Polarisation of bonds

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What is polarity?

Different ways of representing bonds.

Does size affect polarising power?

So does different size and charge mean different polarising power?

So what conditions favour fully ionic bonds?


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What is polarity?

Polarity is the distortion of the electron cloud of one atom by another.

The standard example is often hydrogen chloride (HCl)

This distortion is said to be a dipole.

There are several methods of representing this shift in the position of electrons.

Different ways of representing bonds.

A normal covalent bond, representing a pair of shared electrons.

A representation of a double bond, normally C=C or C=O. This shows the presence of a s and a p bond.

Representation of a dative bond, donation of electrons to B by A.

A polar bond, B is more electronegative, so bonding pair closer to B. Also shown using the d+ d- .

Does size affect polarising power?

Yes, and so does electronegativity. The greater the electronegativity, the greater the polarising power.

So for hydrogen halogen compounds:

Bond polarity has a huge hand in determining chemistry.

The size mismatch of the anions (-ve) and cations (+ve) is of huge importance also.

  • If two ions are similar in size, then they exist quite happily
  • If there is a size mismatch, then is it quite likely that covalent bonding will occur.


  • NaCl melts at 801C, strong attraction between particles in solid lattice structure (Ionic bonding likely)
  • AlCl3 sublimes (goes from solid to gas not via the liquid phase) at 180C, so there are no strong attractions present (Covalent bonding likely).

So does different size and charge mean different polarising power?

In essence yes. Al3+ has a high charge density (3+) and its very small. This gives it a high polarising power.

  • If the cation is small and highly charged, it has a large polarising power.
  • If the anion is large and has a relatively low charge, then it is said to have a large polarisability.

If the above is the case, and the anion is being polarised by the cation, there will be a degree of covalent character to the bond.

So the bonding in AlCl3 is virtually covalent.

Small highly charged cation + large easily polarised anion = covalent character.

There are some ionic compounds that do not exist at all. Aluminium carbonate is one such example.

  • The aluminium 3+ cation is so small and highly polarising that is completely distorts the large CO32- ion into self-decomposition.
  • This leaves instead of Al2(CO32-)3, carbon dioxide is driven off, leaving aluminium oxide.

So what conditions favour fully ionic bonds?

  •   Small charge

  •  Small anions

  •  Large cations

  •  No mismatch in size.

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