1. Guide to Bonding Covalently
Step 1
Determine the arrangement of atoms
Phosphorus trichloride:
Cl P Cl Cl
center atom has least amount of valence e-
Halogen & Hydrogen can’t be center atom
Step 2
Determine the total # of valence e-
Element
# Atoms
Valence e-
Total P 1
x 5 e- =
5 e- Cl 3
x 7e-
21 e-
Total Valence e-
26e-

2. . . Cl .. .. .. : Cl - P - Cl : .. .. :Cl : .. Everyone has 8 e-
Step 3
Attach each bonded atom to the central atom w/ pair of e-
Cl : P : Cl or Cl - P - Cl Cl Cl
Step 4
Place the remaining e- as lone pairs to complete octets
Exception: 2 for H
& 6 for B
26 total e- - 6 bonding e- = 20 remaining ( 10 pairs)
: Cl P - Cl :
:Cl : ..
Step 5
If octets are not complete, form a multiple bond. Convert a lone pair to a bonding pair w/ the central atom.
Everyone has 8 e-
- 18 e- on each
- last 2 e- added to center atom

3. Example Step 1 Carbon dioxide O C O Step 2 = Total Valence e- Atoms

4. Step 3 Step 4 Step 5 O - C - O :O=C=O: .. .. O C O O - C - O
16 total e- - 4 bonding e- = 12 remaining ( 6 pairs)
O C O
Step 5
O has 8 e‑ but C has 4 e‑
The octet is not complete! Share more e-, forms multiple bonds
O - C - O :O=C=O:
12 e- on each
none left to add to center atom

5. Practice
Ammonia (NH3)
2. Carbon Tetrachloride

6. 3. Water 4. Nitrogen trifluoride

7. 5. H2CO 6. C2H2