Drawing Lewis Structures
The Octet Rule
1. Determine the arrangement of atoms in the molecule
•
Some elements work only as terminal atoms (H, F, Cl, etc)
•
Some are par@cularly good as internal atoms (C, N)
2. Determine the total number of valence electrons in all atoms (don’t forget charge!)
•
Add numbers of all valence electrons
•
If nega@vely charged, add the appropriate number of electrons
•
If posi@vely charged, subtract the appropriate number of electrons
3. Place one pair of electrons between each pair of bonded atoms, forming a bond
4. Use remaining electron pairs as lone pairs around each terminal atom, to complete its octet
5. Place electrons around the central atom. If you run out, “share” electron pairs from terminal atoms
Which is the best Lewis structure? [
2) [
1)
3)
C
C
[C
]
N]
N
N
]
Which is the best Lewis structure? [
2) [
1)
3)
[
C
C
C
]
N]
N
N
]
10 e-
C – [He]2s22p2
N – [He]2s22p3
12 e-
Plus 1 e‐
Too many electrons
14 eToo many electrons
Isoelectronic species
(10 electrons)
N
N
N – [He]2s22p3
N – [He]2s22p3
C
O
C – [He]2s22p2
O – [He]2s22p4
[C
N
C – [He]2s22p2
N – [He]2s22p3
2+3+2+3
2+2+2+4
]
Plus 1 e‐
2+2+2+3+1
Which is the best Lewis structure? 1)
[
[
3) [
2)
O
N
O
O
N
O
O
N
]
]
O]
16 e16 e16 e-
Which is the best Lewis structure? 1)
[
[
3) [
2)
O
N
O
O
N
O
O
N
]
]
O]
16 e16 e-
N – [He]2s22p3
O – [He]2s22p4
O – [He]2s22p4
Minus 1 e‐
16 e-
Which is the best Lewis structure? 1)
[
[
3) [
2)
More than an octet
O
N
O
O
N
O
O
N
]
]
O]
16 e16 e-
N – [He]2s22p3
O – [He]2s22p4
O – [He]2s22p4
Minus 1 e‐
16 e-
Which is the best Lewis structure? 1)
[
[
3) [
2)
More than an octet
O
N
O
O
N
O
O
N
]
]
O]
Less than an octet
16 e16 e-
N – [He]2s22p3
O – [He]2s22p4
O – [He]2s22p4
Minus 1 e‐
16 e-
p. 355
[
O
N
O
]
p. 355
[
O
N
O
]
p. 355
[
O
N
O
]
Remove an electron (any electron)
p. 355
[
O
N
O
]
Remove an electron (any electron)
p. 355
[
O
N
O
]
p. 355
[
O
N
O
]
p. 355
[
O
N
O
]
p. 355
[
O
N
O
]
p. 355
[
O
N
O
]
p. 355
• Octet Rule ‐ double count shared electrons
– assumes both atoms get both electrons
[
O
N
O
]
11
• Octet Rule ‐ double count shared electrons
– assumes both atoms get both electrons
[
8
O
8
N
8
O
]
11
Formal Charge
versus Octet Rule
• Octet Rule ‐ double count shared electrons
– assumes both atoms get both electrons
[
8
O
8
N
8
O
]
• Formal Charge ‐ Evenly split electrons in a bond
– one to one atom, one to the other
[
O
N
O
]
12
Formal Charge
versus Octet Rule
• Octet Rule ‐ double count shared electrons
– assumes both atoms get both electrons
[
8
O
8
N
8
O
]
• Formal Charge ‐ Evenly split electrons in a bond
– one to one atom, one to the other
[
O
N
O
]
12
Formal Charge
versus Octet Rule
• Octet Rule ‐ double count shared electrons
– assumes both atoms get both electrons
[
8
O
8
N
8
O
]
• Formal Charge ‐ Evenly split electrons in a bond
– one to one atom, one to the other
[
6
O
4
N
6
O
]
12
Formal Charge
versus Octet Rule
• Octet Rule ‐ double count shared electrons
– assumes both atoms get both electrons
[
8
O
8
N
8
O
]
• Formal Charge ‐ Evenly split electrons in a bond
– one to one atom, one to the other
[
6
O
4
N
6
O
]
Brought to the party
12
Formal Charge
versus Octet Rule
• Octet Rule ‐ double count shared electrons
– assumes both atoms get both electrons
[
8
O
8
N
8
O
]
• Formal Charge ‐ Evenly split electrons in a bond
– one to one atom, one to the other
[
Brought to the party
6
O
4
N
6
O
]
O
6 12
Formal Charge
versus Octet Rule
• Octet Rule ‐ double count shared electrons
– assumes both atoms get both electrons
[
8
O
8
N
8
O
]
• Formal Charge ‐ Evenly split electrons in a bond
