Molecular Geometry
2-2
Ch. 6 – Molecular shape
• Molecules are three-dimensional
objects that occupy a threedimensional world; In general, only
the smallest molecules can be said to
have a fixed geometrical shape; the
icosahedral C60 “soccer ball” is a rare
exception.
VSEPR Theory
• Valence Shell Electron Pair Repulsion Theory
- focuses on the bonding and nonbonding electron
pairs present in the outermost (“valence”) shell
of an atom to which are connected two or more
other atoms.
• Electron pairs orient themselves in order to
minimize repulsive forces.
VSEPR Theory
• Types of e- Pairs
– Bonding pairs - form bonds
– Lone pairs - nonbonding e-
Lone pairs repel
more strongly than
bonding pairs!!!
Determining Molecular Shape
• Draw the Lewis Diagram.
• Shape is determined by the # of
bonding pairs and lone pairs.
Know the 5 common shapes !
1. Common Molecular Shapes
2 bond
0 lone
LINEAR
AB2
180°
Linear molecules: AB2
• Ex: BeCl2 and CO2.
• -If you write out the electron dot formula
for carbon dioxide, you will see that the CO bonds are double bonds.
Double bonds are treated like
single bonds when shape is
determined!!!!!!
Cl
BeCl2
Be
Cl
TWO ELECTRON PAIRS AROUND BERYLLIUM ATOM
180°
270°
Be
Be
90°
180°
The shape of BeCl2 is linear.
THE MOLECULAR SHAPE IS BASED ON
THE POSITION OF THE ATOMS!
• CO2
2 bond
0 lone
O C O
LINEAR
180°
2. Common Molecular Shapes
3 bond
0 lone
AB3
TRIGONAL PLANAR
120°
Trigonal planar: AB3
• In the molecule BF3, there are three
regions of electron density extending out
from the central boron atom. The repulsion
between these will be at a minimum when
the angle between any two is 120°. This
requires that all four atoms be in the same
plane; the resulting shape is called trigonal
planar.
F
BF3
F
B
NO OCTET ON B !
F
THREE ELECTRON PAIRS AROUND THE
BORON ATOM.
MOLECULAR
SHAPE
ATTACH
120°
120°
B
FLOURINES
TO EPG.
F
B
120°
F
F
THE SHAPE OF BF3 IS TRIGONAL PLANAR.
3. Common Molecular Shapes
2 bond
1 lone
AB2E
AB2E2
BENT
<120°
Shape with lone pairs: AB2E2
2 bonding electrons and 2 lone
pairs
or AB2E , 2 bonding electrons and 1
lone pair
• Bent:
• The nonbonding electrons are also in
orbitals that occupy space and repel the
other orbitals.
SeO2
O
Se
O
VSEPR treats double
bonds like a single bond
Se
O
SeO2 IS V-SHAPED, OR BENT
O
RESONANCE!
Se
O
Se
O
O
O
Common Molecular Shapes
2 bond
2 lone
H2O
BENT
104.5°
4. Common Molecular Shapes
4 bond
0 lone
AB4
TETRAHEDRAL
109.5°
Tetrahedral: AB4
• Methane, CH4, contains a carbon atom to
which are connected four hydrogens.
Consequently, the four equivalent bonds
will point in four equivalent directions in
three dimensions.
H
CH4
H
C
H
There are four
H
electron pairs
around the
carbon atom.
90°
90°
C
90°
90°
BUT……….
There is a better arrangement for four electron
pairs:
TETRAHEDRAL
The angle is…..
109.5°
C
Put on the H-atoms…….
There is a better arrangement for four electron
pairs:
TETRAHEDRAL
H
C
H
H
H
4 electron pairs
tetrahedral EPG
The shape of CH4 is tetrahedral.
5. Common Molecular Shapes
3 bond
1 lone
AB3E
TRIGONAL PYRAMIDAL
107°
• PF3
3 bond
1 lone
F P F
F
TRIGONAL
PYRAMIDAL
107°
Polyatomic ions
• The charge will dictate how many less or
extra electrons will be added to the lewis
dot diagrams.
Polyatomic ions
• NO3• the negative charge tells you that you
need to add one more electron to the
diagram.
5 + 3(6) + 1 = 24
Nitrate anion
O
N
O
O
VSEPR treats multiple bonds as effective
single electron pairs.
Trigonal planar is the shape