Drawing Lewis Structures

With a bit of practice, you can easily create Lewis structures for almost any molecule. Use the following procedure as a guide to creating Lewis structures.

Let's consider nitromethane shown below. This is the Lewis structure we are attempting to create. The condensed structure is shown on the left and your goal is to generate the Lewis structure on the right.

1. Calculate the total number of valence electrons in the molecule.

(1) C 1 x 4 = 4
(1) N 1 x 5 = 5
(2) O 2 x 6 = 12
(3) H 3 x 1 = 3
total = 24 valence electrons

2. Adjust the valence electron count for charge

For negative charges add the corresponding number of electrons, for positive charged atoms subtract electrons.

In the case of nitromethane, since the molecule is neutral we do not need to adjust.

3. Draw the molecule with single bonds only

4. Subtract the number of electrons in single bonds from the total valence electrons

We have 6 sigma bonds and each bond holds 2 electrons, so we have 12 electrons in these bonds. So 24 - 12 = 12 remaining electrons.

5. Add the non-bonding electrons

Start with the most electronegative elements filling them first, then to the next most electronegative.

6. Calculate the formal charges

7. Create any π bonds

Use non-bonding pairs on negative atoms to create π bonds with adjacent positively charged atoms. Be sure to recalculate the formal charges.