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.

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:

However, the exceptions are plenty:

Hydrogen only needs one additional electron (for a total of two), to have a noble gas structure (that of helium; the duet rule of the first shell). But symmetric hydrogen bonds again violate the duet rule
Lithium needs to lose one (also the duet rule of the first shell).
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. See hypervalency
Boron is a frequent exception, normally with only 6 electrons in its bonding pairs. Similarly, beryllium normally has 4 electrons in its bonding pairs.
There are a few special cases in very common substances, notably carbon monoxide, ozone and carbenes
Any molecule or ion with an odd number of electrons.
For transition metals the alternative for the octet rule is 18-electron rule.

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See also