1. Resonance and Formal Charge

2. SAMPLE PROBLEM:
Writing Resonance Structures
PROBLEM:
Write resonance structures for the nitrate ion, NO3-.
SOLUTION:
Nitrate has 1(5) + 3(6) + 1 = 24 valence e-
N does not have an octet; a pair of e- will move in to form a double bond.

4. Four criteria for choosing the more important resonance structure:
1. Smaller formal charges (either positive or negative) are preferable to larger charges;
2. A more negative formal charge should exist on an atom with a larger EN value.

5. Resonance and Formal Charge
Formal charge of atom =
# valence e- =
(# unshared electrons +
1/2 the # shared electrons)

6. Nitric acid : .. H O N ..
We will calculate the formal charge for each atom in this Lewis structure. 19 6

7. Nitric acid Formal charge of H : .. H O N ..
Hydrogen shares 2 electrons with oxygen.
Assign 1 electron to H and 1 to O.
A neutral hydrogen atom has 1 electron.
Therefore, the formal charge of H in nitric acid is 0. 19 7

8. Nitric acid : .. H O N Formal charge of O ..
Oxygen has 4 electrons in covalent bonds.
Assign 2 of these 4 electrons to O.
Oxygen has 2 unshared pairs. Assign all 4 of these electrons to O.
Therefore, the total number of electrons assigned to O is = 6. 19 8

9. Nitric acid : .. H O N Formal charge of O .. Electron count of O is 6.
A neutral oxygen has 6 electrons.
Therefore, the formal charge of oxygen is 0. 19 9

10. Nitric acid : .. H O N Formal charge of O ..
Electron count of O is 6 (4 electrons from unshared pairs + half of 4 bonded electrons).
A neutral oxygen has 6 electrons.
Therefore, the formal charge of oxygen is 0. 19 10

11. Nitric acid : .. H O N Formal charge of O ..
Electron count of O is 7 (6 electrons from unshared pairs + half of 2 bonded electrons).
A neutral oxygen has 6 electrons.
Therefore, the formal charge of oxygen is -1. 19 11

12. Nitric acid : .. H O N Formal charge of N – ..
Electron count of N is 4 (half of 8 electrons in covalent bonds).
A neutral nitrogen has 5 electrons.
Therefore, the formal charge of N is +1. 19 12

13. Nitric acid
Formal charges : .. H O N + – .. 19 13

14. Formal Charge An arithmetic formula for calculating formal charge.
Number of
valence
electrons
number of bonds
number of unshared electrons – – 16 14

15. "Electron Counts" and Formal Charges in NH4+ and BF4-.. B F : 1 4 N H + 7 – 4 17 15

16. Formal Charge: Selecting the Best Resonance Structure
An atom “owns” all of its nonbonding electrons and half of its bonding electrons.
Formal charge of atom =
# valence e-
- # unshared electrons
- 1/2 # shared electrons

17. For OB
# valence e- = 6
# nonbonding e- = 2
# bonding e- = 6 X 1/2 = 3
Formal charge = +1
For OC
# valence e- = 6
# nonbonding e- = 6
# bonding e- = 2 X 1/2 = 1
Formal charge = -1
For OA
# valence e- = 6
# nonbonding e- = 4
# bonding e- = 4 X 1/2 = 2
Formal charge = 0

18. Resonance and Formal Charge
EXAMPLE: NCO- has 3 possible resonance forms - A B C

19. Now Determine Formal Charges-2 +1 -1 -1
Forms B and C have smaller formal charges; this makes them more important than form A. (rule 1)
Form C has a negative charge on O which is the more electronegative element, therefore C contributes the most to the resonance hybrid. (rule 2)

20. Exceptions to the Octet Rule

21. Exceptions to the Octet Rule
4b. Expanded Octets – only on period 3 and higher
Expanded octets form when an atom can decrease (or maintain at 0) it’s formal charge
Ex: SF6, PCl5, SO2, SO3, SO4
5a. Electron deficient – have fewer than 8
Ex: BeCl2, BF3
may attain an octet by coordinate covalent bond
Odd number of electrons – aka free radicals
Ex: NO2
May attain an octet by pairing with another free radical

22. Writing Lewis Structures for Exceptions to the Octet Rule.
SAMPLE PROBLEM:
Writing Lewis Structures for Exceptions to the Octet Rule.
PROBLEM:
Write the Lewis structure for BFCl2.
PLAN:
Draw the Lewis structures for the molecule and determine if there is an element which can be an exception to the octet rule.
SOLUTION:
BFCl2 will have only 1 Lewis structure.

23. Resonance and Formal Charge: Let’s see if you can:
Define resonance
Determine resonance structures for a molecule
Calculate the formal charge for an atom
Determine the resonance structure that contributes the most to a compound by using formal charge

24. -1 -1 1 1 1 -1 -1 -1 -1
Is there a better structure??
No!!