|Students use laptops for PhET and iPads to record data.|
Next Generation Science Standards tell us that inquiry always starts with a question. This school year I’ve made adjustments to my lessons to incorporate more student inquiry and less lecture. At the 2014 Northern Illinois Science Educators Conference, Elizabeth Hamann of Lincoln-Way North HS suggested that molecular geometries be taught via inquiry methods. I took her advice and had my kids tackle a PhET Simulation to get acquainted with molecular shapes.
I loved this simulation due to the immediate visual feedback that kids got when they made adjustments to the shape. The ability to rotate molecules helped kids understand the 3-dimensional impact of adding bonded atoms or lone pairs. A handout guided students through a series of questions in which changes were made to the central atom. Students added bonded atoms, changed bond orders, and added lone pairs with the purpose of observing how the adjustment changed the shape and bond angles of the molecule. While playing around, students drew sketches of the molecules that they created and came up with funny words or phrases to describe the shape that they saw.
My students took my request of inventing funny shape names seriously. For the linear molecule, one group described it as “an industrial ear piercing.” The trigonal planar molecule was described as the “Y in the YMCA dance” and it reminded one boy of the “Flux Capacitor” from Back to the Future. One group said tetrahedral molecules looked like “jacks” from the childhood game Several groups described the bent molecule as “a boomerang.”
|An industrial ear piercing. Looks like a linear molecule!|
During the simulation I heard some intuitive comments from kids. While working the room I heard them say things like “the atoms must be spread out equally” and “the lone pair pushes the atoms down.” Without my help, they developed a basic understanding that pairs of electrons repel each other.
I followed up the lab with a homework assignment in which students watch a You Tube video. The 15 minute video explained the actual shape names that correspond to each molecular shape. Upon returning to class, they demonstrated a solid understanding of electron pair repulsion and the origin of shape names.
You Tube video: Shapes of Molecules
|Students coming up with their own shape names.|
NGSS does not specifically mention “molecular geometry” within its physical science performance expectations. However, a strong understanding of shapes and polarity is required to satisfy Performance Expectation PS1-3 under Matter and Its Interactions. PS1-3 expects chemistry students to “plan and conduct an investigation to gather evidence to compare the structure of substances a the bulk scale to infer the strength of electrical forces between particles.” The ability to predict differences in physical properties, such as melting points and boiling points, stems from a strong understanding of intermolecular forces, which depend on molecular geometry.
Disciplinary Core Ideas PS1.A (Structure and Properties of Matter) and PS2.B (Types of Interactions) expect students to make connections between electrostatic attraction/repulsion, molecular structure, and physical properties. To accomplish this task, teachers should be hitting content that includes Lewis Structures, Molecular Geometries, and Polarity. This Molecular Shapes Lab provides an excellent way to introduce shapes and the simulation gets kids thinking about how repulsion between regions of negative charge affects the overall geometry of molecules. From this point, teachers can move into actual shape names and polarity.
Shapes of Molecules- Phet Simulation Worksheet