In my textbook it says in the reaction $\ce{AlCl3 + Cl- -> AlCl4-}$, Aluminium Chloride ($\ce{AlCl3}$) has six valence electrons and accepts a lone pair from $\ce{Cl-}$, but wouldn't that mean the aluminium atom covalently bonded with the three chlorine atoms to get six electrons (which is wrong as it's ionic)?
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2$\begingroup$ Welcome to Chemistry.SE! For formatting help visit the help center and for more information about this page, take the tour. I have beautified your post accordingly, please take a look. $\endgroup$– Martin - マーチン ♦Jun 30, 2014 at 6:23
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1 Answer
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if it's an ionic compound
Actually, it is debatable. $\ce{Al^{3+}}$ is a very small cation and its formation is strongly unfavored. In fact, this cation, just like many others, may exist only in solvates, like $\ce{[Al(OH2)6]^{3+}}$, or unchemical environment.
In particular case of anhydrous $\ce{AlCl3}$ the molecule is a strong Lewis acid, strong enough to coordinate with lone pair of another similar molecule, so it forms either dimer $\ce{Al2Cl6}$ with two chlorine bridges, donating one pair to both Al ions, or polymer, with Al atoms exhibiting octahedral coordination. Unlike many ionic compounds, melted $\ce{AlCl3}$ has poor conductivity.
This, however, does no apply to $\ce{AlCl3.6H2O}$ which is, again, a ionic compound $\ce{[Al(OH2)6]^{3+} (Cl^{-})3}$ and does not exhibit any Lewis acidity (though it does exhibit strong Brønsted acidity, i.e. tendency for hydrolysis)
This said, however, I have to clarify: ionicity of compound does not prevent particular ion from being Lewis acid/base. In fact, cations often are excellent Lewis acids. The only thing needed for a particle to be a Lewis acid is presence of low vacant orbital, and sometimes presence of such orbital is not derivable from octet rule. For example, $\ce{SnCl4}$ is Lewis acid, though in it $\ce{Sn}$ has full octet. The orbital in question is describable in terms of MO theory which I strongly encourage to study.
The most often considered Lewis acid is hydrogen cation. In fact, this Lewis acid is so strong, that it never occurs free, it is always bound to Lewis base (again, except unchemical environment). Another common Lewis acid is $\ce{Na^+}$. This cation is quite weak Lewis acid, but it is sterically accessible, and usually is paired with anion, which is usually a Lewis base. This said, despite mostly ionic nature of $\ce{NaCl}$, there is considerable covalent bonding in it. (of course, on the other hand there is such anion as $\ce{BPh4^-}$ which is not a lewis base of considerable strength, so its salts are almost purely ionic in nature)
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$\begingroup$ What do you mean the hydrogen cation exists in in chemical environments? $\endgroup$ Jun 30, 2014 at 14:30
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2$\begingroup$ @Dissenter It exists in environments that are not domain of chemistry of condensed phases, but physics: plasma, particle beams and similar. $\endgroup$ Jun 30, 2014 at 14:53
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$\begingroup$ @permeakra thank you very much, my doubts have been cleared! $\endgroup$– RishiJul 1, 2014 at 0:11