Topscore GChem 1 - Wrong?

[NumbaOneStunna]

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What am I overlooking here?
Here's my thought process - tell me where Im wrong:
- For Lewis Dot structures, total # e- = total # of valance e- from all atoms
- So here it's 36 (4*7+8). So all the electrons should be used up in bonding the F's to Xe giving A. Where does Xe get the extra electrons from? I know it has an expanded octet, but that just means it can accept more electrons than usual - I always though that for these kind of atoms they only bring their group # of atoms into lewis dot structures.
Thanks a ton! (Answer is D, I thought it was A)


*** Disregard this thread *** wasn't thinking

 

[WOAHHI]

Hey bro - I think it's because the F's will have a formal charge with A

 

[baedero1]

yes, because of the formal charge

 

[NumbaOneStunna]

Holy **** I'm stupid, sorry for the dumb question. 🙂 I guess that's a sign that I should stop studying for today.

 

[OC1314]

Nope, D is correct. First count all valence e-s, which is 36. Then realize that each atom should have the same e-s around it as its valence number. This means you count all the nonbonding e-s BUT for each bond, you only count ONE e-. So in A, the double bonded F each have 4 nonboding e-s and two e-s from the bonds (one from each bond). This would equal 6 in all, but we need to have 7 bc F has a valence of 7. Also, Xe has to have 8 e-s but in A it only has 6 e-s because of the 6 bonds. However in D, all F have 7 e-s and Xe has 8. 🙂

 

[suhshi]

OP

how do you post pics like that?

 

[toothdoc0121]

the formal charge on the F and Xe in choice D is 0 as which would make it the best possible structure.

For choice D
F= (-7) + 1 + 6 = 0
Xe= (-8) + 4 + 4 = 0

overall charge is 0

For choice A

F(w/double bond)= (-7) + 2 + 4 = -1
F(w/o double bond)= (-7) + 1 + 6= 0
Xe= (-8) + 6 + 0= -2

overall charge is -4

hope this helps

 

[OC1314]

Post pics by using attachments.

 

[NumbaOneStunna]

I print screen the problem outta the test on my computer, then I go to paint paste it and select the part I want with the dotted rectangle tool, paste it into a new file that doesnt have any excess space - save it, go on SDN and make a post scroll down and attach the file by clicking manage attachments. Hope that helps.

 

[suhshi]

I print screen the problem outta the test on my computer, then I go to paint paste it and select the part I want with the dotted rectangle tool, paste it into a new file that doesnt have any excess space - save it, go on SDN and make a post scroll down and attach the file by clicking manage attachments. Hope that helps.

thanks

oh yea
remember when drawing Lewis dot structures halogens are usually not doublebonded

as said by my ochem teacher sumbody stop me if im wrong