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Polar Molecules

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

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Polar Molecules

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  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. Electronegativity Difference 0 4 - + X- X Y X Y X- Y+ 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. - Cl + C Cl Cl Cl - - - CCl4 tetrachloromethane Symmetrical Bonds: polar Molecule: non-polar

  15. Trichloromethane • Not symmetrical 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!

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