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.

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+

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

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

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

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

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

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

Not symmetrical Trichloromethane 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!