Why is AlCl3 a Lewis acid? What happens to its geometry after bonding with another atom?

1 Answer
Truong-Son N.
Jan 18, 2016

Having only three valence electrons, aluminum is (along with Boron, for instance) special in that it doesn't need an octet to become stable.

Its electron configuration is [Ne]3s23p1. It thus has an empty 3p atomic orbital (AO) as an atom.

So, aluminum then has an empty 3p valence unhybridized orbital after it needs to bond with three Cl atoms.

Since AlCl3 has an empty 3p valence AO, that AO can accept electrons, making AlCl3 a (good) Lewis acid.

Accepting electrons in this way is still favorable. These electrons can be donated from Cl− (such as when reacting with HCl) in this example, thus forming AlCl−4.

At this point, the molecular geometry around aluminum has changed from trigonal planar to tetrahedral. In other words, the orbital hybridization changed from sp2 to sp3.

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