Editing Octet rule (section)

== Exceptions ==
Many [[reactive intermediate]]s do not obey the octet rule.  Most are unstable, although some can be isolated.  

Typically, octet rule violations occur in either low-dimensional [[coordination geometry|coordination geometries]] or in [[radical (chemistry)|radical species]].  Although hypervalent molecules are commonly taught to violate the octet rule, [[Ab initio quantum chemistry methods|''ab initio'' calculations]] show that almost all known examples obey the octet rule.  The compounds form many [[bond order|fractional bonds]] through [[chemical resonance|resonance]] (see {{slink||Hypervalent molecules}} below).

===Low-dimensional geometries===
In the [[trigonal planar]] coordination geometry, one [[p orbital|''p'' orbital]] points out of the bonding plane, and can only [[orbital overlap|overlap]] with nearby atomic orbitals in a [[π bond]].  If that ''p'' orbital would be empty in an isolated atom, it may be filled through an intramolecular [[dative bond]], as with [[aminoboranes]].  However, in some cases (e.g. [[boron trichloride]] and various [[borane]]s, [[triphenylcarbenium|triphenylmethanium]]), no nearby filled orbital can profitably overlap with the empty ''p'' orbital.  In such cases, the orbital remains empty, and the compound obeys a "sextet rule".  Likewise, linear compounds, such as [[dimethylzinc]], have two ''p'' orbitals perpendicular to the bonding axis, and may obey a "quartet rule".<ref>{{cite book|pp=298-299|year=1985|publisher=Wiley|lccn=84-15310|isbn=0-471-87393-4|last1=Albright|first1=T.&nbsp;A.|last2=Burdett|first2=Jeremy&nbsp;K.|last3=Whangbo|first3=Myung-Hwan|title=Orbital Interactions in Chemistry}}</ref>  In either case, the empty unshielded orbitals tend to attract adducts.  

===Radicals===
Radicals satisfy the octet rule in one [[quantum spin|spin orientation]], with four spin-up electrons in the valence shell, and almost satisfy it in the opposite spin orientation. Thus, for example, the [[methyl radical]] (CH<sub>3</sub>), which has an unpaired electron in a [[non-bonding orbital]] on the carbon atom and no electron of opposite spin in the same orbital. Another example is the radical [[chlorine monoxide]] (ClO<sup>•</sup>) which is involved in [[ozone depletion]].  

[[Stable radical]]s tend to adopt states in which the unpaired electron can [[delocalized electron|delocalize]] through resonance. In such cases, the octet rule can be restored through the formalism of a [[Covalent bond#One- and three-electron bonds|1- or 3-electron bond]].  

Species such as [[carbene]]s can be interpreted two different ways, depending on their spin state. [[Carbene#Singlet-triplet effects|Triplet]] carbenes are best thought of as two radicals localized on the same atom, and obey the octet rule in those radicals' shared spin-up orientation. [[Carbene#Singlet-triplet effects|Singlet]] carbenes tend to adopt a planar configuration, and are best thought of as obeying the planar sextet rule.