Calculate Formal Charge of Chlorine in HClO3

[zhen]

question: Calculate the formal charge on chlorine in HClO3 using the resonance structure in which all the bonds on chlorine are single bonds.

I know the formal charge equals to the valance electron on a free atom minus the valance electron assigned to it in a molecule.

so my answer is +3, unfortunately, that is wrong.
can someone tell me why?

 

[Astronuc]

Oxygen and chlorine would form double bonds in this case, so Cl is +5. Two oxygens form double bonds.
See - http://en.wikipedia.org/wiki/Chloric_acid.

HClO₄, perchloric acid, would have Cl with +7 state.

 

[zhen]

Oxygen and chlorine would form double bonds in this case, so Cl is +5. Two oxygens form double bonds.
See - http://en.wikipedia.org/wiki/Chloric_acid.
HClO₄, perchloric acid, would have Cl with +7 state.

but the correct answer is +2, nor 5 or 7.
why?

 

[Bystander]

"Formal charge" games are just that, games. Ordinarily, as A-nuc has done, one follows a heirarchy of formal charges in which oxygen outranks every other species with a formal charge of -2. The part of the problem statement, "...in which all the bonds on chlorine are single bonds," implies an unnatural situation in which you are asked to calculate formal charge on Cl with -1 formal charge on the three oxygen atoms.

It's artificial, it's silly, and it looks to be something your instructor or text want you to do. There's absolutely no physical reason, quantum mechanical calculation, rationalization, or anything else to support such a picture. Nor, strictly, is there any reason to support a -2 state for oxygen in all cases. "Formal charge" is something you'll need to take seriously in courses that present molecular structures in terms of formal charge, but in the long run, the concept is useless beyond doing the bookkeeping on charge.

 

[Astronuc]

I would think Cl with 5 valence electrons - 3 electrons for the single bonds would give 2.

See - http://www.utdallas.edu/~parr/chm1341/13410709.html - go to bottom of page for the example of HClO₃ with the oxygen atoms each developing a single bond with the Cl atom, and Cl has +2 and O has -1 formal charge.

Below is the correct structure in which two oxygens form a double bond and the formal charges on Cl and each O is 0.

See also the discussion of formal charge - http://www.scientia.org/cadonline/Chemistry/bonding/formalcharge.ASP

 

[zhen]

thanks, I noticed I was drawing the wrong structure, which the H has connected to the Cl.

 

[Astronuc]

I found another discussion for 'formal charge' here -

http://www.wpi.edu/Academics/Depts/Chemistry/Courses/General/concept3.html (just use Find (on this page) "formal charge")