– one to one atom, one to the other
[
Brought to the party
6
O
6
4
N
6
O
6
]
O
N
6 12
5
Formal Charge
versus Octet Rule
• Octet Rule ‐ double count shared electrons
– assumes both atoms get both electrons
[
8
O
8
N
8
O
]
• Formal Charge ‐ Evenly split electrons in a bond
– one to one atom, one to the other
[
Brought to the party
6
4
6
O
N
O
6
5
6
]
O
N
6 12
5
Formal Charge
versus Octet Rule
• Octet Rule ‐ double count shared electrons
– assumes both atoms get both electrons
[
8
O
8
N
8
O
]
• Formal Charge ‐ Evenly split electrons in a bond
– one to one atom, one to the other
[
Brought to the party
Formal Charge
6
4
6
O
N
O
6
0
5
+1
6
0
]
O
N
6 12
5
Formal Charge
versus Octet Rule
Number of electrons brought to the party
Number of electrons ending up with
6
4
6
O
N
O
Brought to the party
6
5
6
Formal Charge
0
+1
0
Ends up with
[
]
O
N
6
Ends up with one fewer than it “came with”
or wants, so N has a formal charge of +1
5
13
Formal Charge
versus Octet Rule
Number of electrons brought to the party
Number of electrons ending up with
6
4
6
O
N
O
Brought to the party
6
5
6
Formal Charge
0
Ends up with
[
+
+1
+
0
]
=
+1
Note: sum of atom formal charges must equal
the overall charge on the molecule
13
Formal Charge
versus Octet Rule
Number of electrons brought to the party
Number of electrons ending up with
6
4
6
O
N
O
Brought to the party
6
5
6
Formal Charge
0
Ends up with
[
+
+1
+
0
]
=
+1
Note: sum of atom formal charges must equal
the overall charge on the molecule
13
Example 8-3, p. 360
Cl
Example 8-3, p. 360
Cl
7
Example 8-3, p. 360
Cl
7
3
4
1
2
5
Example 8-3, p. 360
Cl
7 - 5 = +2
3
4
1
2
5
Example 8-3, p. 360
-1
+2
-1
-1
Example 8-3, p. 360
-1
-1 = +2 - 3(-1)
+2
-1
-1
Example 8-3, p. 360
A Closer Look, p. 360
New concept:
Oxidation Number
18
New concept
[
O
H
]
19
New concept
[
[
O
H
O
H
]
]
Formal Charge
Covalent assumption
Electrons shared,
one to each
19
New concept
[
[
O
H
]
Formal Charge
Covalent assumption
Electrons shared,
one to each
O
H
]
[
O
H
]
Oxidation Number
Ionic assumption
Both electrons
transferred to the
one who wants
them more
19
Brought to the party
Ends up with
6
8
Brought to the party
Ends up with
1
0
A Closer Look, p. 360
21
22
• Octet Rule ‐ double count shared electrons
– assumes both atoms get both electrons
22
• Octet Rule ‐ double count shared electrons
– assumes both atoms get both electrons
• Formal Charge ‐ Evenly split electrons in a bond
– one to one atom, one to the other
22
• Octet Rule ‐ double count shared electrons
– assumes both atoms get both electrons
• Formal Charge ‐ Evenly split electrons in a bond
– one to one atom, one to the other
• Oxida?on Number ‐ Unevenly split electrons in a bond
– the atom that wants electrons more, gets both shared electrons (as in ionic bonds). The other one loses out. An extreme view. 22
Questions?
23
Back to Octet Rule
24
Ozone anion
O3 -
p. 361
Ozone anion
O3 -
Which side gets the double bond?
p. 361
Real molecule - equal bond lengths!! (not interconverting)
p. 361
Nitrite anion - NO2-
Example 8-4, p. 363
Nitrite anion - NO2-
Resonance Structure - two views; neither fully correct
Example 8-4, p. 363
Nitrite anion - NO2Unified Structure - one view; more correct
Resonance Structure - two views; neither fully correct
Example 8-4, p. 363
Nitrite anion - NO2Unified Structure - one view; more correct
But harder to
implement
the octet rule
Resonance Structure - two views; neither fully correct
Example 8-4, p. 363
Exceeding the Octet Rule
using near‐energy d orbitals
5x7 = 35 electrons brought to the party
32 electrons
35 electrons
Example 8-5, p. 366
Table 8‐3, p. 356