1) Formal charges can be used to determine the preferred resonance structure of SCN- by identifying the structure with the most neutral formal charges. 2) Formal charges are also used to determine if a Lewis structure is reasonable by calculating if any formal charges are violated, such as sulfur exceeding its octet in a proposed Lewis structure of SO2. 3) Formal charges identify which part of a polyatomic ion is charged, such as the two single-bonded oxygen atoms having a negative charge and nitrogen having a positive charge in NO3-.

1. FORMAL CHARGE AND
LEWIS DOT DIAGRAMS

2. FORMAL CHARGE AND LEWIS DOT DIAGRAMS
Draw the Lewis Dot Diagram for SCN-
-
S C N
6 + 4 + 5 + 1 = 16 electrons
But this isn’t the only possible way to draw the structure

3. FORMAL CHARGE AND LEWIS DOT DIAGRAMS
Draw the 3 possible Lewis Dot Diagrams for SCN-
-
S C N
- -
S C N S C N
Which of these is the most likely resonance structure?

4. FORMAL CHARGE AND LEWIS DOT DIAGRAMS
Formal charges can be used to determine
1)which resonance structure is preferred over the
others when several resonance structures exist;
2)whether a particular structure is reasonable;
3)which part of a molecule is charged.
Formal Charge = Valence Electrons − (number of nonbonding electrons + number of bonds)
NOTE:
Formal charge is NOT the overall charge of a molecule.
Instead, they add up to the charge of a molecule.

5. FORMAL CHARGE AND LEWIS DOT DIAGRAMS
Assigning formal charges:
Formal Charge = Valence Electrons − (number of nonbonding electrons + number of bonds)
-1 0 0
-
S C N
S: 6 - (6 + 1) = -1 N: 5 - (2 + 3) = 0
C: 4 - (0 + 4) = 0

6. FORMAL CHARGE AND LEWIS DOT DIAGRAMS
Assign formal charges to the other structures for SCN-
Formal Charge = Valence Electrons − (number of nonbonding electrons + number of bonds)
+1 0 -2
- 0 0 -1 -
S C N S C N
S: 6 - (2 + 3) = +1 S: 6 - (4 + 2) = 0
C: 4 - (0 + 4) = 0 C: 4 - (0 + 4) = 0
N: 5 - (6 + 1) = -2 N: 5 - (4 + 2) = -1

7. FORMAL CHARGE AND LEWIS DOT DIAGRAMS
Example #1: Compare the SCN- structures. Which one
has the most neutral formal charges?
+1 0 -2
- 0 0 -1 -
S C N S C N
-1 0 0
-
S C N
This is the most preferred structure. There are the most 0
formal charges and the most electronegative atom has the
negative charge.

8. FORMAL CHARGE AND LEWIS DOT DIAGRAMS
Formal charges can also be used to determine if a
structure is reasonable.
Example #2: Can SO2 have this Lewis structure?
0 0 0
O S O
Sulfur exceeds the octet
O: 6 - (4 + 2) = 0 ALL formal charges
are zero. Thus this
S: 6 - (2 + 4) = 0 structure is
reasonable.
Formal Charge = Valence Electrons − (number of nonbonding electrons + number of bonds)

9. FORMAL CHARGE AND LEWIS DOT DIAGRAMS
Formal charges can also be used to determine if a
structure is reasonable.
Example #2: Can SO2 have this Lewis structure?
In fact, this structure is preferred
0 0 0 0 +1 -1
O S O vs
O S O
Often it is preferred to violate the Octet Rule in order
to achieve a zero formal charge on an atom.

10. FORMAL CHARGE AND LEWIS DOT DIAGRAMS
Furthermore, formal charges can be used to identify
which part of a molecule is charged
Example #3: Which part of NO3- is charged?
-1 -
0
+1
-1
The 2 single-bonded
O: 6 - (6 + 1) = -1 oxygen atoms have
a negative charge,
O’: 6 - (4 + 2) = 0 and nitrogen has a
N: 5 - (0 + 4) = +1 positive charge.

11. FORMAL CHARGE AND LEWIS DOT DIAGRAMS
Furthermore, formal charges can be used to identify
which part of a molecule is charged
Example #3: Which part of NO3- is charged?
Why can’t NO3- be drawn this way so that the central atom does
not have a positive charge?
0
0
0
-1
Nitrogen cannot exceed the octet. Only elements beyond
atomic number 10 (Neon) can exceed the Octet Rule.