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Why is AlCl3 a Lewis acid?
##"AlCl"_3## is a Lewis acid because it can accept a pair of electrons in many reactions.
According to the Lewis definition of , a compound isn't acidic or basic until it does something.
In other words, you can't just say that "X compound is a Lewis acid" unless you've seen it act as an acid in some chemical reaction. In that case, you'd say that "X compound is a Lewis acid in this particular reaction."
For ##"AlCl"_3## to be a Lewis acid, it would have to react in such a way that it accepted a lone pair from some other atom or molecule (from a Lewis base).
##"AlCl"_3## has an electron-deficient aluminum atom. It has only six electrons in its valence shell.
It readily accepts electrons from other atoms, in an attempt to get a full valence shell of eight electrons.
That's why it generally behaves as a Lewis acid.
In the reaction below, the Al atom accepts a lone pair of electrons from a Cl atom.
This completes its octet and forms the ##"AlCl"_4^-## ion.