12/14/2014
Polarity Revisited
Unit 6 – Chemical Bonding
(part 2)
Lesson 6.3: Covalent
Bonding - 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 0.4 or
greater is a polar bond.
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A. Bond Polarity
B. Molecular Polarity
• Polar molecule = one end slightly (+) and
one end slightly (–)
• Most bonds are a
blend of ionic and
covalent
characteristics
• Molecule has 2 poles = dipolar 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.
Polarity helps explain:
Glue
DNA
Insects walking on walls.
Stickiness in general.
Polar vs. Non-Polar Molecules
• Polarity in a molecules determines
whether or not electrons in that
molecule are shared 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 non-polar.
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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
• 0.4—1.7 = Polar
The molecule is non-polar if :
•
each bond in the molecule is non-polar and
there are no
–
•
–
•
lone electron pairs.
each bond in the molecule has the same
polarity and there are no
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.
bonds in the molecule have different
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 Molecules
• Dipole moments are symmetrical and cancel
out.
F
BF3
H
O
F
H2O
H
H
net
dipole
moment
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Polar Bonds vs. Polar Molecules
• 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!
Determining Molecular Polarity
• Therefore, polar molecules have...
– asymmetrical shape (lone pairs) or
– asymmetrical atoms
H
CHCl3
Cl
Cl
Cl
Polar molecules
Polar molecules
• The effect of polar bonds on the polarity
of the entire molecule depends on the
molecule shape
– water has two polar bonds and a bent shape;
the highly electronegative oxygen pulls the eaway from H = very polar!
Polar molecules
• 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!
net
dipole
moment
water has two polar bonds and a bent shape;
the highly electronegative oxygen pulls the eaway from H = very polar!
How to identify a Polar molecule?
• When polar molecules are placed
between oppositely charged plates, they
tend to become oriented with respect
to the positive and negative plates.
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Step 2: Determine the symmetry of the molecule
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:
Everybody wants 8 eExcept
Hydrogen (wants 2 e-)
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+).
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B. Non-Polar molecules have two or more lines of
symmetry. All atoms have the same charge.
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B. Molecular Polarity
• Identify each molecule as polar or nonpolar
–SCl2
–O2
–CS2
–CF4
–CH2F2
Tetrahedral, bent → polar
Nonpolar bonds → nonpolar
Linear → nonpolar
Tetrahedral → nonpolar
Tetrahedral → polar
And Why should we care?
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