Calculate Formal Charge of Chlorine in HClO3

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In summary, the formal charge on chlorine in HClO3 is +2 when all the bonds on chlorine are single bonds, as opposed to +5 or +7 when oxygen and chlorine form double bonds. This is because oxygen has a higher formal charge of -2, and assigning all single bonds to chlorine causes the formal charge on oxygen to be -1. However, this is an artificial situation and does not reflect the true physical nature of molecules. Formal charge should only be used for bookkeeping purposes and does not have any real-world significance.
  • #1
zhen
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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?
 
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  • #2
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.
 
  • #3
Astronuc said:
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?
 
  • #4
"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.
 
  • #5
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 [Broken]
 
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  • #6
thanks, I noticed I was drawing the wrong structure, which the H has connected to the Cl.
 

What is the formula for calculating formal charge?

The formula for calculating formal charge is:

Formal charge = [Number of valence electrons on the atom] - [Number of lone pair electrons] - [1/2 * Number of bonding electrons]

How do I determine the number of valence electrons on an atom?

The number of valence electrons on an atom can be determined by looking at its group number on the periodic table. For example, chlorine is in group 7, so it has 7 valence electrons.

What is the formal charge of chlorine in the compound HClO3?

To calculate the formal charge of chlorine in HClO3, we must first determine the number of valence electrons on chlorine. Since chlorine is in group 7, it has 7 valence electrons. Next, we count the number of lone pair electrons on chlorine, which is 0 in this case. Then, we count the number of bonding electrons between chlorine and other atoms, which is 3 (1 bond with hydrogen and 2 bonds with oxygen). Plugging these values into the formula, we get:

Formal charge = [7 valence electrons] - [0 lone pair electrons] - [1/2 * 3 bonding electrons] = +5

What is the significance of formal charge?

Formal charge is important because it helps us determine the most stable Lewis structure for a molecule or ion. The structure with the lowest formal charges on each atom is typically the most stable.

How can I use formal charge to predict the reactivity of a molecule?

Formal charge can give us an idea of the distribution of electrons in a molecule, which can affect its reactivity. Molecules with high or low formal charges on certain atoms are more likely to be reactive, as they may be more likely to gain or lose electrons to achieve a more stable structure.

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