12/11/09
Molecular Geometry
Oh how I love those shapes!!!
VSEPR
•  The structure around a given atom is
determined principally by minimizing
electron pair repulsions.
•  In other words, spread everything out as
far apart as possible.
Structure is based on electron
domains.
•  An electron domain is a place where
electrons are.
•  Can be either a bond or a pair of nonbonded electrons.
•  Your yellow table lists all possible
structures with 2 through 6 electron
domains.
1
12/11/09
Linear and Trigonal Planar
•  Linear
–  3 atoms, no lone pairs (AB2), ex. CO2
–  Bond Angles = 180o
•  Trigonal Planar
–  4 atoms, no lone pairs (AB3) ex. BH3
–  Bond Angles = 120o
•  See white board for drawing
2
12/11/09
Tetrahedral (in 3D)
•  4 atoms present, no non-bonding electron
pairs
•  General Formula: AB4
•  Ex. CH4 or CCl4
•  Bond angles: 109.5o
Trigonal Bipyramidal
•  5 atoms present, no non-bonding electron
pairs.
•  General Formula: AB5
•  Bond Angles: 90o and 120o
•  Example: PCl5
3
12/11/09
Octahedral
•  6 atoms present, no non-bonding
electrons.
•  General Formula: AB6
•  Bond Angles: 90o
•  Example: SF6
4
12/11/09
What about compounds w/ nonboding electrons??
•  What about H2O??
Bent or Angular
•  3 atoms and one non-bonding pair of
electrons.
•  Bond Angle: about 116o
•  Example: O3
5
12/11/09
Trigonal Pyramid
•  4 atoms, one non-bonding pair of
electrons.
•  Bond Angle: ~ 107o
•  Example: NH3
Bent or Angular (Part 2)
•  3 atoms, two non-bonding pairs of
electrons.
•  Bond Angle: 104o
•  Example: H2O
6
12/11/09
See - Saw
•  Four atoms, one pair of non-bonding
electrons.
•  Bond Angles: 90o and <120o
•  Example: SF4
7
12/11/09
Square Pyramid
•  6 atoms, one pair of non-bonded
electrons.
•  Bond Angles: <90o
•  Example: BF5
Square Planar
•  5 atoms, two pairs of non-bonding
electrons.
•  Bond Angles: 90o
•  Examples: XeF4
8
12/11/09
9