I'm supposed to demonstrate that by adding up the formal charge of each atom I can obtain the overall net charge. 2-.. (Valence e)-(non-bonded e)-(1/2 bonded e) = F.C
Sulfur F.C = 6-0-(8/2) = +2
Oxygen (single bond) F.C = 8-6-(2/2)=-1
Oxygen (single bond) F.C = 8-6-(2/2)=-1
Oxygen (double bond) F.C = 8-4-(4/2)=0
Add these up and you get 0... Not 2- like it should be. Why???
Sulfur F.C = 6-0-(8/2) = +2
Oxygen (single bond) F.C = 8-6-(2/2)=-1
Oxygen (single bond) F.C = 8-6-(2/2)=-1
Oxygen (double bond) F.C = 8-4-(4/2)=0
Add these up and you get 0... Not 2- like it should be. Why???
-
Formal charge is not an actual charge. Formal charge is useful for determining the arrangement of atoms within a molecule, and secondarily, for determining the bond order of individual bonds. You want to draw the Lewis structure so that formal charges are minimized. The sum of the formal charges on the elements in a compound will add up to zero. The sum of the formal charges in a polyatomic ion will add up to the charge on the ion.
First draw the Lewis structure for the polyatomic ion.
SO3^2- has a total of 26 electrons, including three lone pairs on each singly bonded oxygen, two lone pairs on the doubly bonded oxygen and a lone pair on sulfur. The sulfur is hypervalent, meaning that it has more than an octet of electrons, but this will minimize the formal charges, making it more in line with the bond orders which are observed.
'''''''..
O-S=O
.... |
....O
FC = VE - NBE - 1/2 BE .... VE = valence electrons, NBE = non-bonding electrons, BE = bonding electrons
S --> FC = 6 - 2 - 1/2(8) = 0
=O --> FC = 6 - 4 - 1/2(4) = 0
-0 x 2 --> FC = 6 - 6 - 1/2(2) = -1
The sum of the formal charges is 0 + 0 + (-1) + (-1) = -2
This is consistent with the -2 charge on the ion.
You have several problems with your solution.
1. Not enough electrons. I would suggest that your Lewis structure only shows 24 electrons and not 26. Therefore, you omitted a lone pair on sulfur.
2. When you calculated the FC of the oxygen atoms, you wrote the number of valence electrons as 8. Oxygen ats have 6 valence electrons.
First draw the Lewis structure for the polyatomic ion.
SO3^2- has a total of 26 electrons, including three lone pairs on each singly bonded oxygen, two lone pairs on the doubly bonded oxygen and a lone pair on sulfur. The sulfur is hypervalent, meaning that it has more than an octet of electrons, but this will minimize the formal charges, making it more in line with the bond orders which are observed.
'''''''..
O-S=O
.... |
....O
FC = VE - NBE - 1/2 BE .... VE = valence electrons, NBE = non-bonding electrons, BE = bonding electrons
S --> FC = 6 - 2 - 1/2(8) = 0
=O --> FC = 6 - 4 - 1/2(4) = 0
-0 x 2 --> FC = 6 - 6 - 1/2(2) = -1
The sum of the formal charges is 0 + 0 + (-1) + (-1) = -2
This is consistent with the -2 charge on the ion.
You have several problems with your solution.
1. Not enough electrons. I would suggest that your Lewis structure only shows 24 electrons and not 26. Therefore, you omitted a lone pair on sulfur.
2. When you calculated the FC of the oxygen atoms, you wrote the number of valence electrons as 8. Oxygen ats have 6 valence electrons.