polar bonds arranged symmetrically so that the dipoles cancel out
A polar molecule has a net dipole.
This is achieved in the molecule by the molecule being made up of polar bonds arranged unsymmetrically so that the dipoles do not cancel out.
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
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.
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.
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
---------------------->
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 < angle between bonding pair and lone pair < 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 < 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 elctronegative than either carbon or hydrogen. Each hydrogen atom will take on a partial positve 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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