How Do You Identify Resonance Structures for SO4^2- and H3PO4?

[nobb]

Hey
There are two questions asking me to identify the resonance structures for:
SO4 two minus and H3PO4 (sorry, I don't know how to do subscripts)
Could someone please explain to me how to do these resonance structures? Thanks.

 

[maverick280857]

First note that the sulfate anion has a sulfur covalently bonded to four oxygens two of which have a negative charge and are singly bonded and the other two are double bonded. Now shift the pi electrons from either negatively charged oxygen to the S-O bond while shifting a pair of pi electrons from either of the two doubly bonded oxygen-Sulfur bonds to the oxygens to get identical resonance structures (do this for each such oxygen).

I'll leave H3PO4 to you now. I think its

HO-(O=P)-OH
|
OH

Check it though.

Cheers
Vivek

 

[nobb]

Ok...I managed to draw this resonance structure. But could you please explain why two oxygens are double bonded to the sulfur, while two oxygens are singly bonded? How did you know to do this? I am really confused on resonance structures.
By the way...are you sure the H3PO4 you described is right? I did a google image search and it appears that there are 4 oxygen atoms bonded to the phosphorus.

 

[maverick280857]

Please check the structure for H3PO4 it didn't come out correctly in the post. Why google it. Your book would have it (every standard general chem textbook has something--usually--on oxyacids of chlorine, phosphorous).

Sometime back I replied to somebody's query about lewis structures. This link may help you: https://www.physicsforums.com/showthread.php?t=45144&highlight=Clarke's+Method

Structures of acids, cations and anions are very frequently encountered in general chemistry (and more so in theoretical organic chemistry where resonance in these "species" is the core of most ideas) so I suggest you become familiar with them with repeated practice. I just knew how to do it because I've done it before.

Hope that helps...

Cheers
Vivek