Incomplete octets
Boron and aluminum compounds can be electron deficient. BF3 and BCl3 use three bonds around the central atom, leaving six electrons.
Beyond the basic octet
Octet Rule Exceptions
The octet rule is a useful pattern, not a universal law. Recognize electron-deficient centers, odd-electron radicals and expanded-valence classroom structures.
Odd-electron radicals
NO has 11 valence electrons and NO2 has 17. One electron remains unpaired in a representative Lewis contributor.
Expanded-valence models
Introductory Lewis models often place more than eight electrons around third-period or heavier central atoms.
Do not force every structure into the same rule
| Type | Diagnostic clue | Typical response |
|---|---|---|
| Incomplete octet | Electron-deficient group 13 center | Accept fewer than eight electrons when formal-charge alternatives are worse |
| Odd electron | Odd total valence-electron count | Place one unpaired electron and compare formal charges |
| Expanded model | Third-period or heavier central atom | Follow course convention and state model limits |