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Polarity

Chemistry: AQA A Level, topic 1.3.7

Polar Bonds

Some bonds within molecules may be described as polar. This is caused by differences in electronegativities.

Electronegativity

Electronegativity

  • In a bond between two unlike atoms, one atom will have a stronger attraction to the electron pair than the other.
  • This means the electron pair will not be at the centre of the bond.
  • This causes the buildup of a partial charge on one atom.
  • This is written as δ±
  • The ability of an atom to attract the electron pair is called electronegativity.
Pauling electronegativities

Pauling electronegativities

  • The values of electronegativity can be measured on several different scales.
  • The one you need to know about is the Pauling scale (named after Linus Pauling, its inventor).
  • The most electronegative element is fluorine, with a value of 4.0.
  • Other strongly electronegative elements are oxygen (3.4), chlorine (3.2), nitrogen (3.0)
  • Carbon has an electronegativity of 2.5 and hydrogen has an electronegativity of 2.2.
Polarity

Polarity

  • The most electronegative atom in a bond will develop a partial negative charge (δ-) and vice versa.
  • This charge difference is called a permanent dipole.
  • A good example of a polar bond is the HCl molecule.
  • A good example of a non-polar bond is the C-H bond.
  • The electronegativities of hydrogen and carbon are very similar, so the bond is not polar.

Polar Molecules

Molecules can have an overall dipole just like bonds.

Molecular polarity

Molecular polarity

  • Molecules can also be polar.
  • A requirement for a molecule to be polar is that it has polar bonds.
  • If there is a charge separation between one side of the molecule and the other, there will be a permanent dipole.
  • A good example of a molecule like this is chloroform, CHCl3.
Finer details

Finer details

  • A molecule with polar bonds is not necessarily polar.
  • If the bonds are arranged symmetrically about the centre, then the partial charges will be the same on each side of the molecule.
  • Such a molecule is not polar.
  • A good example of a molecule like this is tetrachloromethane (CCl4).

Test yourself on Polarity

  • Electronegativity is labeled using which one of the following greek letters?

  • Which of the following is NOT a polar molecule?

  • A molecule contains multiple polar C-Cl bonds. Is the molecule definitely polar?

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