Summary

In this lesson, students² will explore the atomic level structure of various elements and compounds to determine how arrangement of atoms at the microscopic level _{affects macroscopic} material properties.

Grade Level

Middle School and High School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• HS-PS2-6: Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
• HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
• Scientific and Engineering Practices:
  • Asking Questions and Defining Problems
  • Obtaining, Evaluating, and Communicating Information

Objectives

By the end of this lesson, students should be able to

• Describe how bulk structure at the molecular-level can be used to infer material properties.
• Relate relative electrical attraction between atoms and molecules to boiling and melting point.

Chemistry Topics

This lesson supports students’ understanding of

• Physical properties
• Molecular Structure
• Intermolecular attractions

Time

Teacher Preparation: 15 minutes (to familiarize self with software and plan discussion)

Lesson: 120 minutes

Materials

• Teacher computer connected to projector that can run MolView in Google Chrome browser
• Computers that can run MolView in Google Chrome browser
  • 1 computer per pair of students is recommended
  • A teacher computer and projector is sufficient if students do not have devices
• Copies of ChemMatters article, “The Write Stuff” for each student
• Optional:
  • Samples of halite and calcite
  • A hammer

Safety

• No specific safety precautions need to be observed for this activity.

Teacher Notes

• In this lesson, students will complete 3 activities:
  • Explore the bulk molecular-level structure of graphite and read the ChemMatters article, “The Write Stuff” to evaluate why pencil “lead” is actually graphite.
  • Explore the bulk molecular-level structures of other substances using MolView to find patterns between structure and melting point/boiling point.
    • Note that MolView is a free website that can be used to view 3D structures of chemical compounds. Structures can be rotated in 3D space. MolView should run smoothly on Chromebooks, laptops and tablets.
  • Research the bulk molecular-level structure and common uses of a chosen substance and construct a short presentation relating the two.
• Activity 1: Exploring bulk structure and properties of graphite (~40 minutes)
  • Start by asking students to simply write their name on a piece of paper with their pencils. Ask students to think about how the color is deposited onto the page. After discussing, show students the structure of graphite using MolView.
    • For help with Molview, read this article on ChemEdX.
  • Ask students, “What do you notice about the structure of graphite?” Make sure to rotate the graphite on the screen so students can see the sheet structure.
    • Students should notice that graphite has a sheet structure. The sheets are not connected to each other but they appear to extend infinitely. You can ask students, “Why do you think this sheet structure makes graphite good for pencils?”
    • An additional question to guide discussion is, “What do you wonder about pencils?” Students may wonder things like, “Do pencils really have lead in them?”, “How do erasers work?”, “Why are pencils yellow?” and other questions not related to graphite. These questions just serve to peak student’s interest and might be answered in the article!
  • Once you are satisfied with the discussion on the structure of graphite, hand out the December 2017 ChemMatters article, “The Write Stuff” and ask students to read it. You may have students fill out the attached reading guide and use the questions from the reading guide as part of class discussion.
  • During the discussion, you can pull up the structures of diamond and buckmisterfullerene on MolView to show students the examples of allotropes from the reading. These two substances and graphite are all made of carbon but in very different arrangements, leading to very different properties.
  • The last question on the attached reading guide is about London Dispersion forces. Students may have gathered enough from the reading to define London Dispersion forces as week attraction between molecules. If not, make sure students have the basic definition that all molecules are weakly attracted to each other. That means it takes energy to separate molecules from each other (like during a phase change). At this point, you can let students know that there are two types of intermolecular attractions, weak London dispersion and stronger dipole-dipole but we are just going to focus on intermolecular attractions in general (if you want to, you can give intermolecular attractions the name Van der Waals Forces). Remind students that intermolecular attractions are much weaker than the attractions between atoms in bonds. Ending the discussion with intermolecular attractions will lead into the next activity.
• Activity 2: Exploring bulk structure and properties of other substances (~40 minutes)
  • Make sure each pair of students has a computer that can run MolView and a copy of the Structure and Properties Activity worksheet.
  • Work through the first substance with students to get them acquainted with MolView. Once students are confident in operating the program by themselves, allow students to finish the activity in pairs.
  • NOTE: The worksheet does not distinguish between ionic and covalent bonds, though you could easily add bond type as an extension. To extend even further, you could explore the concept of polar covalent and non-polar covalent bonds as well as permanent dipole-dipole attraction.
  • As students finish, lead a class discussion noting patterns in structure and melting point and boiling point from the worksheet. Remind students that all molecules are weakly attracted to each other, even though these attractions are not depicted in the ball and stick models. The “sticks” represent strong attractions between atoms called bonds.
  • If you have samples of the minerals calcite (calcium carbonate) and halite (sodium chloride), you can show students the rhombic and cubic structures respectively to show how bulk structure affects crystal habit. You can also break these minerals with a rock hammer to show students that they cleave in the same directions they grow. If you do not have access to samples, you can show these YouTube videos of halite and calcite.
  • As an extension, give students the structures of different substances and ask them to predict and explain which substance would have the higher boiling point based on the structure. Use the provided Formative Assessment handout for this.
• Activity 3 (extension): Research other substances and properties (~40 minutes)
  • Ask students to pick one of the substances from Activity 2 to research with their partner or individually. Students should specifically be looking for links between the structure of the substance and its physical properties. Students should connect physical properties to common uses.
  • Students may use the provided PowerPoint template to construct their presentation. Students should make sure to note the types of attractions in their compound and how those attractions or the arrangement of atoms might affect the physical properties. The attached template is completed for water as an example.
• Note: All images used in this classroom resource are from MolView, open source terms can be found here.