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Lec-26: Resonance & Formal Charge

Roy A. Lacey, Stony Brook University; Che 131, Spring 2013

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Lewis Structure

Roy A. Lacey, Stony Brook University; Che 131, Spring 2013

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Covalent Bonds

Roy A. Lacey, Stony Brook University; Che 131, Spring 2013

Bond Polarity

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Polar Covalent Bond:
• Unequal sharing of electrons in covalent bonds result in uneven distribution of charge. • Results from a difference in the
“electronegativity” of bonded atoms
• Polarity indicated by arrow pointing to more negative end, “+” at more positive end. Roy A. Lacey, Stony Brook University; Che 131, Spring 2013

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Bond Polarity



Electronegativity (EN):

• Ability of an atom to attract bonding electrons.
• Periodic trend similar to ionization energy.

Electro-negativities
Roy A. Lacey, Stony Brook University; Che 131, Spring 2013

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Bond Polarity

Roy A. Lacey, Stony Brook University; Che 131, Spring 2013

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Polarity of molecules

Polar molecule

Non-polar molecule

Polar molecule

Bond Polarity and geometry determine the polarity of a molecule Roy A. Lacey, Stony Brook University; Che 131, Spring 2013

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Lewis Structure

Resonance Structures

Roy A. Lacey, Stony Brook University; Che 131, Spring 2013

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Resonance Structures
#VE = 18
-4
--14

Draw the Lewis structure for O3

Then share electrons

O O O

O=O O

O O O

Skeletal structure

Satisfy octet
With remaining electrons O OO

Resonance Structures

Roy A. Lacey, Stony Brook University; Che 131, Spring 2013

Resonance Structures

O3

Which is the Correct structure?

O O O

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O O O

- all structures can exist
 Use double arrow to indicate this

 Sometimes we can draw an “average” or hybrid structure We use formal charges to determine the preferred Lewis structure(s)
Roy A. Lacey, Stony Brook University; Che 131, Spring 2013

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Formal Charge
FC(atom) = (valence e-) - (1/2 bonding e-) - (lone electrons)

O OO
A

B

-1


+1 0

O OO

0

+1 -1

O=O O

C

A = 6 – 1 – 6 = -1
B = 6 – 3 – 2 = +1
C=6–2–4= 0

O
6 – 3 – 2 = +1

 The preferred structure is the one with the most atoms having formal charge = 0
 A negative formal charge should be on the most electronegative atom(s).
Roy A. Lacey, Stony Brook University; Che 131, Spring 2013

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Using Formal Charge
Notation

Draw the other resonance structures with formal charge

Roy A. Lacey, Stony Brook University; Che 131, Spring 2013

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Resonance Structures
Important Note

When drawing resonance structures, only electrons should be moved; Atoms are never moved:
O

O

This: H O C

H O C

O

O

Use "push" curved arrows

Not
This:

O
H

O

O

C

O

O

C
O
H

 You can only move electrons in multiple bonds or lone pairs.
 The overall charge of the system must remain the same.
 The bonding framework of a molecule must remain intact.

We use formal charges to determine the preferred structure(s)
Roy A. Lacey, Stony Brook University; Che 131, Spring 2013

Using Formal Charge
N= 5
3O
18
1e
1
--24
-6
---18

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Draw the Lewis structure for NO3-

Then share electrons

Satisfy octet
With remaining electrons no Lewis diagram is complete without formal charges

Draw the other