Molecular Polarity Notes

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Polarity of Molecules
When we have determined the Molecular Shape of the MOLECULE, we can predict whether the
molecule will be a Polar Molecule or a Non-Polar Molecule.
Polar Molecules: The molecule will have a somewhat positive end and a somewhat negative end. This
can be predicted by examining the nature of the bonds and the three dimensional shape of the
molecule
1.
The 3-D shape will be unsymmetrical.
2.
The bond polarities do not cancel each other out, i.e., the molecule is not
symmetrical.
Non-Polar Molecules: The molecule will not have positive and negative ends.
1.
The 3-D shape will be symmetrical.
2.
All bond polarities will cancel each other out, i.e., the molecule is
symmetrical.
3. Has all nonpolar bonds
It can help to picture polarity as a game of tug-of-war. The electrons all flow from the positive dipole to
the negative dipole. This flow can be thought of as pushing or pulling. In the case of AlCl3, each Cl is
pulling on the central Al evenly, so the whole molecule is symmetrical and nonpolar, even though each
individual bond is polar. If it were a tug-of-war the Al would not move because it is being pulled evenly
from each side. If one Cl is replaced with a F, the F has a higher electronegativity, so it can pull harder –
it is not symmetrical and the Al would move – this is now polar.
Examples

Non-polar Molecules
Only non-polar bonds present
H2
H-H
is non-polar since both hydrogen atoms making up the
molecule have equal electronegativity so there is no net
dipole.
NCl3 is non-polar since the nitrogen atom and the
chlorine atoms making up the molecule have the same
electronegativity so there is no net dipole.
NCl3
Polar bonds arranged symmetrically
CO2
O=C=O
Each C - O bond is polar since oxygen is more
electronegative than carbon, however, these bonds are
arranged symmetrically (all angles are 180o) so that the
two dipoles cancel out resulting in no net dipole for the
molecule.
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
AlCl3
Each Al-Cl bond is polar since chlorine is much more
electronegative than aluminium, however, each Al-Cl
bond in AlCl3 is arranged symmetrically (all angles are
120o) so that the dipoles cancel out resulting in no net
dipole for the molecule.
CH4
Each C-H bond is polar since carbon is more
electronegative than hydrogen, however, each C-H
bond in CH4 is arranged symmetrically (all angles are
109.5o) so that the dipoles cancel out resulting in no net
dipole for the molecule.
Polar Molecules
Polar bonds arranged unsymmetrically
HCN
----------&gt;
Both the C-H and the C-N bonds are polar. Nitrogen is more
electronegative than carbon which is more electronegative than hydrogen.
So that the hydrogen takes on a partial positive charge and the nitrogen
takes on a partial negative charge. This results in an unequal sharing of
the bonding electrons resulting in a net dipole for molecule since the two
dipoles do not cancel out.
H2O
Each O-H bond is polar since oxygen is more electronegative than
hydrogen so each hydrogen takes on a partial positive charge and the
oxygen atom takes on a partial negative charge. The two O-H bonds are
arranged unsymmetrically (angle between bonding pair &lt; angle between
bonding pair and lone pair &lt; angle between lone pair and lone pair)
resulting in a net dipole since the two dipoles do not cancel out.
NH3
Each N-H bond is polar since nitrogen is more electronegative than
hydrogen so each hydrogen takes on a partial positive charge and nitrogen
takes on a partial negative charge. The three N-H bonds are arranged
unsymmetrically (angle between bonding pairs &lt; angle between bonding
pairs and lone pair) resulting in a net dipole since the three dipoles do not
cancel out.
CH3Cl
Each C-H bond is polar since carbon is more electronegative that
hydrogen, and the C-Cl bond is polar since chlorine is more
electronegative than either carbon or hydrogen. Each hydrogen atom will
take on a partial positive charge and the chlorine atom will take on a
partial negative charge resulting in a net dipole since the dipoles will not
cancel out owing to the difference in electronegativities of carbon,
hydrogen and chlorine.
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Predicting Molecular Polarity
Tip-off – You are asked to predict whether a molecule is polar or nonpolar; or you are asked a question
that cannot be answered unless you know whether a molecule is polar or nonpolar. (For example, you are
asked to predict the type of attraction holding the particles together in a given liquid or solid.)
General Steps Step 1: Draw a reasonable Lewis structure for the substance.
Step 2: Identify each bond as either polar or nonpolar. (If the difference in electronegativity for the atoms
in a bond is greater than 0.4, we consider the bond polar. If the difference in electronegativity is less than
0.4, the bond is essentially nonpolar.)
If there are no polar bonds, the molecule is nonpolar.
If the molecule has polar bonds, move on to Step #3.
Step 3: If there is only one central atom, examine the electron groups around it.
If there are no lone pairs on the central atom, and if all the bonds to the central atom are the same,
the molecule is nonpolar. (This shortcut is described more fully in the Example that follows.)
If the central atom has at least one polar bond and if the groups bonded to the central atom are not
all identical, the molecule is probably polar. Move on to Step #4.
Step 4: Draw a geometric sketch of the molecule.
Step 5: Determine the symmetry of the molecule using the following steps.
Describe the polar bonds with arrows pointing toward the more electronegative element. Use the
length of the arrow to show the relative polarities of the different bonds. (A greater difference in
electronegativity suggests a more polar bond, which is described with a longer arrow.)
Decide whether the arrangement of arrows is symmetrical or asymmetrical
If the arrangement is symmetrical and the arrows are of equal length, the molecule is nonpolar.
If the arrows are of different lengths, and if they do not balance each other, the molecule is polar.
If the arrangement is asymmetrical, the molecule is polar.
Predict whether or not the following molecules are polar or not: carbon tetrachloride, CH3Cl, silicon
disulfide, C2H2, PH3, H2O
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