Lewis Dot Structures
1)
2)
3)
4)
5)
Count up all valence electrons
Add an electron for each negative charge, subtract one for each positive charge
Put the atom with the lowest central (usually the one listed first)
Single bond all other atoms to the central one
Satisfy the terminal atoms’ octets by adding in lone pairs of electrons
(H needs only 2, everything else needs 8)
6) Any extra electrons go as lone pairs on the central atom
7) Convert lone pairs on terminal atoms to multiple bonds to satisfy central atoms’ octet
if not enough electrons are present (only CNOS make multiple bonds, halogens never
do)
Examples:
H2O
8 e-1
H
O
H
O
CCl2O
-1
24 e
Cl
C
Cl
-1
O
NO3-1
24 e-1
O
N
O
+1
NO2+1
16 e-1
SF4
34 e-1
O
N
O
F
F
S
F
F
Exceptions to the Lewis Dot rule
1. Coordinate Covalent Bonds- both electrons in a bond come from the same atom
H+ + NH3  NH4+1
+1
+
H
H+
+
H
N H
H
H
N H
H
2. Fewer than 8 electrons
BF3
24 electrons, halogens NEVER multiple bond
F
F
B
F
Boron compounds famous for this and notoriously unstable because of it
3. Expanded valence- more than 8 electrons on central atom
SF6 exists but OF6 has never been found. Why?
SF6 48 electrons
F
F
F
F
S
F
OF6 48 electrons
F
F
F
F
F
O
F
F
Both have more than 8 electrons on the central atom which seems to break the
above rules. So why is it OK for sulfur but not for oxygen?
O: [He] 2s2 2p4
S: [Ne] 3s2 3p4
Sulfur is at a higher energy level than oxygen. So what?
Oxygen:
n=2 l=0
n=2 l=1
ml = 0
ml = +1 0 -1
ms = ½ - ½ 2 electrons
m2 = ½ - ½ 4 out of 6 electrons possible
still only 8 electrons possible in the valence shell
Sulfur
n=3 l=0
n=3 l=1
n=3 l=2
ml = 0
ms = ½ - ½
ml = +1 0 -1 ms = ½ - ½
ml = +2 +1 0 -1 -2 ms = ½ - ½
2 electrons
4 out of 6 electrons possible
0 out of 10 electrons possible
sulfur has 5 more orbitals for bonding that are unused!
Atoms with n =3 or higher can form expanded valences
4. Odd-Electron compounds
Draw Lewis Dot structure of NO2
O
17 e-
N O
Notice how you can’t get the N 8 electrons? You could double bond the other
oxygen as well but that would put 9 electrons on the N which would break its valence
rules. There is no choice but to leave an odd electron on the N. This is why NO2 tends to
form a dimer. A dimer is a molecule made when 2 identical molecules bond together.
O
N
O
O
O
O
N
N O
N O
2 NO2

O
N2O4