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My textbook states that AlCl3 is both an ionic and a covalent compound .How is it so?

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  • $\begingroup$ Strongly related but not a dupe IMO. chemistry.stackexchange.com/questions/19568/… $\endgroup$ – bon May 7 '16 at 15:45
  • $\begingroup$ I thought that there was another question on the nature of bonding in $\ce{AlCl3}$ floating around, but I cannot find it … $\endgroup$ – Jan May 7 '16 at 15:54
  • $\begingroup$ chemistry.stackexchange.com/questions/19715/… $\endgroup$ – Mithoron May 7 '16 at 15:54
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    $\begingroup$ @Mithoron Not dupe-y enough imho o.o $\endgroup$ – Jan May 7 '16 at 15:57
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    $\begingroup$ @Mith the questions aren't dupes if the same answers apply. That's not the only criterion. I think leaving this open to get some answers would be useful. $\endgroup$ – M.A.R. May 7 '16 at 16:09
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The distinction between ionic and covalent compounds is arbitrary and many compounds fall somewhere in the middle. This is explained in a previous question.

In the case of $\ce{AlCl3}$ we have a metal 'cation' and a non-metal 'anion' so in a simplistic view we would expect this to be an ionic compound. However, it actually turns out that the $\ce{Al-Cl}$ bonds display a significant degree of covalent character. In the solid state it adopts an 'ionic lattice' structure with octahedral coordination for the $\ce{Al^{3+}}$ ions but in the liquid and gas phases it exists as a covalent compound, either as $\ce{AlCl3}$ or as a dimer $\ce{Al2Cl6}$.

We can rationalize this by considering the polarizing effect of the aluminium cation on the chloride anions. $\ce{Al^{3+}}$ is a small, highly charged cation and therefore has a high charge density. $\ce{Cl-}$ is a relatively large anion, with a low charge density and is easily polarized by the hard cation, giving the bond significant covalent character. The effect is even more pronounced if we move to a larger anion such as $\ce{I-}$ where the molecule, $\ce{AlI3}$ exists as a covalent dimer, $\ce{Al2I6}$, even in the solid phase.

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  • $\begingroup$ I would thing thrice before calling Al2Cl6 and especiall Al2I6 'covalent' I don't have the numbers on me, but the last time I checked, 'covalent' PbF4 had charge on the central atom around +2 or so. Al2Cl6 is likely to be similar in this regard. $\endgroup$ – permeakra May 7 '16 at 16:54

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