Illustrated Glossary of Organic Chemistry

Octet rule: The concept that compounds containing carbon, nitrogen, oxygen, and fluorine are more stable if these atoms have eight valence electrons. When one of these atoms has less than eight valence electrons it has an open octet. The terms 'full octet' and 'full valence electron count' are not interchangeable (although they are often used this way), as eight is not the preferred valence electron count for all elements. Likewise the terms 'open octet' and 'open valence electron count' are also not interchangeable.

Every carbon, nitrogen, oxygen, and fluorine atom in this molecule has a full octet. All atoms in this molecule have a complete valence electron count.


This carbocation has open valence electron counts on carbon and sulfur. This is an open octet for carbon but not for sulfur, because carbon prefers eight valence electrons whereas sulfur prefers twelve (i.e., sulfur has an expanded octet.)