Octet rule

The octet rule or eight- electron rule is a classic rule of chemistry. It says that the electron configuration of atoms of the second period of the periodic table in molecules is a maximum of eight outer electrons ( valence electrons ) or four pairs. The atoms are so eager to accept the noble gas configuration. The octet rule is thus a special case of the broader noble gas rule.

Atoms, which usually behave according to the octet rule

The octet rule is often only for the main group elements of the second period. This includes the elements carbon, nitrogen, oxygen and fluorine. These elements achieve the electron configuration of the noble gas neon in most of its compounds. The carbon, nitrogen and fluorine atoms also have in their elemental state - as a diamond, or fullerene as dinitrogen N2 trioxygen ( ozone) and Difluormolekül F2 - neon electronic configuration with eight valence electrons. For all of these atoms, they can be indeed surrounded by less than 8 electrons (eg as a carbocation ) but never more than 8

Exceptions

The octet rule applies to most stable compounds of the above elements. But there are also exceptions. Significantly more exceptions there are for elements of higher periods. Elements such as tin and lead, for example, also form cations in the divalent oxidation state with a lone pair of electrons ( Relativistic effect ).

However, there are many molecules in which only the formal electron octet is exceeded ( octet extension). Typical examples are phosphorus pentafluoride ( PF5 ), sulfur hexafluoride ( SF6) or Iodheptafluorid ( IF7 ). Until now, it has been striving for beyond the octet bonding electrons often energetically higher lying unoccupied d- orbitals. More detailed quantum mechanical considerations, however, show that the d- orbitals should not play a significant role because of the enormous difference in energy to the s and p valence orbitals. Alternative descriptions of these molecules use multicenter bonds or partially ionic formulations (e.g., PF4 F, SF42 ( R ) 2, IF43 ( R ) 3).

Molecules, can be set up for which the octet rule compliant Lewis structures, which nevertheless often Formulas with more than four bond lines are used, should not be considered as exceptions. As typical examples here sulfuric acid or sulfur dioxide may be mentioned.

Is hydrogen and the light cations, Li , Be2 and B3 satisfy the octet rule not because they have too few electrons and the corresponding inert gas configuration (helium), which is achieved in compounds that has only two electrons. However, this is more of a formal arrangement, the noble gas rule but they meet.

Nitrogen and oxygen

Exceptions are for example, nitrogen monoxide, the nitrogen oxides NO, also known as nitric oxide and nitrogen dioxide NO2. The molecules of these compounds are stable radicals, that have an odd number of electrons, which in principle is incompatible with the octet rule.

Another exception to the octet rule is the dioxygen molecule O2: measurements show that it contains two unpaired electrons. However, the noble gas configuration requires paired electrons. The dioxygen molecule can pass O2 in the reaction of potassium, rubidium and cesium with air in the superoxide ion; it caused the Hyperoxide KO2 RbO2 and CSO2. The superoxide ion has an odd number of electrons and thus also no octet.

No validity for unstable intermediates

The rule is especially true for isolable compounds. In many reactions are unstable, but detectable intermediates who do not obey the octet rule, such as radical as the chlorine radical or carbocation in which the carbon has only six electrons, for example in the reaction of butyl chloride. Nitrenes and carbenes were also mentioned.

In transition metal compounds is found, an analog control: the 18- electron rule. But this is - if at all - only sufficiently well fulfilled for complexes with predominantly covalently bound ligand, but even here there are countless exceptions.