Possible Error in Chloric Acid's Lewis Structure

[vertciel]

Hello everyone:

I was looking at Wikipedia's page on chloric acid (http://en.wikipedia.org/wiki/Chloric_acid) and was wondering if the Lewis diagram there is correct. I will not link directly to it as it has a high resolution.

It shows Cl with a lone pair of electrons having double bonds with two O atoms and then a single bond to O - H. Wouldn't Cl have 11 electrons in this case, beyond its limit of 8?

Thanks.

 

[Ygggdrasil]

Chlorine would have 12 electrons in its valence shell (10 bonding electrons + 2 in the lone pair) and would indeed seem to violate the octet rule. However, elements in the 3rd row of the periodic table are able to expand their valence shell by involving the 3d orbitals. Other examples of molecules where this occurs are H₂SO₄ and H₃PO₄. Elements in the 2nd row cannot expand their valence shells beyond 8 electrons because there are no 2d orbitals.

 

[vertciel]

Thanks for your reply, ygggdrasil.

Does this exception occur often? If so, what would be the maximum number of valence electrons allowed for chlorine? 20?

 

[Mike H]

vertciel - you may be interested in looking up the topic of hypervalent compounds, of which chloric acid is one. I wouldn't really think of them as all that common, but they do pop up frequently enough that one should be aware of them.

The Wikipedia page notes that chloric acid tends to decompose when not kept under certain conditions (under a certain concentration and cold). It's not something which you keep around on a lab bench. Some hypervalent compounds (particularly those of sulfur and phosphorus) tend to be more stable under ambient conditions, though.