Octet rule
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.
Encyclopedia
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 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 , 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:
Exceptions
- The duet rule of the first shell - the noble gas helium
| 3He || 0.000137%* || colspan="4" | He is stable [i] with 1 neutron [i]
...
has two electrons in its outer shell, which is very stable.
Hydrogen only wants one additional electron to attain this stable configuration, while
lithium needs to lose one
- Electron deficiency occurs when there are too few electrons to form an octet:
- This occurs commonly with boron
| colspan="6" align="center" | *Boron-10 content may be as low as 19.1% and ashigh as 20.3% in natural samp...
, with only 6 electrons in its bonding pairs .
- This also occurs in some reactive species like carbenes.
- Free radicals have an odd number of electrons
- Atoms with shells 3 and higher can expand their outer shell to form more than four bonds :
| Critical temperature [i] || 994 K [i]
...
...
s
See also
