Example: Write the Lewis structure for
carbon tetrachloride (CCl4)
Step 1. Determine the number of valence electrons for all
atoms in the molecule or ion
C: [He] 2s22p2 -- four valence electron
Cl: [Ne] 3s23p5 -- seven valence electrons
total of 32 valence electrons
Lewis structure of carbon tetrachloride
Step 3. Subtract two electrons for each single bond used
in Step 2 from the total number of valence electrons
Lewis structure of carbon tetrachloride
Step 4. Distribute remaining electrons as electron pairs
(pairs of dots) around each atom to give each atom a noble
gas configuration
: :
: : :
: :
: :
:
8 electrons
Place two dots between the carbon and chlorine atoms to
form the covalent bonds
:
Cl
Cl:C:Cl
Cl
-
Cl
Cl:C :Cl
Cl
:Cl:
Cl:C :Cl
:Cl:
8 electrons
32 total
valence
electrons
Step 2. Write the skeletal arrangement of the atoms and
connect them with a single covalent bond (two dots or one
dash)
=
24 electrons
left to complete
the structure
Covalent bonds:
single, double, and triple
In a single bond
• one pair of electrons is shared.
In a double bond
• two pairs of electrons are shared
In a triple bond
• three pairs of electrons are shared
:
C + 4 Cl
Lewis structure of carbon tetrachloride
: :
Lewis structures of compounds
carbon atom has noble gas
configuration (8 electrons)
chlorine atoms have noble gas
configuration (8 electrons each)
Write a Lewis structure for CO2
Step 1. Determine the total number of valence electrons
for the atoms in the compound
C: 4 valence electrons
O: 6 valence electrons
4 + ( 2 x 6 ) = 16 total valence electrons
Write a Lewis structure for CO2
Write a Lewis structure for CO2
Step 2. The two O atoms are bonded to a central C atom.
Write the skeletal structure and place two electrons
between the C and each oxygen.
Step 3. Subtract the four electrons used in Step 2 from 16
(the total number of valence electrons) to obtain 12
electrons yet to be used.
O:C:O
O:C:O
16 valence electrons - 4 electrons = 12 electrons left
Write a Lewis structure for CO2
:O:C:O:
:O:C:O
O::C::O
6 6
6
6
electrons
electrons
electronselectrons
: :
4
electrons
:O:C:O:
Structure I
: :
III
:
: :
::
: :
: :
:O:C:O:
II
:
:
: :
I
Step 5. Remove one pair of unbonded electrons from each O
atom in structure I and place one pair between each O and the C
atom forming two double bonds.
: :
Step 4. Distribute the 12 electrons (6 pairs) around the
carbon and oxygen atoms. Three possibilities exist.
: :
Write a Lewis structure for CO2
Many of the atoms in these structures do not have eight
electrons around them.
Write a Lewis structure for CO2
Examples of multiple covalent bonds
Step 5. Remove one pair of unbonded electrons from each O
atom in structure I and place one pair between each O and the C
atom forming two double bonds.
