Section 5.4—Polarity of
Molecules
Objectives
•examine the trend of electronegativity on the
periodic table
•use electronegativity to determine the
polarity of covalent bonds
Electronegativity
Electronegativity is the attraction an atom
has for the electrons it shares with
another atom in a bond.
Electronegativity has a periodic and group
trend
Periodic Table with Electronegativies
increases
decreases
Polar Bonds
Recall from Sec. 5.1, that covalent bonds
(where electrons are shared between 2
atoms) can be nonpolar or polar.
In nonpolar covalent bond, electrons are
shared equally but in a polar covalent
bond, they are not shared equally.
The electronegativity difference between
the 2 atoms in the bond determines the
type of covalent bond that forms.
Nonpolar: Two atoms sharing equally
N
N
Each nitrogen atom has an electronegativity of 3.0.
There is no electronegativity difference.
They pull evenly on the shared electrons so, the
electrons are not closer to one or the other of the
atoms.
This results in a non-polar covalent bond.
Nonpolar: Atoms sharing almost equally
H
H
C
H
H
Electronegativities: H = 2.1
C = 2.5
Electronegativity difference of each bond: 0.4
The carbon pulls on the electrons slightly more (having a
higher electronegativity), but the difference isn’t enough to
create a polar bond.
This is a non-polar covalent bond.
Polar: Two atoms share unequally
H
C
H
O
Electronegativities: H = 2.1
C = 2.5
O = 3.5
The C-H difference is 0.4; the C-O difference is 1.0.
The C-H bonds are nonpolar.
But, the oxygen atoms pulls significantly harder on the
electrons than the carbon does. This is a polar covalent
bond!
A polar covalent bond results in one end of
the bond (and molecule) having a slight
negative charge and one end having a
slight positive charge.
The atom having a higher electronegativity pulls
the electrons closer to itself and, so, has a
partial negative charge.
The atom with the lower electronegativity has
the electrons farther away and, so, has a partial
positive charge.
Showing Partial Charges
There are two ways to show the partial
separation of charges
Use of “” for “partial”
Use of an arrow pointing towards the partial
negative atom with a “plus” tail at the partial
positive atom
+
H
C
H
-
O
H
C
H
O
Ionic Bonds
Ionic bonds occur when the
electronegativies of two atoms are so
different that they can’t even share
unevenly…one atom just takes them from
the other
How to determine bond type
Find the electronegativies of the two
atoms in the bond
Find the absolute value of the difference of
their values
If the difference is 0.4 or less, it’s a non-polar
covalent bond
If the difference is greater than 0.4 but less than
1.4, it’s a polar covalent bond
If the difference is greater than 1.4, it’s an ionic
bond
Let’s Practice
C–H
Example:
If the bond
is polar,
draw the
polarity
arrow
O—F
F—F
C—Cl
Let’s Practice
Example:
If the bond
is polar,
draw the
polarity
arrow
C–H
2.55 – 2.2 = 0.35
nonpolar
O—F
3.44 – 3.98 = 0.54
polar
F—F
3.98 – 3.98 = 0.0
C—Cl
2.55 – 3.16 = 0.61
non-polar
polar