Tuesday, October 21st, 2014

Student question on Lewis structures and formal charge

I was recently asked the following question by a student from Rhode Island. This type of question comes up a lot, and I thought it would be worthwhile flagging it for general consumption.

Here’s the original question;

……in class on Friday we students ran into a problem. We were reviewing the Lewis dot structure, and, when constructing the diagram for ClO2-, we couldn’t figure out why it can’t be written as Cl-O-O (as opposed to O-Cl-O, the textbook’s answer). Formal charge tells us that the closer the formal charge is to zero, then it is most likely correct. With the diagram Cl-O-O, the formal charge is 0, 0, and -1, respectively. With the diagram O-Cl-O, the formal charge is -1, +1, and -1, respectively. If you could explain why ClO2- ‘s lewis dot structure is O-Cl-O and not Cl-O-O despite the formal charge issue, our class would be really grateful!

and my answer;

LOTS of great lessons here!

Firstly, I am of the opinion (along with several other AP teachers) that formal charge is overused. What do I mean by that? Well, UNLESS an AP question actually asks for it, OR the question somehow implies that you should use it, IF you can follow the regular rules of drawing Lewis structures (putting the least electronegative atom in the center, making up the octets and taking care of all of the valence electrons) then you should not even BOTHER to look for alternative structures. EVEN if alternative structures DO have lower formal charges, there’s really no reason to look for them. IF you were ALWAYS looking for “better” formal charge structures then you could be looking forever! It’s just not a pragmatic thing to do.

I would agree that the Cl-O-O structure DOES have a lower formal charge, but it does NOT have the most electronegative atom in the center, and as such I would have never bothered to look for it. It’s also interesting to note that this structure, O=Cl=O with 2 lone pairs on each atom has an equally “good” formal charge (and it is theoretically OK to expand the octet of Cl), as do O-Cl=O & O=Cl-O with three lone pairs on one oxygen and two pairs on the other atoms. All of these have Cl at the center of the ion which satisfies having the least electronegative atom in that position.

OK, so what does all of this mean? Well, I would say the following;

1. I like the book answer for the reasons I have stated above (no need to look for OTHER structures).

2. I DON’T DISLIKE your answer of Cl-O-O, and I feel that I would be prepared to accept it as an answer.

3. If this were a REAL AP question, I would be happy (as a grader) to accept many different answers, BUT IF I were writing the question I would want to make it clear IF I was looking for a consideration of formal charge.

4. A simple answer in the back of a textbook is often NOT the only way to think about things!

5. You would probably need other data to KNOW for sure which one of the structures is “best”.

Probably more than you wanted to know but I hope this helps.