remind students that our unit test will go back to the start of the unit, so we need to be sure of what we know. I ask them to do it independently with their periodic table but without their notes first.
As students work through, I circulate the room to check their progress. After about 5 minutes, I tell them:
"After you go through once, circle the questions you have not answered. Then use your binder to find those you circled, and check the ones you did answer."
I let students work right up to the bell, and collect the papers if they finished, or will take them the following day if they need to finish.
While checking the reviews, I notice two common errors. Students continue to mix up groups and periods. To help remedy this misconception, I use this visual the following day:
The other common misconception is which elements are metals and which are non-metals. Students understand that the zig-zag line separates them. I share two strategies to remember the difference, allowing students the choice of which resonates with them.
- The elements are listed alphabetically by type. M's (metals) before N's (non-metals) from left to right.
- Choose an element you are sure of. For example, we know gold is a metal, so find gold, and since it is left of the zig-zag, everything left must be a metal.
This student didn't make either error, but did a nice job on the remainder of the review.