Ch. 8
Molecular Shapes
MOLECULAR
GEOMETRY
aka
MOLECULAR GEOMETRY
VSEPR
• Valence Shell
Electron Pair
Repulsion theory.
• Most important factor
in determining geometry
is relative repulsion
between electron pairs.
Molecules
adopt the
shape that
minimizes the
electron pair
repulsions.
Electron clouds of Lone Pairs take up
much more space than electron clouds
of Shared Pairs and, so repel with more
force.
(Shared pairs of electrons are trapped
in a rather restricted cloud between
the nuclei of the atoms sharing them –
they can’t spread out much)
Predicting a VSEPR Structure
Draw Lewis structure.
Put electron pairs as far apart as
possible around atoms.(Shared pairs
must stay together!)
Determine positions of atoms from
the way electron pairs are shared.
Determine the name of geometric
structure (shape!) from #’s of shared
& unshared pairs of e-’s
Structure Determination by VSEPR
If there are only 2 atoms in
the molecule:
Diatomic – 2 like atoms
Binary - 2 different atoms
Can only be Linear in shape
Structure Determination by VSEPR
If there are only 3 atoms in the molecule:
Two shapes are possible (based on #’s of
shared & lone electron pairs around central
atom!)
Linear & Angular (bent)
If: 2 shared pairs & no lone pairs
(around central atom!)
• With three atoms the
farthest they can get
apart is 180º.
• Shape called
Linear.
Ex. CO2
180º
O C O
.O
.
C
.
O.
If: 2 shared pairs & 1 or 2
lone pairs (around central atom!):
Ex. Water, H2O
104.5°
••
H O H
••
2 shared
pairs
2 lone
pairs
The molecular
geometry is
BENT or
ANGULAR.
Common Molecular Shapes
2 bonding pairs
1 (or 2) lone pairs
SO2
BENT or Angular
<120°
If: 3 atoms attached to a central atom:
2 shapes are possible:
Trigonal Planar (3 shared, 0 unshared)
Trigonal Pyramidal (3 shared, 1 unshared)
If: 3 shared pairs & no lone pair
the bonds are all identical, so push apart
with equal force (equidistant apart)
• The farthest you
can the electron
pairs apart is 120º.
H
H
C O
H
H
C
120º
O
•Shape is flat and called
TRIGONAL PLANAR.
Common Molecular Shapes
3 bonding
0 lone
BF3
B
TRIGONAL PLANAR
A
B
B
120°
If: 3 shared pairs & 1 unshared pair
around central atom:
One lone pair
3 shared pairs
Ammonia, NH3
H N H
H
H
N
H
H
<109.5º
The MOLECULAR GEOMETRY — the
positions of the atoms — is TRIGONAL
PYRAMIDAL.
The lone pair’s electron cloud
takes up more space then the
shared pairs’ clouds
3 bonding
1 lone
NH3
TRIGONAL PYRAMIDAL
107°
Examples
• PF3
3 shared
pairs
1 lone
pair
F P F
F
TRIGONAL
PYRAMIDAL
107°
If: 4 atoms around a central atom:
4 shared pairs and no lone pair
(all bonds the same – so equidistant apart)
• Basic shape is
Tetrahedral.
• A pyramid with a
109.5º
triangular base.
• Same basic shape
for everything
with 4 pairs.
H
H
Ex. CH4
C
H
H
B
4 bonds
0 lone pair
A
B
B
B
CH4
TETRAHEDRAL
109.5°
Methane building blocks
CH4
Bonding and Shape of Molecules
Number
of Bonds
Number of
Unshared Pairs
0
3
0
4
0
3
1
2
2
Shape
Examples
-Be-
Linear
BeCl2
Trigonal planar
BF3
Tetrahedral
CH4, SiCl4
Pyramidal
NH3, PCl3
Angular (Bent)
H2O, H2S, SCl2
B
C
:
2
Covalent
Structure
:
N
O:
Polar Molecules
(Not Polar Bonds!)
(We already did those!)
Molecules with slightly charged ends!
Polar Molecules (Dipoles)
• Molecules with a partially positive end
and a partially negative end
• Requires two things to be true
 The molecule must contain polar bonds
(This can be determined from
differences in electronegativity.)
Symmetry cannot cancel out the effects
of the polar bonds.
So….Must determine geometry first.
Polarity
1. Draw structural formula in correct shape;
2. Add partial charges
(based on electronegativity difference);
3. Can you divide it into a + half & a – half?
+
-
H F
+

Polar Molecules
• Symmetrical shapes are those
without lone pair on central atom
– Tetrahedral
– Trigonal planar
– Linear
• Will be nonpolar if all the atoms
(around central atom) are the same
• Shapes with lone pair on central
atom are not symmetrical
• Will be polar
Determining Molecular Polarity
• Polar molecules have...
– asymmetrical shape (lone pairs) or
– asymmetrical atoms
+
CHCl3
Cl
-
Shows direction electrons
are attracted to
H
Cl
Cl
Yes! So molecule is Polar
Molecular Polarity
• Can you draw a straight
line through this molecule
& get a + & a – side?
Yes! So molecule is Polar
Nope! SO molecule is non-polar
even though bonds are polar!
Nonpolar Covalent Molecules
• Equal distribution
of charge around
a central atom.
• Molecule has a
symmetrical shape
Polar Molecules
..
F
N
O
Cl
H
H
Polar
H
F
Polar
H
B
Polar
Cl
F
F
Cl
Polar
F
H
C
Xe
F
Cl
F
Cl
F
H
F
Nonpolar
H
F
Nonpolar
C
Cl
Cl
Nonpolar
H
H
Polar
Diatomic molecules - Only 100%
pure Nonpolar Covalent bonds• Diatomic molecules include:H2, N2, O2,
F2, Cl2, Br2, I2
• These seven elements ALWAYS exist
as diatomic molecules if they are:
– NOT part of a compound AND
– NOT an ion.
BrINClHOF or HON 7
3. Helpful hints and practice:
A. Hints to help you decide if a molecule is
POLAR:
1. Does it have at least one polar bond
If so, it's probably polar.
2. Does it have any unshared pair of electron
around the central atom? If so, it is
probably polar.
3. Can the molecule act like a magnet? If so,
it is probably polar.
8.4
Polar Molecules
• A hydrogen chloride molecule is a dipole.
-
+
+ -
H-F
Molecular Polarity
• This is why oil and water will not
mix! Oil is nonpolar, and water
is polar.
• The two will repel each other,
and so you can not dissolve one
in the other
Molecular Polarity
• “Like Dissolves Like”
–Polar dissolves
Polar
–Nonpolar
dissolves
Nonpolar