Molecular Geometry
Lewis Structures and VSEPR Theory
Lewis Structures
Also known as electron dot
structures
Shows the positions and
bonds of each atom in a
molecule
Example below will be used
to illustrate the 6 steps for
creating Lewis Structures.
Draw the Lewis structure of
iodomethane, CH3I.
Lewis Structures: Step 1
Determine the type and number of
atoms in the molecule.
CH3I
•1 carbon atom
•3 hydrogen atoms
•1 iodine atom
Lewis Structures: Step 2
Write the electron-dot structure for
each type of atom in the molecule.
C
H
I
Lewis Structures: Step 3
Determine the total number of valence electrons
in the atoms to be combined.
1 carbon atom  4 valence electrons  4 valence electrons
3 hydrogen atoms  1 valence electrons  3 valence electrons
1 iodine atom  7 valence electrons  7 valence electrons
4  3  7  14 valence electrons
Lewis Structures: Step 4
Arrange the atoms to form a skeleton structure for
the molecule. If carbon is present, it is the central
atom. Otherwise, the least-electronegative atom is
central (except for hydrogen, which is never
central). Then connect the atoms by drawing
straight-line bonds. Each line represents two
valence electrons.
H
H C H
I
Lewis Structures: Step 5
Make sure the central atom has four bonds
going to it – if it doesn’t add double or triple
bonds to get there. Add unshared
electrons so that each hydrogen atom
shares a pair of electrons and each other
nonmetal is surrounded by eight electrons.
H
H C H
I
Lewis Structures: Step 6
Count the electrons in the structure to be
sure that the number of valence electrons
used equals the number available.
4 bonds  2 electrons each  8 electrons
3 lone pairs  2 electrons each  6 electrons
8  6  14
H
H C H
I
Resonance Structures
Sometimes we can draw two or more
equally valid Lewis structures for a
given molecule.
O
O
O
O
O
O
Resonance Structures
In the past, chemists thought the actual
molecule switched between these two
structures – the molecule resonated
between the two options
Modern tests show that the molecule has
two bonds of equal length (a 1.5 bond)
We can’t draw a 1.5 bond, so we draw the two
resonance structures instead
Put a double-head arrow between the two
choices
VSEPR Theory
Valence Shared Electron Pair
Repulsion theory
Electrons have negative charges
Negatives repel other negatives
This repulsion means the electron
pairs around an atom spread out as
much as possible
We can use this to predict the shape of
any molecule
Number of atoms Number of lone
bonded to the
pairs on the
central atom
central atom
4
0
Shape
tetrahedral
Tetrahedral
Number of atoms Number of lone
bonded to the
pairs on the
central atom
central atom
Shape
4
0
Tetrahedral
3
0
Trigonal planar
Trigonal Planar
Number of atoms Number of lone
bonded to the
pairs on the
central atom
central atom
Shape
4
0
Tetrahedral
3
0
Trigonal planar
3
1
Trigonal pyramidal
Trigonal Pyramidal
Number of atoms Number of lone
bonded to the
pairs on the
central atom
central atom
Shape
4
0
Tetrahedral
3
0
Trigonal planar
3
1
Trigonal pyramidal
2
0
Linear
Linear
2 atoms only
Number of atoms Number of lone
bonded to the
pairs on the
central atom
central atom
Shape
4
0
Tetrahedral
3
0
Trigonal planar
3
1
Trigonal pyramidal
2
0
Linear
2
2
Bent
Bent
Predict these shapes
NH3
H
••
N
H2O
H
H
trigonal
pyramidal
H
••
O H
••
bent
CO2
••
•• O
•• •
C O•
linear