Molecular Geometry
Molecular Geometry Investigating Molecular Shapes with VSEPR About this Lesson This activity is intended to give the students opportunities to practice drawing Lewis structures and then build the corresponding model. This lesson is included in the LTF Chemistry Module 4. Objective Students will: Draw Lewis structures of selected substances.
Build models using molecular model kits or using toothpicks and gumdrops.
Use the models to visualize the molecular geometry of the molecule and determine the hybridization.
T E A C H E R
Level Chemistry
Code
Standard
(LITERACY) RST.9-10.3
Follow precisely a multistep procedure when carrying out experiments, taking measurements, or performing technical tasks, attending to special cases or exceptions defined in the text. Translate quantitative or technical information expressed in words in a text into visual form (e.g., a table or chart) and translate information expressed visually or mathematically (e.g., in an equation) into words. Write arguments to support claims in an analysis of substantive topics or texts, using valid reasoning and relevant and sufficient evidence.
(Literacy) RST.9-10.7
(LITERACY) W.1
Level of Thinking Apply
Depth of Knowledge II
Apply
II
Apply
II
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
P A G E S
Common Core State Standards for Science Content LTF Science lessons will be aligned with the next generation of multi-state science standards that are currently in development. These standards are said to be developed around the anchor document, A Framework for K–12 Science Education, which was produced by the National Research Council. Where applicable, the LTF Science lessons are also aligned to the Common Core Standards for Mathematical Content as well as the Common Core Literacy Standards for Science and Technical Subjects.
Molecular Geometry
Connections to AP* AP Chemistry: I. Structure of Matter B. Chemical Bonding 2. Molecular models a. Lewis structures b. Valence bond; hybridization of orbitals, resonance, sigma and pi bonds c. VSEPR *Advanced Placement and AP are registered trademarks of the College Entrance Examination Board. The College Board was not involved in the production of this product.
Materials and Resources Each lab group will need the following: gumdrop paper towels toothpicks Additional teacher materials:
Assessments The following types of formative assessments are embedded in this lesson: Assessment of prior knowledge. Visual observations of models built during the laboratory experience. Guided questions while facilitating.
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
P A G E S
The following additional assessments are located on the LTF website: Chemistry Assessment: Bonding 2007 Chemistry Posttest, Free Response Question 2; 2008 Chemistry Posttest, Free Response Question 2; 2010 Chemistry Posttest, Free Response Question 2 AP Style Free Response Short Lesson Assessment: Molecular Geometry
T E A C H E R
bag, zipper-lock, quart
Molecular Geometry
Teaching Suggestions Each student pair will need a kit containing 12 gumdrops—two must be the same color and 10 of a different color. If you have model kits with 4, 5, and 6 holed central atoms, they may also be used. If you are using model kits it is a good idea to explain the relationship between the number of holes on the central atom and the sites of electron density in a Lewis structure. Constructing double bonds should also be discussed. This lesson is designed to follow an introduction to Lewis structures for covalent compounds. Students should also have been introduced to the concept of hybridization. During a pre-lab discussion you should demonstrate the Lewis structures and corresponding geometries for several of the example compounds in the reference table on the student pages.
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
P A G E S
These regions get increasingly more repulsive moving down the list. You will find a table of basic VSEPR molecular geometries, along with examples of molecular species that exhibit that molecular geometry, on the student instruction page. Note that lone pairs are more repulsive than any of the bonds. This is because they are only influenced by one nucleus rather than two nuclei. For this reason, lone pairs take up more space and will cause the other bond angles to be smaller. In general, each lone pair will collapse the bond angle by approximately 2o per lone pair.
