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    ##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.
    OK.

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