Octet rule

The bonding in carbon dioxide

The octet rule is a simple chemical rule of thumb 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. The rule is applied to the main-group elements, especially carbon, nitrogen, oxygen, and the halogens. In simple terms, molecules tend to be more stable when the outer shells of their constituent atoms are empty or full, that is, have 8 electrons in the outer shell. See electron shells.

Note: "Full" in this case means that there is the highest number of electrons in the valence shell, before the next shell starts filling. However, higher subshells ("d", "f", etc.) have not been filled. There can be at most 8 valence electrons in a ground-state atom because "p" subshells are always followed by the "s" subshell of the next shell. So once there are 8 valence electrons ("p" subshell is filled), the next additional electron goes into the next shell, which then becomes the valence shell.

The octet rule also states that atoms react generally by gaining, losing, or sharing electrons in order to achieve a complete octet of 8 valance electrons. An octet of electrons results in a very stable electron configuration. This stability is the reason that the noble gases are so unreactive. 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
• Halogens, F, Cl, Br, I

Exceptions

• The duet rule of the first shell - the noble gas helium has two electrons in its outer shell, which is very stable. (Since there's no "1p" subshell, "1s" is followed immediately by "2s", and thus shell 1 can only have at most 2 valence electrons.) Hydrogen only needs one additional electron to attain this stable configuration, while lithium needs to lose one (but symmetric hydrogen bonds are an exception)
• Electron deficiency occurs when there are too few electrons to form an octet:
  • This occurs commonly with boron, with only 6 electrons in its bonding pairs (e.g. BF₃).
  • This also occurs in some reactive species like carbenes.
• Free radicals have an odd number of electrons (e.g. nitric oxide)
• Atoms with shells 3 and higher can expand their outer shell to form more than four bonds (hypervalency):
  • Phosphorus (eg phosphorus pentachloride)
  • Sulphur (eg sulfur hexafluoride)
• The 18-Electron rule overrides the octet rule in transition metals

See also

• Lewis structure
• Electron counting
• 18-Electron rule