## Supplement to Lewis Dot Diagrams

Try the activity with 30 practice molecules, which can be found here.

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

```
step 5: Attribution for the above supplement is:
Teaching a model for writing Lewis structures
Juan Quilez Pardo
J. Chem. Educ., 1989, 66 (6), p 456
I based my original set of rules for drawing structures on the rules I learned as a student at the University of Southern Maine. I have modified them (May 1, 2012) according to the following JChemEd article:
Lewis structures, formal charge, and oxidation numbers: A more user-friendly approach
John E. Packer and Sheila D. Woodgate
J. Chem. Educ., 1991, 68 (6), p 456
Basic Lewis Diagrams Instructions
Lewis Diagrams Exercises
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