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!