Chemical Bonding

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Objective

SWBAT compare and contrast ionic and covalent bonding based on the types of elements involved, the attractions between the elements' nuclei and their electrons, and the behavior of their valence electrons.

Big Idea

Electron sharing or transferring is based on the strength of attraction between two elements' nuclei and their valence electrons.

Background

This lesson connects to the following 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.
  • Science and Engineering Practice 2: Develop and use models
  • Stability and Change Cross-Cutting Concept: Much of science deals with constructing explanations of how things change and how they remain stable.

Students will learn which elements' atoms form certain types of bonds, and about the ability of some elements' atoms to covalently share more than one pair of electrons.  The emphasis is on how atoms bond to reach a more stable state with respect to their valence shells.  Students will focus on the type of element: metal vs non-metal; along with the Lewis Dot diagram models to determine the number and types of bonds between atoms.

Our district decided to not include drawing Lewis structures of molecules in chemical bonding this year.  Students had been working with ions for a while, so the day before this lesson they visited ExploreLearning to complete the Ionic Bonds Exploration.  I felt there was enough background information present in the Exploration for students to understand the basics of how electrons transfer to create ions and the resultant attractions between oppositely charged ions.  The background knowledge from the Exploration and our previous work in this unit should have the students prepared to take the notes on chemical bonding.

Opener

6 minutes

When students enter the room the PowerPoint for the day is already projecting, to signal them to get their binders out to take notes.

I pass back the Ionic Bonds ExploreLearning packet from the day before and tell students we will be taking notes.  Some students re-arrange seats now for notes to get a clearer view, or simply to accommodate poor eyesight.  While students are getting out their binders, I finish passing out the returned work.

When students are settled and ready, I refer to their work the day before on Ionic Bonds, and explain that chemical bonding is the last major concept of the unit.  So far they have explored one of the three types of bonding with Ionic, and that we will cover the rest today.

Bonding Notes

25 minutes

As we begin the Chemical Bonding notes, I refer back to what students already know.  Mainly that atoms bond due to attractions and to become more stable by transferring or sharing electrons.  In addition to information about how covalent bonds are formed, the Powerpoint has diagrams and cartoons to help illustrate concepts and add some humor and context to the notes.

I reinforce that the goal to become stable is referred to as the octet rule, because it is a full valence shell that makes atoms stable.  I ask, "Which elements are already stable with 8 valence electrons?"  Student responses vary from the incorrect "Group 8" to "group 18" or "The Noble Gases".  I then ask if these stable elements will transfer or share electrons, to reinforce with the students that noble gases are unreactive.

Next, we define compounds.  Last year I forgot this part, and since we didn't do the organization of matter at the start of the year, students were lost when we began discussing it.  I refer back to the NaCl formation video we saw when learning about ions to discuss how compounds' properties are different from those of the elements that compose them.

When I discuss ionic compounds, I am careful to refer back to our attraction trends (e.g. electronegativity, with non-metals attracting electrons more strongly than metals.  This difference of attraction helps students understand why non-metals steal electrons from metals.

As we discuss covalent bonding, I begin the slide by asking what it means to "cooperate".  Students explain that we work together, and I point out that "operate" means to work.  So "Cooperate means to share work, what does 'Covalent' mean then?"  Students may struggle, but someone will say "Share valent?" which we morph to "share valence electrons".  We focus again on attractions, and use an analogy of a tug-of-war, where both sides are equally attractive, so they share the rope.

Teaching multiple bonds without Lewis structures is a challenge, one that I won't repeat next year.  Here we focus on how many bonds an element can make.  If they have multiple free electrons, they can share more than one pair in multiple bonds.  Students will need to practice with this before moving forward.

Bonding Activity

19 minutes

I then encourage students to keep their notes out, and pass out the Bonding Activity.  I ask students to use their notes and the periodic table to complete the activity.  I point out that they should only finish the front side today, and we will finish the back in class tomorrow.

While students work, I circulate the room, checking their work and answering questions.  Students tend to leave out the idea that the bonds form for two reasons -- due to different attraction differentials and to become stable.  I push them to include extra detail -- recalling and using what we just did with the notes in order to process the information on a deeper level.

When students complete the chart at the top of the page, I then ask them to look across each row and find three ways both bond types are similar.  If they struggle, I refer them back to their notes to find more information for their chart.  If they continue to struggle, I direct them to what's missing.  Expected responses are:

  • Both bonds form to become stable
  • Both involve coulombic attractions
  • Both involve the octet rule

Next I have them look for differences.  Often these are easier to find, so students will do it first.  Common responses are:

  • Ionics have a metal with a non-metal, covalents are only non-metals
  • Ionic bonds involve transferred electrons, covalent shares
  • Ions bond from attraction between cations and anions, covalent bonds form from attractions between the nuclei and each other's valence electrons.

Here is a completed piece by a student who was really struggling in the unit.

Students use the information from the front of the page to help them on the back side of the page.

This student didn't finish the last piece of the activity.

If students prefer me to hold their paper for the following day, I do, or they keep it.  The following day, they will finish this activity and work on the ExploreLearning for Covalent Bonds.