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

Sep 9, 2017

This is representable as

${\text{AlCl}}^{2 +}$

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.
• ${\text{Cl}}^{-}$ is similarly pulled by $\text{Al}$'s $+ 3$ oxidation state.

These are good approximations because ${\text{AlCl}}^{2 +}$ is very much a diatomic ion with respect to water, and has a nearly ionic bond within itself---the electronegativity difference is $E {N}_{C l} - E {N}_{A l} = 3.0 - 1.5 = 1.5$.

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

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