Name:
Lab Section:
HOMEWORK 11—MOLECULAR MODELS
Draw best Lewis dot structures, considering formal charge and octet rules. Remember, the least
electronegative atom is most often the central atom.
a. CS2
b. CH3−
c. BrO3-
(Continued) Draw best Lewis dot structures, considering formal charge and octet rules. Remember, the
least electronegative atom is most often the central atom.
d. O3
e. NO3
f.
XeF42+
11
MOLECULAR MODELS
BACKGROUND
Valence Shell Electron Pair Repulsion (VSEPR) theory provides a way to predict the shape of molecules.
The underlying theory suggests that there is mutual repulsion of electron pairs between electrons that
connect atoms and electrons that are non-bonding pairs. Basically, the atoms and electron pairs want to
spread out as far as they can around the central atom. As a scientist, the shape of molecules matter. There
is a big difference between electronic geometry and molecular geometry.
For determining the electronic geometry, bonding electron pairs and non-bonding electron pairs are
treated the same way. For example, ammonia, :NH3, has four groups (see figure 11.1a), three hydrogens
and one electron pair around nitrogen. Each group repels, or pushes the other groups as far as they can
away from each other around the atom. (See figure 11.1b.) These four groups create a tetrahedral
electronic geometry where each group sits approximately 109.5° from each other. In real life, the nonbonding electron pairs actually occupy a larger volume of space than bonding pairs. As a result the angle
between the non-bonding electron pair and the hydrogen atoms is larger than predicted. The H-N-H
angles become smaller at ~107°.
For molecular geometry, the electron pair is still physically present and affects the shape of the molecule,
but we can’t actually see the electrons, only the atoms. Therefore, we only look at how the atoms are
placed relative to each other. The central nitrogen atom has three atoms and one electron pair, so the
molecular geometry is trigonal pyramidal. (See Figure 11.1.c.)
Notice how in Figure 11.1, the drawings have the hydrogen atoms below the nitrogen. This is a more
accurate drawing of how the real life tetrahedral shape is. Conventionally, to show a 3D drawing, chemists
use wedges and dashes. A wedge means the atom is coming toward the viewer, a dash means the atom
is going away from the viewer, and a line show the atom in the plane of the paper. (See Figure 11.1.d.)
This type of drawing is the most accurate to display shape.
These rules result in a relatively small number of specific arrangements of electron groups around the
central atom, regardless of whether they are atoms or non-bonding electron pairs. In this simulation, you
will be able to see what molecules look like in 3D and what those electron and molecular geometries are
called.
Figure 11. 1. a. A Lewis structure drawing of ammonia, NH3. b. A representation of the electron clouds
taking space around the central nitrogen atom. c. Drawing of ammonia representing the molecular
geometry. d. A 3D VSEPR drawing of ammonia.
PROCEDURE
You will use the molecule shapes simulation for the lab. Click on the link below and then on the play
button. http://phet.colorado.edu/en/simulation/molecule-shapes Choose “Model”. Using this screen,
fill out the tables below. For the electron geometry, write the name of the geometry and draw wedge and
dash images to show 3D, for molecular geometry, write the name and also the bond angles.
Number of
Groups
Use the Model Simulation to fill in the table below.
Electron Geometry
Molecular Geometry
Number of Lone
Draw Wedge and Dash
Write name
Pairs
Write name
Write bond angles
2
0
3
0
3
1
4
0
4
1
4
2
Part I (cont). Use the Model Simulation to fill in the table below.
Electron Geometry
Molecular Geometry
Number of
Number of Lone
Draw Wedge and Dash
Write name
Groups
Pairs
Write name
Write bond angles
5
0
5
1
5
2
5
3
6
0
6
1
6
2
PART 2: COMPARING MODEL VS. REAL MOLECULES
Use same simulation program, this time, choose “Real Molecules”. It may be found at the bottom of
the screen. For the electron geometry, write the name of the geometry and draw wedge and dash
images to show 3D, for molecular geometry, write the name and also the bond angles.
Molecule
H2O
CO2
SO2
XeF2
BF3
ClF3
Part II. Use the Real Molecules Simulation to fill in the table below.
Electron Geometry
Number
Molecular Geometry
Number
Draw Wedge and
of Electron
Write name
of Groups
Dash
Groups
Write bond angles
Write name
Molecule
NH3
CH4
SF4
XeF4
BrF5
PCl5
SF6
Part II (cont). Use the Real Molecules Simulation to fill in the table below.
Electron Geometry
Number
Molecular Geometry
Number
Draw Wedge and
of Electron
Write name
of Groups
Dash
Groups
Write bond angles
Write name
ANALYSIS
PART I: Model Simulations Questions
a. How did adding an atom affect the position of existing atoms or lone pairs?
b. How did adding a lone pair affect the position of existing atoms and lone pairs?
c. Is the effect of adding bonded atoms and lone pairs to the central atom similar or different?
Explain why the changes happen.
PART 2: Real Molecule Simulations Questions
a. Compare the tables you created for the “Model” and “Real” section. List the molecules that show
a difference in bond angle between the two models. Note: differences in bond angle may be small.
b. What conclusions can you draw from your list?
c. For your homework, you drew the best Lewis dot structures of the following molecules.
Draw these molecules again, this time in 3D with wedges and dashes. Based on your
knowledge, assign “real” bond angles. Also state the electron and molecular geometries.
CS2
O3
CH3−
NO3
BrO3-
XeF42+