Why is Aluminium Chloride a Simple Covalent Compound

AlCl3 is a compound formed between a metal and non-metal.

We would expect AlCl3 to be an ionic compound but it is a simple molecule instead.

230424 orbital overlap btw Al and Cl

Al3+ is very polarising due to its high charge density.

It is able to pull the electron cloud of chloride to such a great extent that there is orbital overlap between Al and Cl.

The bond between Al and Cl is covalent and hence AlCl3 is a simple molecule.

1. Compare MgCl2 and AlCl3

230424 MgCl2 vs AlCl3

Mg2+ has less polarising power than Al3+ and distorts electron cloud of chloride to a smaller extent.

There is no orbital overlap between Mg2+ and Cl- and the attraction between the ions is ionic bond with covalent character.

Hence MgCl2 is an ionic compound.

2. Compare Aluminium Halides

230424 compare AlX3

Down Group 17, the electron cloud size of halides increases and become more polarisable.

For AlF3, flouride is smaller and less polarisable than chloride.

There is no orbital overlap between Al3+ and F- hence AlF3 is an ionic compound.

For AlBr3, bromide is larger and more polarisable than chloride.

Since there is orbital overlap between Al3+ and Cl-, we can deduce there must be orbital overlap between Al3+ and more polarisable Br-.

Hence AlBr3, and similarly AlI3, are simple covalent compounds.

Topic: Chemical Bonding, Physical Chemistry, A Level Chemistry, Singapore

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