octet rule

framed octet rule is a simple chemical theory that states that atoms tend to combine in such a way that they each 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. 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 (phosphorus falls under this as well as the rule below)
the octet rule is inapplicable to atoms in periods greater than 2 because their electron shells can hold more than eight electrons.For Transition metals an alternative for the octet rule is electron counting.
boron is an exception with extreme frequency
there are a few special cases in very common substances, notably carbon monoxide and ozone

Octet Rule

See also