Octet rule

History

In the late 19th century it was known that coordination compounds (formerly called “molecular compounds”) were formed by the combination of atoms or molecules in such a manner that the valencies of the atoms involved apparently became satisfied. In 1893, cubical atom theory.

Overview

In short, an element's valence shell is full and most stable when it contains eight electrons (this stability is the reason that the noble gases are so unreactive). Note that a "full shell" means that there are the eight electrons in the valence shell when the next shell starts filling, even though higher subshells (d, f, etc.) have not been filled. There can be at most eight valence electrons in a ground-state atom because p subshells are always followed by the s subshell of the next shell. This means that once there are 8 valence electrons (when the p subshell is filled), the next additional electron goes into the next shell, which then becomes the valence shell.

A consequence of the octet rule is that atoms generally react by gaining, losing, or sharing electrons in order to achieve a complete octet of valence electrons. Reaction of atoms occurs primarily in two ways: covalently.

Some of the atoms for which the octet rule are most useful are:

• Carbon, C
• Oxygen, O
• The Halogens, F, Cl, Br, I

Exceptions

• The duet rule of the first shell - the noble gas lithium needs to lose one.
• Electron deficiency occurs in covalent compounds when an atom has fewer than eight electrons, and has no unpaired electrons with which to make more bonds. This is frequently seen in carbenes.
• nitric oxide) contain one or more atoms which have an odd number of electrons.
• Atoms with 3 or more electron shells can accommodate more than eight electrons in their outer shell (hypervalency). Examples include:
  • phosphorus pentachloride, PCl₅.
  • sulfur hexafluoride, SF₆.
• The transition metals.

See also

• Lewis structure
• Electron counting
• 18-Electron rule