Lab: Shapes of Molecules

Lab: Shapes of Molecules

For the Teacher

Summary

In this lab, students will investigate the VSEPR geometry of covalent compounds. They will draw Lewis structures, use molecular models, and determine the geometry of covalent compounds. There is a lot of repetition so students gain a lot of practice.

Resource Type

Lab

Grade Level

High school

Objectives

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

·  Recognize from a Lewis structure the molecular geometry of a compound.

·  Understand VSEPR theory.

Chemistry Topics

This lesson supports students’ understanding of

·  Molecules and bonding

·  VSEPR theory

·  Molecular geometry

·  Resonance

·  Covalent bonding

Time

Teacher Preparation: 10 minutes

Lesson: 1 class period and complete for homework

Materials

For each group:

·  A set of molecular models

Safety

No safety considerations need to be observed for this investigation.

Teacher Notes

·  It’s helpful to have samples of each geometry at the front of the class so students can familiarize themselves with how to assemble molecular models.

o  Tetrahedral (four atoms around one central atom)

o  Trigonal pyramidal (three atoms, one unbonded pair of electrons around one central atom)

o  Trigonal planar (three atoms and no unbonded electrons around one central atom)

o  Bent (two atoms and two pairs of unbonded electrons around one central atom)

o  Linear (two atoms and no unbonded electrons around one central atom)

·  This investigation only covers compounds that satisfy the octet rule. A possible extension could be to add more complicated molecules that extend past the octet rule.

For the Student

Student Activity Sheet: Shapes of Molecules

Lesson

Background

The valence shell electron pair repulsion (VSEPR) theory is how the geometry of a molecule is determined. It’s called “vesper” theory for short. The shapes that are possible are tetrahedral, trigonal planar, trigonal pyramidal, bent, and linear. To determine the shape of a molecule, you must look at the central atom. Unbonded electrons around the central are not accounted for in the geometry, however they are important because they determine the geometry. Unbonded electrons around atoms that are not the central atom have little effect on the geometry.

In this experiment, you will draw Lewis structures for a number of compounds and use them to determine how the molecular models need to be assembled. From the models, you will determine the geometry of the compounds. After completing a few examples, you should start to see how the two dimensional drawings really exist in three dimensions.

Procedure

Complete each column in order. Compare your model to the samples at the front of the room if you are confused about which geometry your model makes.

Molecule
(write the chemical formula) / Total valence e- / Lewis structure
(check the box if a resonance structure is possible) / Lewis structure with proper geometry (use the models to help here) / VSEPR geometry
(the name of the shape)
Water /
Nitrogen /
Carbonate /
Sulfite /
Carbon tetrachloride /
Ammonium /
Bromine /
Carbon monoxide /
Dinitrogen monoxide /
Ozone /
Nitrate /
Nitrite /
Bromate /
Chlorite /
Phosphate /
Acetic Acid (try your best! Both carbons are in the middle.) /

Analysis

Without using the models, determine the geometry of these compounds (you can draw Lewis structures to help you):

NF3 H2S OCl2 HCN F2 SO2 SO42- ClO3- SO3 (not sulfite!)

Conclusion

All of the compounds in this exercise are what kind of compound? Explain why this is important.