The octet rule is a simple chemical theory that states that atoms tend to combine in such a way that they have eight electrons in their valence shells, similar to the electronic configuration of a noble gas. (In simple terms, molecules are more stable when the outer shells of their constituent atoms are empty, full, or have a multiple of 8 electrons in the outer shell. See electron shells.)

This combination occurs primarily in two ways, electrovalent bonding and covalent bonding.

However, the octet rule is often inapplicable. Some of the atoms for which the octet rule are most useful are:

• carbon, C
• oxygen, O
• the halogens

However, the exceptions are plenty:

• hydrogen only needs one electron to have a noble gas structure (that of helium), and lithium needs to lose one.
• any molecule or ion with an odd number of electrons
• any atom that forms more than four bonds
• the octet rule is inapplicable to atoms in periods greater than 2 because their electron shells can hold more than eight electrons