Help Request : formal charge

[DieselPetrolGrl]

Hi
Can someone please help me hash out the concept of formal charge?


Is it the just the charge on a atom in a molecule

?Formal charge = # valence electrons- # valence electrons bound?

Why doesn?t that work when I try it..
For example in Nitromethane (CH3NO2)

why is the formal charge on Nitrogen +1
?when I tried it I came up with -3 (nitrogen has 5 valence e- and 8 are bound to it so 5-8 = -3)

Thank you for any help!

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[willthatsall]

only half of the bonded pairs count. that's all. so it is:

Formal charge = V - 1/2 bondend electrons - nonbonding electrons

 

[kiahs]

I have Formal charge as valence - (number of nonbonding elect + number of bonds) for the ch3no2 the nitrogen has one double bond to oxygen right? When I did it my way I got +1 as well. Becuse one of the oxygens will have a negative charge and it will be balanced by the positive of the nitrogen. I think that has to be it because I tried it a bunch of other ways and am getting weird answers hope that helps.

 

[DieselPetrolGrl]

I have Formal charge as valence - (number of nonbonding elect + number of bonds) for the ch3no2 the nitrogen has one double bond to oxygen right? When I did it my way I got +1 as well.

i dont know about your way - yes you got the write answer though wills method worked for the Oxygen also should be -1
(Willthatsall's works)

(This is all from Ochem by McMurry pg 35 - Sorry if i am not supposed to comp problems its not really a problem its a diagram thing/example)

 

[Assembler]

It's probably because you don't have the correct formula.

Formal Charge = V - 1/2(B) - L = Valence e minus 1/2 (bonded electrons) minus number of lone pairs.

So, FC = 5 - 1/2(8) - 0 = 5-4 = +1

 

[thegenius]

only half of the bonded pairs count. that's all. so it is:

Formal charge = V - 1/2 bondend electrons - nonbonding electrons

Exactly - another way to look at it is Valence electrons - # of bonds - nonbonding electrons. Note that it's # of bonds (not # of electrons in bonds.)

Both ways will always give you the right formal charge.

Now, oxidation state - I had trouble with that one on a previous exam (althought I got it right with an educated guess).

What is the oxidation state of Arsenic in As2S3? I wasn't sure how to figure this one out.

 

[willthatsall]

Exactly - another way to look at it is Valence electrons - # of bonds - nonbonding electrons. Note that it's # of bonds (not # of electrons in bonds.)

Both ways will always give you the right formal charge.

Now, oxidation state - I had trouble with that one on a previous exam (althought I got it right with an educated guess).

What is the oxidation state of Arsenic in As2S3? I wasn't sure how to figure this one out.

+3, S is -2

 

[DieselPetrolGrl]

+3, S is -2

i put +6 because i thought they meant in the compound not per molecule..was my thinking ok if the Q was pertaining to compound...how will i know if they mean compound or just the molecule as it is in the compound?

 

[thegenius]

+3, S is -2

How do you know that S is -2? Because it's just left of CL?