Developmental Lesson FC #1.6: Molecular Shapes – Lewis Dot Structures and VSEPR Theory Teacher Notes and Materials 1.6: Ways atoms are held together
Goal Facet 03: The student understands that bonding and non-­‐bonding electrons around a central atom will repel each other as much as possible, leading to 3D molecular shapes. Materials:
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Student Handout
Beans or peas or coins (to represent electrons)
Magnetix or model kit
Internet access for Part III (optional)
Background:
Molecular compounds are composed of two or more nonmetals sharing electrons. Recall that
because electrons are negatively charged they repel each other and it is this repulsion that
determines the shape of a molecule. In this lesson you will learn how to use the Valence Shell
Electron Pair Repulsion Theory, VSEPR Theory, to predict molecular shapes, also called
molecular geometries.
The first step in predicting a molecule’s shape is to draw a valid Lewis dot structure for the
molecule.
Rules for writing Lewis Dot structures:
Read the handout titled: Lewis Dot Structure and Molecular Geometry Tutorial
This handout was written by Dr. Steve Borick at Scottsdale Community College and is provided
with his permission. (http://faculty.scottsdalecc.edu/borick/)
Activity:
Part I: Lewis dot structures
1. Cut out the element symbols from the last page. These will represent the nuclei and inner
electrons for each element.
2. Using the beans (or other small objects) to represent electrons and the cutouts of elements
symbols, create Lewis dot structures for the following molecules. Record the structures in
the table on next page.
BeBr2
CH4
H2S
AsCl3
NH2-
BCl3
1 Developmental Lesson FC #1.6: Molecular Shapes – Lewis Dot Structures and VSEPR Theory Teacher Notes and Materials Part II: Molecular Geometries (Magnetix)
Part II: Molecular Geometries (Model Kits)
1. Using the Magnetix magnets, build the
molecules from Part 1. Use a metal ball to
represent the central atom. Use one color
of magnet sticks to represent lone-pair
electrons, and then other colors to
represent the atoms attached to the central
atom.
2. Remove the lone-pair electrons from each
molecule.
3. Predict the geometric shape of each
molecule and record the shapes in the table
below. Use the attached chart and
Magnetix model to help.
1. Using the Model kit, build the molecules
from Part 1. Use the ball with 4 holes to
represent the central atom. Connect the other
atoms. Then use a stick with no atom
attached to the other end to represent lonepair electrons.
2. Remove the lone-pair electrons from each
molecule.
3. Predict the geometric shape of each
molecule and record the shapes in the table
below. Use the attached chart and Magnetix
model to help.
Data Table
Molecule
Lewis Dot Structure
VSEPR Geometry
BeBr2
CCl4
H2S
AsCl3
NH2-
BCl3
2 Developmental Lesson FC #1.6: Molecular Shapes – Lewis Dot Structures and VSEPR Theory Teacher Notes and Materials VSEPR Geometries Chart
*Steric Number can be thought of as the number of places where electron pairs
can exist around the central atom. A double bond is considered one place where electrons exist.
For example, in carbon dioxide with the structure show here, the steric number for the carbon is 2.
The steric number for each oxygen is 3.
Part III (optional): Exploring 3-D computer models
Visit the website listed here to explore 3-D models of simple molecules. Once the page has loaded,
you can rotate the molecules, zoom in and out and view or hide lone pair electrons. Have fun
exploring!
http://undergrad-­‐ed.chemistry.ohio-­‐state.edu/VSEPR/3d.html 3 Developmental Lesson FC #1.6: Molecular Shapes – Lewis Dot Structures and VSEPR Theory Teacher Notes and Materials Notes to the teacher:
Magnetix can be bought at a toy store or Amazon.com in
various sizes from 35 pieces to 150 pieces. A set of 85 pieces
(makes ~10 tetrahedral models at once) costs ~$30. Remember
to select a color to represent lone pair electrons before
beginning the lesson.
Examples of Magnetix molecules (pink was selected to
represent lone pairs):
H2O with lone pairs
H2O after removing lone pairs
BrCl3
NH3 with lone pair
NH3 after removing lone pair
You can have students build models using molecular model kits however, not all atoms in the kit
have room to put in lone-pair electrons. Only Carbon atoms can be used as the central atom.
Another advantage of using Magnetix is that you can change the bond angles to represent the fact
that the lone pair electrons create stronger repulsive forces than bonding electron pairs.
Here is a great website for having students look at 3-D models of molecules. They can input the
formula to view different molecules and can look at the ball-and-stick model or space-filling
model. They can look at bond lengths, bond angles and dipoles. For students whom are very
curious they can explore vibrational modes and other aspects of molecules.
http://www.chemeddl.org/resources/models360/
4 Developmental Lesson FC #1.6: Molecular Shapes – Lewis Dot Structures and VSEPR Theory Teacher Notes and Materials *Rules for writing Lewis Dot structures can be explained by many different handouts
available online or by using what you already have. The one referred to and used in this lesson,
was reviewed by ChemFacets. It was written by Dr. Steve Borick at Scottsdale Community
College and is provided with his permission. (http://faculty.scottsdalecc.edu/borick/)
VSEPR Geometries for Expanded Octet Molecules and Ions
Note about hybridization in expanded octets:
Research suggests that expanded octets do NOT actually involve empty d orbitals.
1. Xiaoping Sun (2002). "Pentacoordinated AB5-­‐Type Main Group Molecules Favorably Adopt sp2 Hybridization in the Central Atom: Bonding without d-­‐Orbital Participation". Chem. Educator 7: 11–14. doi:10.1007/s00897010525a. 2. Xiaoping Sun (2002). "The Three-­‐Center, Four-­‐Electron Bond in Hexacoordinated AB6-­‐Type Main Group Molecules: An Alternative Model of Bonding without d-­‐
Orbital Participation in the Central Atom". Chem. Educator 7: 261–264. doi:10.1007/s00897020598a
It is important to prepare your students to succeed on standardized tests and therefore necessary
to teach them the material that will be tested. It is also valuable for them to understand the nature
of science, as well as the process whereby new scientific discoveries and new models become
incorporated into the awareness of the greater scientific community and eventually into
textbooks. You may want to have students look into the current discussion about this topic.
An alternative model being considered to explain the bonding in hypervalent molecules such as
SF6 and PCl5 is the Three-center, Four-electron bond model. For further information you can
refer to some of the references listed at this website.
http://www.enotes.com/topic/Three-­‐center_four-­‐electron_bond 5 Developmental Lesson FC #1.6: Molecular Shapes – Lewis Dot Structures and VSEPR Theory Teacher Notes and Materials ANSWER KEY Geometric Predictions
Molecule
VSEPR Geometry
BeBr2
Linear
CCl4
Tetrahedral
H2S
Bent/Angular (one lone pair)
AsCl3
Trigonal Pyramid
NH2-
Bent/ Angular (two lone pairs)
BCl3
Trigonal Planar
6 Developmental Lesson FC #1.6: Molecular Shapes – Lewis Dot Structures and VSEPR Theory Teacher Notes and Materials Atoms for creating Lewis Dot structures with peas or beans:
H
H
H
H
N
C
As
Br Br
B
B
Cl Cl Cl
Cl Cl Cl Cl Cl Cl Cl
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