Experiment
10
Molecular Geometry and Molecular Models
molecular geomometry background.w pd
INTENT
The purpose of this experiment is to introduce to you some of the basic theories and techniques used by
chemists to predict the electronic structure and geometries of molecules and complex ions. These
include Lewis structures and Valence Shell Electron Pair Repulsion (VSEPR) Theories, as well as the
concepts of hybridization, isomerism, and resonance.
INTRODUCTION
You have been studying the rules which help us to predict whether combinations of atoms produce ionic
or covalent substances and what the formulas of those substances might be. Equally important to the
chemical and physical properties of many substances is the arrangement of their atoms in space (their
geometries). Biology courses have taught you that the shapes of DNA, proteins and other biological
molecules determine their biological behavior. Earlier in CHM1025, you saw that a variation in the
arrangement of carbon atoms produces the great difference in properties of transparent, colorless
diamond and opaque, black graphite. Similarly, the arrangement of atoms in a simple molecule like
carbon dioxide can determine its molecular polarity, and by this, its melting temperature (mp), boiling
temperature (bp), heat of fusion, heat of vaporization, and solubility characteristics.
DISCUSSION
To start, you will be asked to draw Lewis diagrams for specific molecules and certain ions that contain
covalent bonds. As you go through these exercises, some new concepts will be introduced. You will
have to consider the existence of isomers and resonance forms in some of the examples. You will then
predict the geometry of the ions and molecules in terms of Valence Shell Electron Pair Repulsion
(VSEPR) theory (see Class Handout) and to describe the orbitals used for bonding by the central atom
using atomic orbital hybridization theory (CHM2045 topic). You will also actually assemble 3-D
models of all of the examples and from these draw conventional 2-D pictures of the 3-D structures.
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Data / Report
Experiment 10
Name:
Partner:
CHM 1025
Section:
Date:
Molecular Geometry and Molecular Models
EXPERIMENTAL
Part A: Lewis Diagrams and Geometry
Follow the procedure outlined below, first by practicing with the lettered, worked-out examples, A!E,
on page 101 and then with the numbered, unknown chemical formulas, 1-25, beginning on page 102.
1. To begin, count up the total number of valence electrons possessed by the atoms in your formula.
Place the number in the column labeled "# of val. e& 's."
2. On a separate sheet of scrap paper, draw a correct Lewis of your formula. Use a dash for each
shared pair of valence electrons and a pair of dots for each non-bonding pair of valence electrons.
Check for the correct number of atoms and the number of valence electrons. Draw only those
structures which have minimum energies, as determined by formal charge considerations,
possible obeyance of the noble gas configuration, and the tendency to avoid small ring
strain.
Transpose this diagram to the third column, labeled "Structure of one isomer or resonance form."
When the formula you are working on shows the existence of isomers or resonance forms, draw
each of these in a column labeled " Additional isomer or resonance form."
3. For each isomer construct a molecular model with your kit, including the non-bonding electrons
(one plastic connector for each pair of electrons). Use a short connector for a single bond and
two long connectors if a double bond exists. Using conventional atomic symbols--dashes, dotted
lines, wedges and dots (see the examples on page 101)--carefully sketch the model of one of your
isomers or resonance forms in the column labeled "3-D Structure of one isomer or resonance
form."
4. For each of your 3-D structures, observe the number of electron "zones" (sigma bonds and lone
pairs only) around the central atom and use the principles of the VSEPR theory to determine
the spatial geometry of the electron zones. Enter the name of this geometry in the column
labeled "Name of molec. geometry."
5. Now, for each of your 3-D structures, determine the hybridization of the central atom(s) (e.g.,
sp3, sp3d2, etc.) and place this information in the column labeled "Central atom(s) hybridization."
6. Finally, to determine the overall polarity of each MOLECULE, begin by determining each bond
dipole moment by comparing the electronegativities of the two atoms in the bond. Then, using
your 3-D VSEPR geometry, determine whether the overall molecule is polar or non-polar. If the
molecule is polar, label that structure with a "P," and if it is non-polar, label it with an "NP."
The concept of molecular polarity applies only to molecules and, therefore, if the species is an
ion it should be labeled as "DNA" for "does not apply."
