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MSJChem – Topic 4 – Polar and non-polar molecules
Polar and non-polar molecules
Describe the two factors that determine the polarity of a molecule.
Explain how a molecule can have polar bonds but overall have no net dipole moment (non-polar).
Classify the following molecules as polar or non-polar.
a) H 2 O
b) CH 4
c) NH 3
d) CCl 4
e) CHCl 3
- From the molecular structures below, classify the molecules as polar or non-polar.
a) Boron trifluoride (BF 3 ) b) Hydrogen cyanide (HCN)
MSJChem – Topic 4 – Polar and non-polar molecules
Answers:
The two factors that determine the polarity of a molecule are the presence of polar bonds within the molecule and the geometry (shape) of the molecule. Polar bonds are caused by unequal sharing of electrons in a bond due to a difference in electronegativity of 0.5 units or greater between the atoms in the bond.
A molecule can have polar bonds but be non-polar overall if the bond polarities cancel out. This is due to the shape of the molecule, for example, CO 2 has two polar bonds and a linear shape – the bond polarities cancel out leaving the molecule with no net dipole moment.
a) H 2 O polar
b) CH 4 non-polar
c) NH 3 polar
d) CCl 4 non-polar
e) CHCl 3 polar
a) BF 3 is non-polar, it has no net dipole moment. The B-F bond is a polar bond (because of the difference in electronegativity), but due to the shape of the molecule (trigonal planar) the bond polarities cancel out making the molecule non-polar overall.
b) HCN is a polar molecule – it has a net dipole moment. The C-H is a non-polar bond and the C-N bond is a polar bond (because of the difference in electronegativity). The shape of the molecule is linear, therefore the bond polarities do not cancel, making the molecule polar overall.
