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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: lttoler on April 14, 2012, 10:10:16 PM

Title: Formal charge of phosphorus in phosphate ion?
Post by: lttoler on April 14, 2012, 10:10:16 PM

In my General Chemisty class, this was one of the homework questions this past week:

What is the formal charge on phosphorus in a Lewis structure for the phosphate ion that satisfies the octet rule?

A: -2
B: -1
C:  0
D: +1
E: +2

The teacher has the answer marked as D: +1, but I get 0 as my answer.  I have Phosphorus in the middle with a double bond and 3 single bonds giving it a formal charge of 0.

Am I doing something wrong?  Any help would be appreciated :)

Title: Re: Formal charge of phosphorus in phosphate ion?
Post by: UG on April 14, 2012, 11:21:48 PM

Quote from: lttoler on April 14, 2012, 10:10:16 PM

I have Phosphorus in the middle with a double bond and 3 single bonds

But note that this structure doesn't satisfy the octet rule. The central P atom has too many electrons.

Title: Re: Formal charge of phosphorus in phosphate ion?
Post by: lttoler on April 14, 2012, 11:37:23 PM

okay, I was thinking this must be the case.  But I was under the impression that is was not a valid structure unless the Phosphorus got the extra electrons.

Title: Re: Formal charge of phosphorus in phosphate ion?
Post by: UG on April 14, 2012, 11:38:52 PM

I think you need the one with four single P-O bonds.

Title: Re: Formal charge of phosphorus in phosphate ion?
Post by: grs35 on April 16, 2012, 12:04:53 AM

phosphate is an polyatomic ion and the charge for the whole thing is -3

so there are no single or lone pair electrons. P is in the middle and a double bond with an O and 3 single bonds with the other O atoms.

formal charge is :

FC = valence electrons - number of bonds - lone pairs

I won't give the answer for P but you can use this to check.