What is the octet rule in chemical bonding?

The octet rule states that atoms tend to form bonds in such a way that each atom has eight electrons in its valence shell, resembling the electron configuration of a noble gas. It explains the bonding behavior of most main-group elements. For example, oxygen forms two bonds in H₂O to complete its octet. Hydrogen follows the duet rule instead. While useful, the octet rule has exceptions like BF₃, which has six valence electrons, and expanded octets in elements like sulfur and phosphorus, which can hold more than eight electrons.