How many individual atoms make up one unit of aluminum chloride, #AlCl_3#?

1 Answer
Dec 30, 2015

Macromolecular crystal OR #4# atoms OR #8# atoms


The compound #"AlCl"_3# takes different forms depending on the temperature of its surroundings and the state that it is found in.

As a solid, the aluminium central atoms exhibit octahedral coordinate geometry: this is because the #"AlCl"_3# unit functions as a Lewis acid, that is it will accept lone pairs of electrons donated from another source (the Lewis base) to form a dative covalent bond / coordinate bond between the two. A chlorine atom from one #"AlCl"_3# unit can donate their lone pairs to the central aluminium atom of another unit in this way. This results in a giant covalent structure, but the formula remains to be #"AlCl"_3#.

Source: chemguide

It is as a liquid that #8# atom complexes exist. As a liquid, aluminium trichloride exists in dimers, where the #"AlCl"_3# monomers exist in groups of two thanks to aluminium's ability to support tetrahedral geometry (once again through coordinate bonding). Observe that the central aluminium atoms support fewer bonds as a liquid than as a solid: this is because the constituent #"AlCl"_3# units have more energy and so do not require bond formation to stabilise to as great an extent. Recall that the formation of bonds liberates energy.

Source: Wikipedia

Aluminium trichloride dimers also exist in the vapour phase. In either of the liquid or vapour phases, aluminium trichloride dimers will dissociate into aluminium trichloride molecules, with a trigonal planar geometry. It is in this state that a unit of #"AlCl"_3# certainly does have #4# atoms.

Source: scbt

If you would like to know more about aluminium chloride (#"AlCl"_3#), click here to visit its corresponding Wikipedia article.