octet rule

Short definition from Wiktionary

Noun

1. 1
   A rule stating that atoms lose, gain, or share electrons in order to have a full valence shell of 8 electrons. (Hydrogen is excluded because it can hold a maximum of 2 electrons in its valence shell.)
2. 2
   The statement that when atoms combine to form molecules they generally each lose, gain, or share valence electrons until they attain or share eight; also called