O O
O C O
carbon dioxide
( CO2 )
Each atom now has 8
electrons around it
H
double bond
: :
: :
O::C::O
double bond
Carbon is sharing 4
electron pairs
H
C C
ethene
( C2 H 4 )
diatomic oxygen
( O2 )
H
H
N N
diatomic nitrogen
( N2 )
Example: Nitrate (NO3-)
Na
Sodium nitrate has both ionic and covalent bonds
• covalent
bonds
present
the
ionic bonds
exist are
between
the between
sodium ions
nitrogen
and oxygen
and the nitrate
ions atoms within the nitrate
ion
Na
+
:O
:O
N O:
covalent
bond
covalent
bond
+
:O
-
:O
N O:
When sodium nitrate is dissolved in water, the
ionic bond breaks
The sodium
ionswhich
and nitrate
separate
•• The
nitrate ion,
is heldions
together
by
from
each
other
forming
separate
sodium
covalent bonds, remains as a unit
and nitrate ions
-
: :
:
:
: :
ionic
bond
nitrate ion
sodium ion Na+
: :
:
:
: :
A polyatomic ion is a stable group of atoms
that has either a positive or negative charge
and behaves as a single unit in many
chemical reactions
Sodium nitrate ( NaNO3 ) contains one
sodium ion and one nitrate ion
Na
+
Na
+
:O
-
:O
: :
:
:
:: ::
:
:
: :
Compounds containing polyatomic ions
:O
-
O: N O:
N :O
covalent
bond
Molecular shape
The 3-dimensional arrangement of the atoms within a
molecule significantly affects its chemical properties
Molecular shape
The 3-dimensional arrangement of the atoms within a
molecule significantly affects its chemical properties
A relatively simple model can be used to predict the
shape of a molecule from its Lewis structure
• remember that the Lewis structure itself does not give
any information about the three-dimensional shape
of a molecule
The valence shell electron pair
repulsion model (VSEPR)
Method for determining the shape of a molecule
Step 1: Write the Lewis structure of the molecule
Step 2: Count the number of electron groups (bonds and
lone pairs) around the central atom
The VSEPR model is based on the following:
• electron pairs in a molecule repel each other
electrically and will seek to minimize this repulsion
• single bonds and multiple bonds (double, triple) are treated as
one electron group
Step 3: Arrange the electron groups around the central atom
to minimize repulsion
• to accomplish this, the electron pairs will be
arranged as far apart from each other as possible
around the central atom
• two groups: linear
• three groups: trigonal planar
• four groups: tetrahedral
Step 4: Determine the shape of the molecule from the
number of lone pairs of electrons
Two electron groups: linear
Two electron groups: linear
To minimize repulsion, the arrangement of the two electron groups
is 180° around the central atom (directly opposite each other)
To minimize repulsion, the arrangement of the two electron groups
is 180° around the central atom (directly opposite each other)
Example: BeCl2
Example: CO2
180° electron group arrangement
Cl Be Cl
Two electron groups
around central atom
Cl
Be
Cl
The shape of the molecule is linear
Three electron groups: trigonal planar
To minimize repulsion, the three electron groups are arranged
at the vertices of a triangle centered on the central atom
Example: BF3
F
F B F
trigonal planar electron
group arrangement
F
120°
F
Three electron groups
around central atom
180° electron group arrangement
O
B
F
The shape of the molecule is
trigonal planar
C
O
Two electron groups
around central atom
(double bonds count
as one electron group)
C
O
O
The shape of the molecule is linear
Three electron groups: trigonal planar
To minimize repulsion, the three electron groups are arranged
at the vertices of a triangle centered on the central atom
Example: SO2
trigonal planar electron
group arrangement
O S
~120°
O
lone pair
O
S
O
Three electron groups
around central atom
The shape of the molecule is bent
Four electron groups: tetrahedral
To minimize repulsion, the four electron groups are arranged at the
vertices of a tetrahedron centered around the central atom
•Four electron groups
•No lone pairs
Molecule shape is tetrahedral
Ammonia, NH3, has four electron pairs
around nitrogen.
The arrangement
of electron pairs
around nitrogen is
tetrahedral
NH3 has one
unbonded pair
of electrons
The NH3 molecule
is pyramidal
Water has four electron pairs around oxygen
The arrangement
of electron pairs
around oxygen is
tetrahedral
H2O has two
unbonded pairs
of electrons
The H2O molecule
is bent
VSEPR summary
Number of
electron
pairs
Electron pair
arrangement
Number of
non-bonding
electron pairs
(lone pairs)
Lewis structures
Molecular
shape
2
linear
0
Linear
3
trigonal planar
0
trigonal planar
3
trigonal planar
1
bent
4
tetrahedral
0
tetrahedral
4
tetrahedral
1
pyramidal
4
tetrahedral
2
bent
Homework Assignment
Chapter 5:
Problems 5.59c-f, 5.61, 5.64, 5.65, 5.67, 5.68a-e,
5.71, 5.72
Molecular models (3-D shapes based on VSEPR model)