1. Lewis Dot Structures: Electron dot structures of compounds
Q: What is the formula for water?
H2O
Q: How many valence electrons does each hydrogen have? 1
Q: How many valence electrons does the oxygen have? 6
Q: How many valence electrons are there in each water molecule?
#VE = 2H + 1 oxygen =
2(1) + 6 = 8
More examples
#VE = 2(1) + 2(6) = 14 H O
1. How many VE in H2O2?
2. How many VE in H2SO4?
#VE = 2(1) (6) = 32 H S O

2. Simple Lewis Dot Structures
1. Determine the central atom. This is usually the atom that you have the least of in the formula.
Ex:
H2O
BF3
NCl3
C2H6
SiF4
2. Draw the electron dot structure of the central atom.
•O• ••
•N• •• •
•C C• • ••
•Si• •
•B• •
3. Wherever there are unpaired electrons on the central atom, pair them up with unpaired electrons on the attached atoms. Si ••
●F● •
C C •• H● ●H • H B ••
●F● • N ••
●Cl● • O •• H● • ●H 8 24 26 14 32
H2O: 2(1)+6 = 8
NCl3: 5 + 3(7) = 26
SiF4: 4 + 4(7) = 32
BF3: 3 + 3(7) = 24
C2H6: 2(4) + 6(1) = 14
4. Finally, check your electron count.

3. O O O N• N• N N What to do when you end up with unpaired electrons:
Example: Draw the Lewis structure for O2
#VE = 2(6) = 12 O •• • O •• • •• O • •
Pair up the remaining unpaired electrons between the two oxygen atoms.
Because each shared pair of electrons represents a chemical bond, there is a double bond between the oxygens in O2.
What would the Lewis structure of N2 look like?
•N• •• • N• •• • N• •• • N N ••
Note: 1 stick = 2 electrons in a bond

4. The ‘Keep ‘em Happy’ approach to Lewis structures:
1. Add up the number of valence electrons present
2. Draw the stick-skeleton of the molecule
3. Satisfy the octet rule for all atoms in the molecule
Exceptions: H only needs 2e- and B only needs 6e-
4. Count up the number of electrons present in the Lewis structure
- If there aren’t enough e- (the molecule is ‘unhappy’), add the missing electrons to the central atom
- If there are too many e- (the molecule is ‘too happy’), take the excess away from the central atom and then form double bonds with the terminal atoms to satisfy the octet.

5. Examples: SF4 F F S #VE = 34 F F 32 34 SO2 O S O #VE = 18 20 18
Note: 1 stick = 2 electrons in a bond
SF4
This molecule isn’t ‘happy’ because it doesn’t have enough electrons. F F S
#VE = 34 F F
This is the only possible structure because Fluorine never forms double bonds 32 34
SO2
This molecule is too happy, take away an electron pair from the central atom… O S O
#VE = 18
but now sulfur is unhappy with only 6 e- 20 18
Make sulfur happy by using one of the pairs on oxygen to form a double bond.