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Molecular Geometry
MOLECULAR GEOMETRY
VSEPR stands for:
Valence Shell Electron
Pair Repulsion theory.
• Most important factor in
determining the shape
of the molecule is the
repulsion between
electron pairs.
Molecules form
their shapes to
minimize the
repulsions of the
electrons.
*They get as far
away from each
other as possible!
VSEPR Charts
• Use the Lewis Structure to determine the
geometry of the molecule.
• Focus only on the CENTRAL atom for all
molecules! Count the number of electron regions.
• Look for REGIONS WHERE ELECTRONS ARE
FOUND rather than number of bonds.
(Ex: Double and Triple bonds count as 1 region)
Lets look at Methane (CH4). The molecule is
really three dimensional, not planar and flat.
H
C
H
H
H
When models are constructed, they show the
bonds, but not the lone pairs of electrons.
..
109.5° (109.5°)
109.5° (107°)
..
..
109.5° (104.5°)
Some Common Geometries
Structure Determination by VSEPR
Water, H2O • 1) Looking at the
central atom,
••
how many
regions are
••
there?
H O H
The electron
pair geometry is
TETRAHEDRAL
2 bond pairs
2 lone pairs
• 2) How many
bonded atoms?
How many lone
pairs?
The molecular
geometry is
BENT.
Structure Determination by VSEPR
Ammonia, NH3
The electron pair geometry is Tetrahedral , and
the Geometry name is TRIGONAL PYRAMID.