Electric Crystals

Read the first two comic pages to meet the tour guide, Krista, and enter the crystal factory. Students in the comic have brought items they think might be a crystal. Students are prompted to imagine what item they might bring to a tour of the crystal factory.

Students will read and sort the analysis summary cards generated by the SPEX 3000 machine: ♧ SPEX 3000 Cards. Each card lists the name of the sample and examples of its visual, microscopic, and chemical structure. The worksheet prompts students to sort these cards in order of personal interest and usefulness.

In the comic, Krista explains the definition of a crystal, and students will use the "computed chemical structure" section of each SPEX 3000 summary to determine if the mystery solids are crystals or not. Krista also introduces the concept of an amorphous solid, which is not microscopically ordered in the same way as a crystal, and therefore has different kinds of uses. Students will use the worksheet to summarize their understanding of crystals and amorphous solids, and the differences between them.

The worksheet asks students to reflect on what they learned, what they found surprising, and what they're still curious about.

Read the first page of the worksheet to learn that unit cells are the building blocks of a crystal. They can't be divided any farther, or they won't grow into a full crystal. However, unit cells contain lots of molecules and atoms inside them.

Each student should obtain 14 gumdrops or grapes of each color, as well as 42 toothpicks.

Panels on the worksheet define the parts of an atom-- protons, electrons, and neutrons-- and the concept of electron shells. The process of ionic bonding is also introduced, and students are guided through some comprehension questions to solidify their understanding of these definitions.

The worksheet guides students through building four NaCl molecules to start, then the process of connecting the molecules together to begin forming a crystal structure. The symmetry rules of the chemical bonds are summarized, and students are prompted with several comprehension questions on these rules. Students finish building an entire NaCl unit cube, and reflect on its shape and usefulness.

Panels on the worksheet introduce the concept of covalent bonding, then explain that the chemical bonds inside SiO2 molecules are more complex than those in NaCl.

More advanced students might enjoy the challenge of building several SiO2 molecular models then attempting to fit them together as a unit cell.

Projections of the SiO2 unit cell are discussed to lay the foundation for concepts covered in P4. Students can also explore a 3D model of the quartz unit cell, and notice that viewing the cell from different angles produces different projections.

Following the instructions on the worksheet, cut out the unit cell meshes: ➚ Unit Cell Handout. These models will be used throughout the rest of the lesson.

The worksheet shows some examples of rotational and mirror symmetries. Also, check in with a video where Madelyn explains how to observe the symmetries of our unit cell models: ▶ Exploring Crystal Structure and Symmetry (Part 1)

The newest employee at The Crystal Factory lost the labels for two of the unit cell files. Students will identify the symmetries of each unit cell model and fill out tables with the relevant information. They'll use the information they gather to determine the missing labels for the file cards.

Students will work together (or build more unit cell models, if they're working individually) to create a large crystal structure from their paper unit cells. They'll then reflect on the process of building a crystal from unit cells as opposed to individual molecules. Students can check in again with a video where Madelyn explains how symmetry restricts the ways crystals can form: ▶ Exploring Crystal Structure and Symmetry (Part 2)

Things can go wrong if a crystal doesn't have the proper symmetries. Students are prompted to create a unit cell without the symmetries we discuss in this lesson. The comic gives a hint as to how crystals grow from their unit cells, and the special properties their symmetry imparts!

The comic introduces the concept of piezoelectricity: the phenomenon where a mechanical deformation results in a small charge on the surface of a crystal.

Read the worksheet's explanation of dipole moment, and how geometry affects electric charge just as much as charge magnitude does. Students are guided through an example dipole moment calculation for an undeformed NaCl configuration. They're then prompted to calculate the dipole moment for the SiO2 configurations.

In this video Madelyn explains the source of piezoelectricity and how the symmetry of atoms inside a unit cell determines whether the material is piezoelectric. Students will then reflect on their NaCl and SiO2 calculations with this new explanation in mind.

The properties of piezoelectric crystals mean they can be used as a a sensor or an actuator. Listed in a wordbank are six devices which use actual piezos. Students will use the wordbank to determine which devices are represented by the schematic diagrams.

On this feedback form students can reflect on their favorite part of the tour, any remaining questions, and topics they're excited to learn more about. The comic ends with Krista geeking out about the amazing applications of symmetry and crystals, and the students on the tour seem to really get the message!