Polar Molecules
• We already know that it is the difference in electronegativities between elements that determines the type of bonding.
• If there is a large difference in electronegativities . . .
• Ionic bonding• Complete transfer of electrons• The more electronegative element takes the
electrons
• No difference in electronegativities• Pure covalent bonding• Equal share of electrons
• In real life though, most molecules are somewhere in between these 2 extremes.
• They are polar covalent• They share electrons, but not equally• The more electronegative element has a
greater share of the electrons.
X Y X Y Y+X- Y+X-
Electronegativity Difference0 4
Pure covalent Polar covalentElectrons not equally shared
Polar ionicDistorted ions
Pure ionic
Polarisation of covalent bonds
Polarisation of ions
Favoured by small, highly charged +ve ions, e.g. Li+, Be2+
- +
Non-polar bond
Polar bond
• We can show that a bond is polar in a number of ways:
This is the commonest way.The funny symbol is pronounced “delta”
This symbol actually shows that there is an OVERALL imbalance in the distribution of charge within a molecule.
The molecule is a dipole
• It is important to know • (because the IB keep asking about it!)• That even though it may contain individual bonds
that a polar, a molecule may not be a dipole.
• Consider CO2
• Each C=O bond is polar (how do we know?)• But the symmetrical shape of the molecule means
electrons are pulled equally in both directions• And the molecule is not a dipole
CO+-
CO2
Bonds: polarMolecule: non-polar
O-
• Compare this with H2O• This also contains three atoms , but this time
the arrangement is not symmetrical,• So the molecule IS a dipole
OHH
+ +
-
H2O
Bonds: polarMolecule: polar
• Comment on whether tetrachloromethane, CCl4, is a dipole
• How about trichloromethane, CHCl3 ?
CCl4
Bonds: polarMolecule: non-polar
C
- -
+
Cl-
ClCl
Cl-
tetrachloromethane
Symmetrical
• Not symmetricalTrichloromethane
CHCl3
Yes – it’s a dipole!
How can we tell if a liquid is polar or non-polar?
• Could you think of an experiment to tell if an unknown liquid is polar or not?
• The charge we have talked about isn’t just theory – it’s a real charge.
• If we hold an electrostatically charged rod near a thin stream of the liquid . . .
• It will be either attracted or repelled if it is a polar liquid.
• We can do this with water!
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