Lewis' Rules for Drawing Lewis Dot Structures

The complete rules for drawing LDS's can be stated as below. As we go through the rules we will see how it applies to three different structures (CH₄, CHClFBr, and CO₂). Follow the examples VERTICALLY as you read the rules.

Step 1: Count the number of electrons in the picture (this includes the valence electrons for each atom and the electrons lost or gained for any charge (if it is an ion).

Step 2: Arrange the atoms. This is done remembering that atoms that need to make a lot of bonds are in the middle and those that need fewer (or only one) will be on the outside. In addition, Lewis said that the more electronegative an atom, the more likely it is to be on the outside of the molecule. It should also be noted that nature prefers chains to rings and single central atoms with other atoms surrounding it than chains. When nature does make rings they tend to be of 5 or 6 atoms, although 4's occur sometimes and 3's can be made (although they aren't very stable)

Step 3: Single bond everything. This draws a basic skeleton of the molecule.

Step 4: Fill in the rest of the electrons (total minus what you just added in the single bonds) in pairs, from the outside in (add to outside atoms until they have octets).

Step 5: Fix octets if needed and if possible. This is done by moving electrons from lone pairs on outside atoms into bonds to satisfy the octet of any central atom that doesn't have an octet yet.

Now try these: (answers are here)

SiS₂
C₂H₄O
C₃H₈
C₃H₆
C₃H₄
CH₂O
SO₄^-2

On to Octet Rule Violators