NCEA L2 - Drawing Lewis structures
(using the octet rule & duet rule)
Lewis formula (electron dot or Lewis structure)
Diagrams that show the bonding between atoms of a molecule or
polyatomic ion and the lone pairs of electrons that may exist in the
molecule or polyatomic ion.
One pair of dots represents two electrons a covalent (single) bond or
non-bonding pair (lone pair) of electrons
Step 1
NH4+
Example H2O
Count total # of valence electrons around the atoms, .
O=6
(if a polyatomic ion : add 1 e- for each –ve charge or subtract 1 e- for H = 1
each positive charge)
H=1
N=5
H=1
H=1
H=1
TOTAL = 8
-1 for + charge
TOTAL (9-1) = 8
Step 2
Connect atoms with single bonds
H
4 e- s remaining
O
H
H
none
remaining
Step 3
Place remaining e- s in pairs around the atoms , starting with the
outer atoms
Finished !
O
H
Step 4
Check valence shell of all atoms are complete (octet).
If not move non-bonding pairs to form double or triple bonds
Finished !
H
H
N
H
+
H
+
Exceptions to octet rule
A) Electron deficient molecules
E.g. Gaseous BeCl2 and BCl3
Cl
Cl
Be
Cl
B
Cl
Cl
Exceptions to octet rule
B) Odd electron molecules
C) Expanded valence shell
F
F
F
F
F
S
F
F
Xe
F
F
F
Cl
Cl
P
Cl
Cl
Cl
Examples
Cl
• CCl4
Cl
C
• CO2
Cl
• HCN
• HOCl
• H2O2
H C
• PH3
• H2CO (central atom C)
• COCl2 (central atom C)
• SO2
• OF2
• O3 , NO2+
O
C
H
O
P
Cl
H
H
O
O
O
H
C
N
H
Cl
O
O
H
H
O
F
O
H
O
Cl
C
O
F
O
S
Cl
O