Question

Why is AlCl3 a lewis acid if a lewis acid is a species that contains an atom that is at least two electrons short of a closed outer shell?

Answer 1

Where did you get that definition from? A Lewis acid accepts electrons from an electron donor, and that's the extent of the definition. You're implying more content in the definition than there actually is.

You can see more information about `"AlCl"_3` here:
http://socratic.org/questions/why-is-alcl3-a-lewis-acid-1

The gist of it is:

• Aluminum has one empty hybrid orbital that it "recruits" to use once a fourth atom donates electrons, attaining an `sp^3` hybridization.
• The empty orbital can accept electrons. (If it couldn't when it's empty, when could it? If there is one electron already in the orbital, it can repel incoming electrons and make it harder for them to come in. And obviously if an orbital is full, there is no way it can accept any more electrons.)

Therefore, it follows the definition of a Lewis acid---aluminum within `"AlCl"_3` can accept two electrons.