Resonance and Formal Charge

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.

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.

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

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

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

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 2 + 4 = 6. 19 8

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

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

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

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

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

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

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

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

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

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

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)

Exceptions to the Octet Rule

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

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.

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

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