Octet Rule
Definition
The guideline that atoms tend to gain, lose, or share electrons to end up with eight electrons in their outer shell, like a noble gas.
Technical definition
The octet rule states that main-group atoms tend to form bonds so as to achieve eight valence electrons, a noble-gas configuration. It is a useful guideline for Lewis structures but has many exceptions, including incomplete octets (beryllium, boron), expanded octets (period 3 and below), and odd-electron species.
Example
Carbon forms four bonds in methane to reach 8 valence electrons. Sodium loses one electron to reach neon's octet.
Common confusion
The octet rule is a rule of thumb, not a law. Phosphorus in PCl5 has 10 valence electrons and boron in BF3 has only 6, and both are perfectly stable.