1. Polar Molecules

2. 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

3. 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.

4. Favoured by small, highly charged +ve ions, e.g. Li+, Be2+
Electronegativity Difference 4 - + X Y X Y X- Y+ X- Y+
Pure covalent
Polar ionic
Distorted ions
Pure ionic
Polar covalent
Electrons not
equally shared
Polarisation of covalent bonds
Polarisation of ions
Favoured by small, highly charged +ve ions, e.g. Li+, Be2+

5. Non-polar bond
Polar bond

6. We can show that a bond is polar in a number of ways:

7. This is the commonest way. The funny symbol is pronounced “delta”

8. This symbol actually shows that there is an OVERALL imbalance in the distribution of charge within a molecule. The molecule is a dipole

9. 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

10. CO2 - + - O C O
Bonds: polar
Molecule: non-polar

11. Compare this with H2O
This also contains three atoms , but this time the arrangement is not symmetrical,
So the molecule IS a dipole

12. H2O - O H H + +
Bonds: polar
Molecule: polar

13. Comment on whether tetrachloromethane, CCl4, is a dipole
How about trichloromethane, CHCl3 ?

14. CCl4 tetrachloromethane Cl Symmetrical Bonds: polar- + Cl
Bonds: polar
Molecule: non-polar

15. Not symmetrical
Trichloromethane
CHCl3
Yes – it’s a dipole!

16. 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 . . .

17. It will be either attracted or repelled if it is a polar liquid.
We can do this with water!