Here is another way to approach things.

• The following is a methodical way to draw Lewis structures of molecules whose atoms follow the octet rule.
• It’s nice because it tips you off to exceptions to the octet rule like the expanded octet.

1. Step 1: Count the electron pairs needed
   * Hydrogen wants 1 pair
   * Group II’s want 2 pairs
   * GroupIII’s want 3 pairs
   * Everyone else wants 4 pairs (unless you have an expanded octet)
2. Step 2: Find the total number of covalent bonds possible
   * Total # of possible covalent bonds = sum of the valence e^– divided by 2
   * Remember to add or subtract electrons according to any ionic charge
3. Step 3: Calculate the actual number of covalent bonds
   * Actual C.B.s = step 1 - step 2
   If you don’t get enough bonds to draw the molecule then it’s an expanded octet.
4. Step 4: Calculate the number of σ bonds, π bonds, and non-bonding pairs
   * # of σ bonds = # of atoms in the molecule - 1
   * # of π bonds = Actual C.B.s (step 3) - # of σ bonds
   * # of non-bonding pairs = possible C.B.s (step 2) - Actual C.B.s (step 3)
5. Step 5: Draw the molecule (Here are some typical examples)

        CH4      NH3     H2O    CO2

step 1: 8    7   6   12

step 2: 4    4   4   8

step 3: 4    3   2   4

step 4:

σ       4    3   2   2

π       0    0   0   2

nbp     0    1   2   4