12/16/2015
Polarity Revisited
Unit 6 – Chemical
Bonding (part II) Covalent Bonding
Lesson 6.9: Molecular
Polarity
Remind Me Polarity of a bond deals with…..
the distribution of electrons in a covalent
bond. Polar bond has an unequal
distribution of electrons, so it has partially
charged (positive and negative) areas.
Any bond between 2 non-metal atoms
that have an electronegativity difference of
greater than 0.3 is a polar bond.
20
1
B. Molecular Polarity
A. Bond Polarity
• Polar molecule = one end slightly (+)
and one end slightly (–)
• Most bonds are a
blend of ionic and
covalent
characteristics
• Molecule
o ecu e has
as 2 po
poles
es = d
dipolar
po a
molecule or dipole
• Difference in
electronegativity
determines bond
type
Why should we care about polarity of a molecule?
Polarity tells us a bunch of things about a substance:
1. How high or low the melting/boiling points are.
2. How easily the liquid phase evaporates.
3. If the substance will dissolve in water, or other
solvents.
solvents
Polarity helps explain:
Glue
DNA
Insects walking on walls or water.
Stickiness in general.
Polar vs. Non‐
Polar vs. Non‐Polar Molecules
• Polarity in a molecules determines
whether or not the electron cloud in
that molecule is distributed equally.
• When determining the polarity of a
molecule, it is all about bond polarity and
symmetry.
• Asymmetric molecules tend to be polar.
• Symmetric molecules are always nonpolar.
20
7
1
12/16/2015
When determining the polarity of a molecule, follow these steps:
• Draw the Electron Dot structure of the
molecule.
• Using Reference Table S determine the
difference in electronegativity for each bond.
• 0 - 0.3 = Non-polar
• >0.3 - 1.7 = Polar
The molecule is non‐polar if :
•
each bond in the molecule is non-polar
and there are no
–
•
each bond in the molecule has the same
polarity
p
y and there are no
–
•
lone electron pairs.
lone electron pairs on the central atom.
There is no net dipole moment (all
moments cancel out)
The molecule is polar if:
•
There is a net dipole moment
–
•
each bond in the molecule is non-polar,
but there are lone electron pairs on the
central atom.
Dipole Moment
• Direction of the polar bond in a molecule.
• Arrow points toward the more
electronegative atom.
b d iin th
bonds
the molecule
l
l h
have diff
differentt
polarities - assymetry, and/or there are
–
+
lone electron pairs on the central atom.
Nonpolar Molecules
– Dipole moments are asymmetrical and
don’t cancel .
– Molecule has a net dipole moment.
B
F
-
Cl
Polar M
Polar Molecules
olecules
• Dipole moments are symmetrical and
cancel out.
F
BF3
H
O
F
H2O
H
H
net
dipole
moment
2
12/16/2015
Polar Bonds vs. Polar Molecules
Polar molecules
• The effect of polar bonds on the
polarity of the entire molecule depends
on the molecule shape
• The effect of polar bonds on the
polarity of the entire molecule depends
on the molecule shape
– carbon dioxide has two polar bonds, and
is linear = nonpolar molecule!
– water has two polar bonds and a bent
shape; the highly electronegative oxygen
pulls the e- away from H = very polar!
How to identify a Polar molecule?
Determining Molecular Polarity
Summary
• When polar molecules are placed
between oppositely charged plates, they
tend to become oriented with respect to
the positive and negative plates.
• Therefore, polar molecules have...
– asymmetrical shape (lone pairs) or
– asymmetrical atoms
H
CHCl3
Cl
Cl
Cl
net
dipole
moment
How to determine Polarity
Step 1: Determine the shape of the molecule.
A. Draw the molecule. Use lines to represent
shared pairs of electrons, and dots to
represent unshared electrons.
Remember:
e e be
Everybody wants 8 eExcept
Hydrogen (wants 2 e-)
21
9
Step 2: Determine the symmetry of the
molecule
A. Polar molecules have one line of symmetry (the
“dipole moment”). The atom with the highest Eneg is
partially negative (δ-). The rest are partially positive
(δ+).
22
0
3
12/16/2015
B. Non-Polar molecules have two or more lines of
symmetry. All atoms have the same charge.
B. Molecular Polarity
• Identify each molecule as polar or nonpolar
–SCl2 Tetrahedral, bent → polar
Nonpolar bonds → nonpolar
–O2
Li
→ nonpolar
l
–CS
CS2 Linear
–CF4 Tetrahedral → nonpolar
–CH2F2 Tetrahedral → polar
And Why should we care?
22
1
4