Octet & Duet Rules
Why atoms want eight (or two), shown with real structures and the exceptions that break the rule.
Why it matters
The octet rule is the reason atoms bond at all. Sharing, gaining, or losing electrons all serve one goal: a full, noble-gas outer shell of eight (or two, a , for hydrogen). Almost every structure you draw is atoms chasing that eight.
When you use it
Every time you place and , or decide whether an atom needs a double bond to finish its shell.
Where it leads
, multiple bonds, , and the honest exceptions, and incomplete octets.
The rule, drawn out
Most main-group atoms are most stable with eight valence electrons, a filled s and p shell. Hydrogen and helium are the exception: their shell holds only two, so they aim for a duet.
Two exceptions, side by side
Boron is happy electron-deficient rather than pulling a positive charge onto fluorine. Sulfur sits in Period 3 with room to expand past eight. Same rule, opposite directions.
Why eight (or two)?
Quick check workflow
- 1Count the total valence electrons (adjust for any charge).
- 2Draw single bonds connecting the central atom to each outer atom.
- 3Distribute the remaining electrons to the outer atoms until each has 8 (or 2 for hydrogen).
- 4Put anything left on the central atom, then check it against the exception list below.
Targets at a glance
| Category | Typical atoms | Target electrons |
|---|---|---|
| Duet | H, He, Li⁺, Be²⁺ | 2 |
| Octet | C, N, O, F, Ne | 8 |
| Expanded | P, S, Cl (Period 3+) | 10, 12, or 14 |
| Incomplete | B, Al, Be | 4 or 6 |
Memory trick
Common mistake
Practice