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 OO
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
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
13
Draw the Lewis structure for NO3-
Then share electrons
Satisfy octet
With remaining electrons no Lewis diagram is complete without formal charges
