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1 Determining Molecular Shape 10.7.00 10:09 PM
Constructing Molecular
Shapes
A Tutorial on Writing the Shape of MoleculesA Tutorial on Writing the Shape of Molecules
Dr. Fred Omega Garces Dr. Fred Omega Garces
Chemistry 100Chemistry 100
Miramar CollegeMiramar College
VSEPR- VSEPR- Valence Shell Electron-Pair Repulsion TheoryValence Shell Electron-Pair Repulsion Theory
Main premise of model-
Valence electron pair repel each other in molecule with shapes the molecule Valence electron pair repel each other in molecule with shapes the molecule
Molecule assumes Geometry that minimizes electrostatic
repulsion:
Occurs when electron pair are far apart as possible.
Driving force is the Pauli exclusion principle :
2 electrons with same spin can't occupy the same space.
Electronic Geometry is the geometry around the central
atom in which electron-electron repulsion is minimize.
AEn (system)
Molecular Geometry is geometry around central atom
when electron pairs are replace by bonding atoms and
the nonbonding electrons are ignored.
ABmEn (system)
3 Determining Molecular Shape 10.7.00 10:09 PM
VSEPR-VSEPR-^ Procedural StepsProcedural Steps
1) Determine the1) Determine the Lewis StructureLewis Structure..
a) Valence electrons for each atom in the structure. a) Valence electrons for each atom in the structure.
b) Determine the atomic sequence, the number of bonds, remainingb) Determine the atomic sequence, the number of bonds, remaining
electronselectrons
c) Write Lewis structure with each atom obeying the octet rulec) Write Lewis structure with each atom obeying the octet rule
Example: HNOExample: HNO 33 ((See Lewis Structure TutorialSee Lewis Structure Tutorial))
HH
OO NN OO
OO
VSEPR- VSEPR- (^) Procedural StepsProcedural Steps
2) 2) DetermineDetermine electronic geometryelectronic geometry ((AEAEnn systemsystem)) from Lewis structure.from Lewis structure.
a) Count the electron domain (region) around the central atom.a) Count the electron domain (region) around the central atom.
b) Arrange electron domain to minimize electron-electron repulsion.b) Arrange electron domain to minimize electron-electron repulsion.
Occurs when electron pair are far apart as possible.Occurs when electron pair are far apart as possible.
c) 2-domainc) 2-domaingglinear, 3-domainlinear, 3-domainggtrigonal, 4-trigonal, 4-domaindomainggtetrahedraltetrahedral
Example: Example: (^) HNOHNO 33
Central Atoms Central Atoms ,, NN andand OO
N: Three electron domainN:Three electron domain
AE AE 33 TrigonalTrigonal
O: Four electron domain O:Four electron domain
AE AE 44 TetrahedralTetrahedral
HH
OO NN OO
OO
HH
OO NN^ OO
OO
HH
OO NN^ OO
OO
7 Determining Molecular Shape 10.7.00 10:09 PM
VSEPR Table VSEPR TableVSEPR Table
VSEPR Table: QuickTime MovieVSEPR Table: QuickTime Movie VSEPR Table: QuickTime Movie
SummarySummary Summary
Key to determine molecular GeometryKey to determine molecular Geometry
- Lewis Structure• Lewis Structure
Determine electron domain in structure. Determine electron domain in structure.
- Electronic Geometry• Electronic Geometry
Establish the AE Establish the AEnn system.system.
- Molecular Geometry• Molecular Geometry
Establish the AB Establish the ABmmEEnn system.system.
Don’t count lone pairs in the molecular.Don’t count lone pairs in the molecular.
