MOLECULAR MODELS
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This exercise considers the six molecular geometries of the central atoms C, N and O. You will start with chemical formulas for a variety of molecules and then draw their Lewis structures and build molecular models. For each molecule, you will determine the electron region geometry and the molecular geometry of each central atom. You should also notice how the overall 3-dimensional shape of a molecule depends on the molecular geometries of its central atoms.
The electron region geometry is the 3-dimensional arrangement of electron containing regions around each central atom in the chemical species of interest. An electron containing region is any of the following:
1) single bond (),
2) double bond (),
3) triple bond (),
4) lone pair of electrons (··), or
5) single lone electron (·) in the case of a radical species.
Each of these features counts as one and only one electron containing region. The three electron region geometries that interest us are given in the following table. These geometries result from the electron containing regions staying as far away from each other as possible while still connected to the central atom.
The molecular geometry about a single central atom is based on the electron region geometry about that atom and depends on the number of bonding regions and the number of nonbonding regions. The following table gives a list of the six molecular geometries that are of interest to us in this exercise. Note that these six molecular geometries are based on only three electron region
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# of e- regions |
Electron Region Geometry |
# of bonding domains |
# of nonbonding domains |
Molecular Geometry |
Ideal Bond Angles |
Representative molecules and polyatomic ions |
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2 |
linear |
2 |
0 |
linear |
180 |
HCN, Both C's in C2H2 |
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3 |
trigonal planar |
3 |
0 |
trigonal planar |
120 |
CH2O and CHO2 |
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3 |
trigonal planar |
2 |
1 |
bent |
120 |
O3, NO2 Each of these has only one central atom |
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4 |
tetrahedral |
4 |
0 |
tetrahedral |
109.5 |
CH4, CH2Cl2, NH4+
Both C's in C2H6 |
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4 |
tetrahedral |
3 |
1 |
pyramidal |
109.5 |
NH3, H3O+ The N in N(CH3)3 |
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4 |
tetrahedral |
2 |
2 |
bent |
109.5 |
H2O The
O in CH3OH and CH3OCH3 |
MOLECULAR STRUCTURE AND THREE DIMENSIONAL GEOMETRY
Using a molecular model kit, assemble the following molecules and complete the table below.
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For Each
Central Atom |
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Molecule
and Lewis Structure |
central
atom(s) |
#
of bonding domains |
#
of nonbonding domains |
ideal
bond angle(s) |
molecular
geometry |
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HCN |
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CO2 |
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CHCH,
ethyne |
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CH2O,
methanal |
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CHOOH,methanoic
acid |
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CH2CH2,
ethene |
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CHONH2,
formamide |
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For Each
Central Atom |
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Molecule and Lewis Structure |
central atom(s) |
# of bonding domains |
# of nonbonding domains |
ideal bond angle(s) |
molecular geometry |
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CH4, methane |
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CH3OH, methanol |
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C6H6, benzene (cyclic) |
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CH3CH2CH2CH2CH3 pentane |
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CH3CHCHCH2CH3 2-pentene (note cis and trans) |
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CH3CCCH2CH3 2-pentyne |
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CH3C(O)CH2CH2CH3 2-pentanone |
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Additional Exercises:
A. Build a model of the molecule shown below and answer the following questions. Remember that the octet rule is obeyed by all of the C, N, O and halogen atoms. Note that all of the atoms are given but none of the lone electrons are showing.
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Instructor signature |
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a) How many central atoms have a tetrahedral molecular geometry?
b) How many central atoms have a pyramidal molecular geometry?
c) How many central atoms have a trigonal planar molecular geometry?
d) How many central atoms have a bent (120) molecular geometry?
e) How many central atoms have a bent (109) molecular geometry?
f) How many central atoms have a linear molecular geometry?
g) How many pairs of lone electrons should be added to the picture?
h) Which four-atom portion of the molecule is linear?
i) Which twelve-atom portion of the molecule is planar? Find the other portions of the molecule (4 atoms or more) that are planar.
B. The Lewis structures shown below look like different molecules. Make a model of one of the Lewis structures and show that you can obtain all of the others by rotating the carbon-carbon single bonds.
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C. How many different molecules (isomers) have the chemical formula C2H2Cl2?
D. Make a model of CHBrClI and its mirror image. Note that these two molecules really are different because you can not simultaneously superimpose all of the corresponding atoms. These two mirror image isomers are called enantiomers and the carbon is called a chiral center. Make a model of CH2BrCl and its mirror image. Are these two molecules enantiomers? Are the carbons chiral?
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Show models to your instructor. Instructor signature _________________