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Formula
# of
val.
e& 's
Lewis structure of one
isomer or resonance form
Lewis structure of an
additional isomer or
resonance form
Lewis structure of an
additional isomer or
resonance form
(IF NECESSARY)
(IF NECESSARY)
3-D Structure
of one isomer or
resonance form
If molecular, indicate
polarity with P or NP
Name of molec
geometry
Central
&
atom(s)
hybridization
Tetrahedral
A. CH4
8
sp3
Nonpolar
Tetrahedral
on C
B. C2 H4 Cl2
26
sp3
Polar
Trigonal Planar
on C
C. C2 H2 Cl2
24
sp2
Polar
Trigonal Planar
on C
D. [CO3 ]–2
24
sp2
Octahedral
on S
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E. SF6
Nonpolar
sp3d2
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Formula
# of
val.
e& 's
Lewis structure of one
isomer or resonance form
Lewis structure of an
additional isomer or
resonance form
Lewis structure of an
additional isomer or
resonance form
(IF NECESSARY)
(IF NECESSARY)
1. CH3 Cl
2. CH2 Cl2
3. NH3
4. [H3 O]+
5. H2 O
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3-D Structure
of one isomer or
resonance form
If molecular, indicate
polarity with P or NP
Name of molec
geometry
Central
&
atom(s)
hybridization
Formula
# of
val.
e& 's
Lewis structure of one
isomer or resonance form
Lewis structure of an
additional isomer or
resonance form
Lewis structure of an
additional isomer or
resonance form
(IF NECESSARY)
(IF NECESSARY)
6. C2 H6
7. C4 H10
8. C3 H8 O
9. C2 H2
10. N2
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3-D Structure
of one isomer or
resonance form
If molecular, indicate
polarity with P or NP
Name of molec
geometry
Central
&
atom(s)
hybridization
Formula
# of
val.
e& 's
Lewis structure of one
isomer or resonance form
Lewis structure of an
additional isomer or
resonance form
Lewis structure of an
additional isomer or
resonance form
(IF NECESSARY)
(IF NECESSARY)
11. BeCl2
B e is central atom
12. CO2
C is central atom
13. [NO3 ]–
N is central atom
14. N2 O
N is central atom
15. O3
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3-D Structure
of one isomer or
resonance form
If molecular, indicate
polarity with P or NP
Name of molec
geometry
Central
&
atom(s)
hybridization
Formula
# of
val.
e& 's
Lewis structure of one
isomer or resonance form
Lewis structure of an
additional isomer or
resonance form
Lewis structure of an
additional isomer or
resonance form
(IF NECESSARY)
(IF NECESSARY)
3-D Structure
of one isomer or
resonance form
If molecular, indicate
polarity with P or NP
16. SO2
S is central atom
W ithout m aking a m odel
17. CH3 CO2 &
T his ion is called
the A cetate ion
18. BrF3
B r is central atom
19. [XeF5 ]+
Xe is central atom
20. [BrF4 ]–
B r is central atom
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Name of molec
geometry
Central
&
atom(s)
hybridization
Formula
# of
val.
e& 's
Lewis structure of one
isomer or resonance form
Lewis structure of an
additional isomer or
resonance form
Lewis structure of an
additional isomer or
resonance form
(IF NECESSARY)
(IF NECESSARY)
21.
22.
23.
24.
25.
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3-D Structure
of one isomer or
resonance form
If molecular, indicate
polarity with P or NP
Name of molec
geometry
Central
&
atom(s)
hybridization
Data / Report
Experiment 10
CHM 1025
Name:
Partner:
Section:
Date:
Molecular Geometry and Molecular Models
1) As the bonding between two atoms changes from single to double, then to triple, what happens to the strength holding the two atoms together?
#1
2) Given the VSEPR notation for the following compounds state if each is polar or nonpolar. Explain why for each.
a) SF2 , AX2E2
b) PF41! , AX4E
c) SCl2F4 , AX6
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Part B: Modeling of Chemical Reactions
For each of the following examples, begin by filling in the blanks to balance the chemical equation.
Then construct the reactants (all of them) with your model kit and sketch them beneath their respective
formulas. Now tear them apart and, using the same balls and connectors, construct the products (all of
them) and sketch them. (You may wish to substitute a small connector for a long connector but do not
use any additional connectors or balls.) You have now visualized the conservation of mass. You may
also now see the necessity in a balanced equation for the correct formulas and coefficients which provide
the same number and type of atoms on both side of the reaction.
1. Combustion of hydrogen:
(unbalanced) ____ H2
____ O2
+
)
xxv
____ H2O
2. Combustion of methane:
(unbalanced) ____ CH4
+
____ O2
)
xxv
____ H2O
+
____ CO2
In the following examples it will be necessary to use a different-colored ball for the phosphorus
atom whenever the number of bonds to phosphorus changes. Use a blue-colored ball for
phosphorus in PF3, and then substitute a brown-colored ball for phosphorus in PF5. Finally,
substitute a silver-colored ball for phosphorus in PF6& .
3. Reaction of fluorine and phosphorus trifluoride:
(unbalanced) ____ PF3
+
____ F2
xxv
____ PF5
4. Reaction of fluoride ion and phosphorus pentafluoride:
(unbalanced) ____ PF5
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+
____ F&
xxv
____ PF6&
Finally, remember that you have been working with models (models that emphasize molecular geometry
only). Atoms are not hard, plastic spheres and electron pairs are not plastic connectors. With this mind,
describe below what your models do not reveal about, or how they give the wrong impression of, real
molecules and polyatomic ions.
OK:
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