Octet Rule

 The octet rule is a chemical rule of thumb that reflects the theory that main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The rule is especially applicable to carbon, nitrogen, oxygen, and the halogens, although more generally the rule is applicable for the s-block and the p-block of the periodic table. Other rules exist for other elements, such as the duplet rule for hydrogen and helium, or the 18-electron rule for transition metals.

The valence electrons can be counted using a Lewis electron dot diagram as shown below for carbon dioxide:

The electrons shared by the two atoms in a covalent bond are counted twice, once for each atom. In carbon dioxide, each atom shares four electrons with the central carbon, two (shown in red) from the oxygen itself, and two (shown in black) from the carbon. All four of these electrons are counted both in the carbon octet and the oxygen octet, so that both atoms are considered to obey the octet rule.

In 1864, English chemist John Newlands classified the sixty two known elements into eight groups, based on their physical properties.

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 valencies of the atoms involved apparently became satisfied. In 1893, Alfred Werner showed that the number of atoms or groups associated with a central atom (the "coordination number") is often 4 or 6; other coordination numbers up to a maximum of 8 were known, but less frequent. In 1904, Richard Abegg was one of the first to extend the concept of the coordination number to a concept of valence in which he distinguished atoms as electron donors or acceptors, leading to positive and negative valence states that greatly resemble the modern concept of oxidation states. Abegg noted that the difference between the maximum postive and negative valances of an element after his model is eight. In 1916, Gilbert N. Lewis referred to this insight as Abegg's rule and used it to help formulate his cubical atom model and the "rule of eight," which began to distinguish between valence and valence electrons. In 1919, Irving Langmuir refined these concepts further and renamed them the "cubicle atom octet" and "octet theory." The octet theory evolved into what is now known as the "octet rule."

Walther Kossel and Gilbert N. Lewis saw that the noble gas did not have the tendency of taking part in chemical reactions under ordinary conditions. On the basis of this observation, they concluded that atoms of noble gases are stable and on the basis of this conclusion they proposed a theory of valency known as "electronic theory of valency" in 1916:

During the formation of a chemical bond, atoms combine together by gaining, losing, or sharing electrons in such a way that they acquire nearest noble gas configuration.

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