VSEPR
Theory
– Electron dot structures fail to reflect the
three-dimensional shapes of molecules.
– The electron dot structure and structural
formula of methane (CH4) show the molecule
as if it were flat and merely two-dimensional.
Methane
(electron dot structure)
Methane
(structural formula)
VSEPR
Theory
• In reality, methane molecules
are three-dimensional.
• The hydrogens in a methane
molecule are at the four
corners of a geometric solid
called tetrahedral shape.
• In any 5 atom molecule the
shape will be tetrahedral.
VSEPR
Theory
In order to explain the three-dimensional
shape of molecules, scientists use
valence-shell electron-pair repulsion
theory (VSEPR theory).
– VSEPR theory states that the repulsion
between electron pairs causes molecular
shapes to adjust so that the valence-electron
pairs stay as far apart as possible.
VSEPR
Theory
– Unshared pairs of electrons
are also important in
predicting the shapes of
molecules.
Ammonia (NH3)
– The nitrogen in ammonia
(NH3) is surrounded by four
pairs of valence electrons.
– However, one of the
valence-electron pairs is an
unshared pair.
VSEPR
Theory
– No bonding atom is vying for
these unshared electrons; thus,
they are held closer to the
nitrogen than are the bonding
pairs.
Unshared
electron
pair
– The unshared pair strongly repels
the bonding pairs, pushing them
together.
– The shape of a molecule with 4
atoms and 1 pair of unshared pair
of electrons is pyramidal.
107°
•Another shape for a four
atom molecule occurs when
the middle atom has no
unshared pair of electrons to
do any repelling.
•The bonds spread out evenly
to produce a shape called
trigonal planar.
• In a water molecule, the two
the two unshared pairs of
electrons repel the bonded
pairs.
• Thus, the water molecule is
angular or bent shape.
• All 3 atom molecules where
the middle atom has 2 pairs
of electrons repelling, will
produce a bent shape
molecule.
Unshared
pairs
• A 3 atom molecule where the middle
atom has no pairs of electrons to repel
because it has double bonds
• An example is CO2 The double bonds
joining the oxygens to the carbon are
farthest apart when the O=C=O bond
angle is 180°.
– Thus, CO2 is a linear shape.
Carbon dioxide (CO2)
180°
No unshared
electron pairs
on carbon
•Another linear shape
molecule would be a
molecule with only 2
atoms.
Here are some common molecular shapes.
Linear
Tetrahedral
Trigonal planar
Trigonal
bipyramidal
Bent
Octahedral
Pyramidal
Square
planar