An octet is a stable arrangement of eight electrons in an atom's outermost (valence) shell, matching the configuration of a noble gas. Achieving an octet is energetically favourable, which is why atoms tend to gain, lose, or share electrons to reach it. This tendency is summarised in the octet rule and underlies much of bonding.

八隅体是原子最外层(价层)中八个电子的稳定排布,与稀有气体的组态相同。达到八隅体在能量上有利,这正是原子倾向于通过得失或共享电子以达成它的原因。这一倾向概括为八隅体规则,是成键的重要基础。