Polarisation of bonds

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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- .

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

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 CO₃^2- ion into self-decomposition.
This leaves instead of Al₂(CO₃^2-)₃, carbon dioxide is driven off, leaving aluminium oxide.

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