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Why does ice float?
POLAR BONDS AND
MOLECULES
Electronegativity
Polar Bonds


When involved in a bond, atoms of some elements
attract the shared electrons to a greater extent than
atoms of other elements – This property is called
Electronegativity (EN)
The following chart is used to determine the
electronegativities of each atom
Difference in Electronegativity = ΔEN


In general, electronegativity increases from left to
right and from the bottom up
As atomic radius increases, electronegativity
decreases.


Based on the difference in electronegativities of
atoms we can predict the type of bond that will
form
Formula:

∆EN = |ENA – ENB |

Chart:
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Examples

Potassium Fluoride, KF



Oxygen, O2



∆EN = ENF – ENK = 3.98 – 0.82 = 3.16
IONIC BOND
∆EN = ENO – ENO = 3.44 – 3.44 = 0
NON-POLAR COVALENT

has EN = 3.0
has EN = 2.5
 From this, we say that chlorine has stronger attraction
for electrons than carbon
 Thus, electrons will spend more time around the Cl than
C
C
Carbon Tetrachloride, CCl4
(look at the ∆EN for one of the C-Cl bonds)



Example: Carbon Tetrachloride (CCl4)
 Cl


With respect to polar covalent bonds, the
differences in electronegativity tell us about the
sharing of electrons
∆EN = ENCl – ENC = 3.0 – 2.5 = 0.5
POLAR COVALENT
This results in a slight separation of positive and
negative charges which we call “partial charges”
and represent them as δ+ or δ-
Equal sharing of electrons
ΔEN = 0
Unequal sharing of electrons
ΔEN > 0
Example: CCl4
 Chlorine
with greater EN will have greater attraction of
e- and thus will have partial negative charge δ Carbon with lower EN will have less attraction of eand thus will have partial positive charge δ+
 Shown as
δ+C-Cl δ-
Dipoles in Molecules



When the bond is separated into partial positive
and negative charges we call this bond a polar
bond
We represent dipole bonds with a vector arrow that
points to the more electronegative atom
Example CCl4
δ+C-Cl δ-
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With your group, build the
following molecules…
Examples

Remember to

 Determine
the bond type (by finding ∆EN)
 Assign the partial charges
 Place the dipole moment




Carbon and Oxygen

δ+C-O δ
Carbon and Fluorine


CCl4
NH3
CO2
HCN
H2O
COCl2
BF3
δ+C-F δ-
Polar Molecules

Water H20
We use our information on polar bonds to
predict whether molecules will be polar or nonpolar



We also must know our VSEPR shapes in order
to do this!!

Back to the question:
Why does ice float?

Density of water=1g/mL
Density of ice = 0.92g/mL
Determine bond type
 ∆EN = ENO – ENH = 3.44 – 2.20 = 1.24
 Thus is POLAR COVALENT
Determine partial charges
 O has higher EN and H has lower EN
 Our partial charges are:
If we include the dipoles
What causes this difference in
density?

The polar bonds in water.
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VSEPR Theory




VSEPR Shapes of Molecules
Valence Shell Electron Pair Repulsion Theory
This theory predicts the shapes of molecules based
on the number of areas of electron density around
the central atom
Electron density can be a lone pair or a bonding
pair of electrons
The areas of electron density want to be as far
apart as possible and as such form predictable
molecular shapes
Carbon Dioxide




This is where VSEPR is important! -- You must know the
shape of the molecule in order to determine it’s
polarity
Water has two partially positive ends
and one partially negative end


Because the dipoles do not cancel each other a net
dipole is produced and we say that the molecule is
POLAR
Determine bond type
 ∆EN = ENO – ENC = 3.44 – 2.55 = 0.89
 Thus is POLAR COVALENT
Determine partial charges
O
The two dipole arrows point in the same direction. If
we add these together we can see the molecule will
have an overall net dipole
CO2
has greater EN than C
partial charges are:
 Our

If we include the dipoles
Hydrogen Cyanide HCN


The dipoles created in this molecule are pointing in
opposite directions and thus will cancel each other


This molecule has no net dipole and therefore is
said to be NON-POLAR
Determine bond type
 ∆EN = ENN – ENC = 3.04 – 2.55 = 0.49
 Thus is slightly POLAR COVALENT
 ∆EN = ENC – ENH = 2.55 – 2.20 = 0.35
 Is also slightly POLAR COVALENT
Determine partial charges
 N has greater EN than C – N will have δ C has greater EN than H – C will have δ-
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Note the Difference!