T E A C H E R
VSEPR (Valence Shell Electron Pair Repulsion) is a simple model that employs the concept that electrons, being negatively charged, are repulsive. Therefore, regions of electron densities will attempt to position themselves as far away from one another as possible. Regions of electron density are as follows: Single bond Double bond Triple bond Lone pair
QUESTIONS 2. BF4−
1. CO2
-
F F
O
C
B
C
F
O
H
F
O
3. H2CO H
Molecular geometry: linear
Molecular geometry: tetrahedral
Molecular geometry: trigonal planar
Hybridization: sp
Hybridization: sp3
Hybridization: sp2
4. PF5
5. SiH4 F
F
P
6. SeF6 F
H
F F
F
F H
Si
H
H
F
Se F
Molecular geometry: trigonal bipyramid
Molecular geometry: tetrahedral
Molecular geometry: octahedral
Hybridization: sp3d
Hybridization: sp3
Hybridization: sp3d2
F F
7. IF4−
8. F2CO
9. XeF4
F
F
F
I
F
F
C
F
O
F
Xe
F
F F
Molecular geometry: square planar
Molecular geometry: trigonal planar
Molecular geometry: square planar
Hybridization: sp3d2
Hybridization: sp2
Hybridization: sp3d2
10. NO2−
11. O3 -
N O
O
-
N O
O
O
O O
O
Molecular geometry: bent (2 resonance structures)
Molecular geometry: bent (2 resonance structures)
Hybridization: sp2
Hybridization: sp2
O
O
12. ClO3−
13. I3−
14. IOF5 (I is the central atom) F
Cl
O
O
F
I
O
I
I
F I
F
F
O
Molecular geometry: trigonal pyramid
Molecular geometry: linear
Molecular geometry: octahedral
Hybridization: sp3
Hybridization: sp3d
Hybridization: sp3d2
15. NH2− -
N H
H
Molecular geometry: bent Hybridization: sp3
Molecular Geometry
Molecular Geometry Investigating Molecular Shapes with VSEPR The shape of a molecule will dictate many physical and chemical properties of a substance. In biological systems many reactions are controlled by how substrate and enzyme molecules fit together. Physical properties of substances, such as solubility and boiling point are also influenced by molecular geometry. PURPOSE In this activity you will draw Lewis structures for a set of molecules and ions. You will then build the molecules or ions from gumdrops and toothpicks to model the correct VSEPR molecular geometry and determine the hybridization of the molecules. MATERIALS Each lab group will need the following: gumdrop paper towels toothpicks
Safety Alert Keep the gumdrops on the paper towel at all times. When you are finished, you may eat the candy if your teacher allows. PROCEDURE 1. All of the substances on your student answer page are covalent molecules or polyatomic ions. 2. Draw Lewis dot structures in the space provided on your student answer page. Use the VSEPR theory to predict the molecular geometry of each molecule or ion listed on your student answer page. 3. Use the gumdrops and toothpicks provided to build each chemical species. Be sure that you use one toothpick for each pair of electrons on the central atom. 4. Write the hybridization of the orbitals in the space provided for each substance.
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
Molecular Geometry
Molecular Geometry Investigating Molecular Shapes with VSEPR VSEPR (Valence Shell Electron Pair Repulsion) is a simple model that employs the concept that electrons, being negatively charged, are repulsive. Therefore, regions of electron densities will attempt to position themselves as far away from one another as possible. Regions of electron density are as follows: Single bond Double bond Triple bond Lone pair These regions get increasingly more repulsive moving down the list. The following table is provided as a reference for basic VSEPR molecular geometries. In the table that follows, M represents the central atom, X represents the terminal or surrounding atoms and E represents lone pairs of electrons. Regions of Electron Density 2 3 3 4 4 4 5 5 5 5 6 6 6
Representative Formula
Example
MX2 MX3 MX2E MX4 MX3E MX2E2 MX5 MX4E MX3E2 MX2E3 MX6 MX5E MX4E2
CO2 BF3 SO2 CH4 NH3 H2O PF5 SF4 ICl3 I3 − SCl6 XeF5+ ICl4−
Molecular Geometry
Hybridization
Linear (180o) Trigonal planar (120o) Bent (118o) Tetrahedral (109.5o) Trigonal pyramidal (107o) Bent (105o) Trigonal bipyramidal See-saw T-shaped Linear Octahedral Square pyramidal Square planar
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
sp sp2 sp2 sp3 sp3 sp3 sp3d sp3d sp3d sp3d sp3d2 sp3d2 sp3d2
Molecular Geometry
QUESTIONS 1. CO2
2. BF4−
3. H2CO
Molecular geometry
Molecular geometry
Molecular geometry
Hybridization
Hybridization
Hybridization
4. PF5
5. SiH4
6. SeF6
Molecular geometry
Molecular geometry
Molecular geometry
Hybridization
Hybridization
Hybridization
7. IF4−
8. F2CO
9. XeF4
Molecular geometry
Molecular geometry
Molecular geometry
Hybridization
Hybridization
Hybridization
10. NO2−
11. O3
12. ClO3−
Molecular geometry
Molecular geometry
Molecular geometry
Hybridization
Hybridization
Hybridization
13. I3−
14. IOF5 (I is the central atom) 15. NH2−
Molecular geometry
Molecular geometry
Molecular geometry
Hybridization
Hybridization
Hybridization
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
