1. Chapter 8 – Lewis Structures of Covalent Molecules
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2. Lewis Structure of Molecules
Add up all the valence electrons.
If it is an anion, add one electron for each negative charge.
If it is a cation, subtract one electron for each positive charge.
PCl3
(7) = 26
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3. Lewis Structure of Molecules
The central atom is the least electronegative element (Never hydrogen!).
Connect the outer atoms to it by single bonds. (Each line represents 2 shared electrons.)
Keep track of the electrons:
= 20
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4. Lewis Structure of Molecules
4. Fill the octets of the outer atoms.
Keep track of the electrons:
= 20; = 2
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5. Lewis Structure of Molecules
5. Fill the octet of the inner atoms.
Keep track of the electrons:
= 20; = 2; = 0
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6. Lewis Structure Practice
Add up all the valence electrons.
CF4
(7) = 32
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7. Lewis Structure Practice
The central atom is the least electronegative element (Never hydrogen!).
Connect the outer atoms to it by single bonds. (Each line represents 2 shared electrons.)
Keep track of the electrons:
= 24
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8. Lewis Structure Practice
4. Fill the octets of the outer atoms.
Keep track of the electrons:
= 24; = 0
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9. Lewis Structure of Molecules Double and Triple Bonds
1. Add up all the valence electrons.
CO2
(6) = 16
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10. Lewis Structure of Molecules Double and Triple Bonds
The central atom is the least electronegative element (Never hydrogen!).
Connect the outer atoms to it by single bonds. (Each line represents 2 shared electrons.)
Keep track of the electrons:
= 12
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11. Lewis Structure of Molecules Double and Triple Bonds
Fill the octets of the outer atoms.
HOWEVER, Carbon only has 4 valence electrons!
Keep track of the electrons:
= 12; = 0
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12. Lewis Structure Practice
5. To solve this problem, Carbon and Oxygen will share 2 pairs of electrons, a DOUBLE BOND.
Keep track of the electrons:
= 12; = 0
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13. Lewis Structure of Molecules Double and Triple Bonds
1. Add up all the valence electrons. N2
2(5) = 10
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14. Lewis Structure Practice
Connect the atoms by a single bond.
12 electrons are needed to complete the octets with a single bond, but only 8 are left.
N - N
Keep track of the electrons:
= 8
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15. Lewis Structure Practice
In order to achieve the octet, each nitrogen will share 3 pairs of electrons, a TRIPLE BOND.
Keep track of the electrons:
= 8,
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16. Ammonium, NH4+ Total valence electrons = 8
Polyatomic Ions
Ammonium, NH4+ Total valence electrons = 8
Total number of electrons around each atom: 8 + 4x2 = 16
Total number of shared electrons: 16-8=8
The polyatomic ammonium ion (NH4), present in ammonium sulfate, is an important component of fertilizer for field crops, home gardens, and potted plants.

17. Resonance Draw the Lewis Structure for NO3-
However, NO3- can be written with the double bond on each O.
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18. Resonance
In the actual structure of NO3- all three bonds are the same length.
We use multiple structures, resonance structures, to describe the molecule.
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19. Resonance This is the Lewis structure we would draw for ozone, O3.
But…
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20. Resonance
… this is at odds with the true, observed structure of ozone, in which…
…both O-O bonds are the same length.
…both outer oxygens have an equal negative charge.
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21. Resonance
One Lewis structure cannot accurately depict a molecule like ozone.
We use multiple structures, resonance structures, to describe the molecule.
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22. Aromatics
An aromatic organic molecule is a ring of carbon atoms with resonance structures.
The term “aromatic” originally referred to their pleasant smells.

23. Aromatic Compounds Benzene, C6H6
Benzene, C6H6, is a perfect hexagon and all carbon-carbon bonds have the same length: between the length of single and double bonds.
Benzene, C6H6
The actual structure is an average of the resonance
structures.

24. Aromatic Hydrocarbons
The three symbols below can be used to represent the benzene ring. If the bonds are shown as alternating single and double bonds (on right), it is important to remember that bonds don’t actually alternate and that all six bonds are identical.

25. Resonance Practice
Which is predicted to have the shorter sulfur-oxygen bonds, SO3 or SO32-?
SO3 has shorter S-O bonds because of the double bond in the resonance structure.
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26. Resonance in Polyatomic Ions
Resonance in a carbonate ion:
Resonance in an acetate ion:

27. Formal Charge
If there are several options for the Lewis structure, how do you know which one is correct?
ANSWER: Determine the formal charge of each atom
Definition: The formal charge is the charge the atom would have if all atoms had the same electronegativity.
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28. Formal Charge
Formal Charge =
[# valence e-] – [# non-bonded electrons + number of bonds]
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29. Lewis Structures & Formal Charge
The best Lewis structure…
…is the one with the fewest charges and/or
…puts a negative charge on the most electronegative atom.
In the example below: The first structure with 2 double bonds is the best structure.
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30. Formal Charge
Formal Charge = [# valence e-] – [# non-bonded electrons + number of bonds]
Determine the formal charge of each atom in carbon monoxide, CO:

31. Lewis Structures & Formal Charge
Calculate the formal charges for the atoms below and determine the best Lewis structure:
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32. Lewis Structures & Formal Charge
The correct structure has the fewest charges and/or negative charge on the most electronegative.
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33. Lewis Structures & Formal Charge
Calculate the formal charges for the atoms below and determine the best Lewis structure:
Answer: The triple bond between the nitrogen and carbon gives the best Lewis structure.
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