Lewis Dot Diagrams
for
Covalent Molecules
3 Types of Representations
1. Lewis Structures of compound
2. Molecular Formula:
H2
3. Structural Formula:
H-H
[- represents 1 pair of shared electrons]
Rules for drawing Lewis Diagrams
Step 1: arrange symbols
– BINARY molecules are easy
• put next to each other
H H
Step 2: count # valence electrons
• each H has 1 val e- so total = 2
Step 3: distribute val e-, starting with single
bonds between all atoms
H:H
Step 4: test for validity
Two Tests for Lewis Structures
Must Pass Both!
1. # dots = # valence electrons found in
rule 2
2. every atom has octet of electrons
around it (except H: needs 2)
Bonding electrons get counted 2
times – once for each atom sharing
them
Testing Lewis Diagrams
H :H
Test 1: 2 dots = 2 val e-
✔
Test 2: each H has 2 val e- (H needs 2)
✔
Diatomic Halogens
share 1 pair (2 electrons): form single covalent bond
Testing Lewis Diagrams
Step 1
Step 2
Step 3
Step 4
✔
F F
(2)(7) = 14 valence electrons
.. ..
: ..F : F.. :
Try HF
Step 2
Step 3
Step 4
✔
HF
7 + 1 = 8 valence electrons
..
:
H F
..
:
Step 1
Types of Covalent Bonds
• Single bond:
– 2 atoms share 1 pair of electrons
• Double bond:
– 2 atoms share 2 pairs of electrons
• Triple bond:
– 2 atoms share 3 pairs of electrons
Try O2
Step 1:
OO
Step 2:
(2)(6) = 12 valence electrons
Step 3:
.. ..
: O :O :
..
Step 4: Test failed

No Good!
Assessing Lewis Diagrams
• If dot structure passes both tests, you’re
finished
• If fails one or both tests, try again
• If single bonds don’t work - try multiple
bonds
– Single bond = 2 electrons shared
– Double bond = 4 electrons shared
– Triple bond = 6 electrons shared
Try O2 Again!
Step 1:
Step 2:
(2)(6) =12
.. ..
::O
O
.. ..
Step 3: distribute e(single bond didn't work so try double bond)
Step 4: Test!
✔
Try N2
Step 1:
Step 2:
..
:N
:
:
N
..
No Good!
2 X 5 = 10 electrons
Step 3: Distribute electrons starting with a single
bond between the nitrogen atoms
Step 4: Test

Try N2 with a double bond
: N :: N :
..
Step 4: Test

No Good!
Try N2 with a triple bond
: N ::: N :
Step 4: Test
✔
Ternary Molecules
more than 2 elements:
- atom with lowest electronegativity placed in center
- hydrogens always terminal (placed on outside)