When we assign the dipoles





We see that they are both pointing the same direction
Thus they will not cancel, but will result in an overall
net dipole
This molecule is said to be POLAR

When we had a linear molecule with the same
atoms attached to the central atom the molecule
was non-polar ex. CO2
When we had a linear molecule with two different
atoms attached to the central atom, the molecule
was polar Ex. HCN
It is very important to look at the electronegativities
associated with the atoms and not just the VSEPR
shape
Sulfur Trioxide SO3

Determine bond type
 ∆EN = ENO – ENS = 3.44 – 2.58 = 0.86
 Thus is POLAR COVALENT




Determine partial charges
O
has greater EN than S
partial charges are:
 Our





Look at the horizontal and vertical
components of the vectors (red and
green arrows)
The red arrows will cancel
The green arrows can add together
This green arrow will cancel with the
blue vector created by the top O



When we assign dipole arrows
All the dipoles are pulling away
from the central atom
You may think that because there
are three dipoles they will not
cancel and will result in a polar
molecule
This is not correct however!!
Similar to our linear molecule, difference will occur
when the atoms attached to the central atom are
different
We must be sure to look at the electronegativities
of each atom when comparing the dipole vectors
Ex. CCl2O


Therefore all dipole vectors will
cancel in this molecule creating no
net dipole and therefore the
molecule is NON-POLAR


O has higher EN than Cl and will
therefore have a greater dipole
The two dipoles from Cl will add
together but they will still be less than
that of O
Overall net dipole will result and thus
molecule is POLAR
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Ammonia NH3


Determine bond type
 ∆EN = ENN – ENH = 3.04 – 2.20 = 0.84
 Thus is POLAR COVALENT


Assign dipole vectors
The three vectors will add together
to create an overall net dipole
Determine partial charges
N
has greater EN than H
partial charges are:

 Our
This will result in a POLAR
molecule
Carbon Tetrachloride CCl4


Determine bond type
 ∆EN = ENCl – ENC = 3.16 – 2.55 = 0.61
 Thus is POLAR COVALENT
Determine partial charges
 Cl



has greater EN than C
partial charges are:
 Our

When we assign dipoles
We can see that all the dipoles are of
the same magnitude because the EN
differences are all the same
There are equal amounts of dipoles in
opposite directions and thus they will all
cancel
This results in no net dipole and
therefore the molecule is NON-POLAR
Chloroform CHCl3

Determine bond type
 ∆EN = ENCl – ENC = 3.16 – 2.55 = 0.61
 Thus is POLAR COVALENT
 ∆EN = ENC – ENH = 2.55 – 2.20 = 0.35
 Thus is slightly POLAR COVALENT




Determine partial charges
 Cl
has greater EN than C
has greater EN than H
 Our partial charges are:
C


Assign dipoles (blue arrows)
We can see that the dipoles to Cl will
all add up to create the larger green
dipole vector
This is opposite to the dipole vector
created by H-C but does not have the
same magnitude
Thus, it will not cancel and result in a net
dipole
This molecule is POLAR
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Summary of Polarity of Molecules

Linear:

 When
the two atoms attached to central atom are
the same the dipoles will cancel, leaving no net
dipole, and the molecule will be Non-Polar
 The
dipoles created from this molecule will
not cancel creating a net dipole and the
molecule will be Polar

dipoles created from this molecule will
not cancel creating a net dipole and the
molecule will be Polar
the two atoms are different the dipoles will
not cancel, resulting in a net dipole, and the
molecule will be Polar
Summary of Polarity of Molecules
Trigonal Planar:
the three atoms attached to
central atom are the same the dipoles
will cancel, leaving no net dipole, and
the molecule will be Non-Polar
Pyramidal:
 The
 When

Bent:
Summary of Polarity of Molecules

Tetrahedral:
 When
 When
 When
 When
the three atoms are different the
dipoles will not cancel, resulting in a net
dipole, and the molecule will be Polar
Summary of Polarity of Molecules
the four atoms attached to the central
atom are the same, the dipoles will cancel,
leaving no net dipole, and the molecule will
be Non-Polar
the four atoms are different, the
dipoles will not cancel, resulting in a net
dipole, and the molecule will be Polar
With your group,

Read through the tutorial on pg 106-107 and
answer question 1 on pg 107
Homework
 Read pg 102-108
Questions pg 108 # 1, 2, 5
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Examples to Try

Determine whether the following
molecules will be polar or non-polar
SI2
CH3F
AsI3
H2O2
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