How can one describe aluminum monochloride cation with a covalency model?

1 Answer
Truong-Son N.
Sep 9, 2017

This is representable as

#"AlCl"^(2+)#

in aqueous solution.

The water molecules are just interacting electrostatically, and not actually bonding.

  • Their partially negative end is pulled by aluminum's #+3# oxidation state in an ion-dipole interaction.
  • #"Cl"^(-)# is similarly pulled by #"Al"#'s #+3# oxidation state.

These are good approximations because #"AlCl"^(2+)# is very much a diatomic ion with respect to water, and has a nearly ionic bond within itself---the electronegativity difference is #EN_(Cl) - EN_(Al) = 3.0 - 1.5 = 1.5#.

For a two-atom bond that is nearly ionic, the covalency is at best #1# since in #"AlCl"^(2+)#, aluminum at best owns #1# valence #3s# electron as #"Al"^(2+)#, and at worst, it owns no #3s# valence electrons as #"Al"^(3+)#. In reality, less than half the electron density is "owned" by #"Al"# atom.

As such, one cannot accurately describe this with covalency as a model.

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