1. Guidelines: Drawing Lewis Structures
Count total valence electrons in the molecule.
Arrange atoms into a "skeleton" molecule.
The less electronegative element is usually the central atom.
The element written first in the formula is usually the central atom.
C bonds to other C atoms readily and forms 4 bonds.
H forms 1 bond only
Connect all atoms with single bonds (dashes).
Distribute e-s to fill octets for all atoms (except H).
recount your e-s
If total e-s used (bonding and nonbonding) = valence e-s counted in step #1 you're done
If you used more e-s to fill octets than were counted in step #1 create double or triple bond
Make sure to check your structure!!!
Total e-s used in the Lewis Struct. should = total valence e-s counted in step #1

2. Examples: Drawing Lewis Structures
Molecular formula
NF3
N e-
5e- x =
Sum of valence e-
F 7e-
3 = 21e- x
Atom placement
Total e-
Remaining valence e- : : : F : : F : : N
Lewis structure
Total e-s needed = 26 : F : :

3. Examples: Drawing Lewis Structures
PROBLEM:
Write a Lewis structure for CCl2F2, one of the compounds responsible for the depletion of stratospheric ozone.
SOLUTION:
Step 1: C has 4 valence e-, Cl and F each have 7.
The sum is 4 + 4(7) = 32 valence e- C Cl F
Steps 2-4: Carbon has the lowest EN and is the central atom. The other atoms are placed around it.
Make bonds and fill in remaining valence
electrons placing 8 e- around each atom. C Cl F :

4. Examples: Drawing Lewis Structures
PROBLEM:
Write the Lewis structure for methanol (molecular formula CH4O), an important industrial alcohol that is being used as a gasoline alternative in car engines.
SOLUTION:
Hydrogen can have only one bond so C and O must be next to each other with H filling in the bonds.
There are 4(1) = 14 valence e-
C has 4 bonds and O has 2. O has 2 pair of nonbonding e- C O H :
14 e-s total used to fill all atom's octets

5. Examples: Drawing Lewis Structures (cont)
PROBLEM: Write Lewis structures for ethylene ( C2H4 ), the most important reactant in the manufacture of polymers
PLAN:
C atoms easily bond to each other and H gets 1 bond only so the C atoms are central and H are terminal
If you do not have enough e-s to fill all atoms octets (exc H), create multiple bonds. Form a double bond if short 2 e-s, and 2 double or a triple bond if short 4 e-s. Dbl bonds between C, N, S, O and triple bonds between C, N, O
SOLUTION:
There are 2(4) + 4(1) = 12 valence e-
You need 14 e- but only have 12. You’re short 2 e-s and have to create a double bond C H : C H :

6. Examples: Drawing Lewis Structures
PROBLEM: Write Lewis structures for the following:
Nitrogen ( N2 ), the most abundant atmospheric gas
SOLUTION:
N2 has 2(5) = 10 valence e-
14 e- are needed to fill octets
10 e- – 14 e- = 4 e-s short to fill octets
A triple bond is required to make the octet around each N : : :
N N : :
N N : : :

7. Examples: Drawing Lewis Structures
SO2
CCl4
NH3
SO3
H2O
CO2
PBr3
C2H5OH
H2CO3
H2CO
BeF2
BF3