The octet rule is a chemical rule of thumb that states that atoms of low (<20) atomic number tend to combine in such a way that they each have eight electrons in their valence shells, giving them the same electronic configuration as a noble gas. The rule is applicable to the main-group elements, especially carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium or magnesium.
The valence electrons can be counted using a Lewis electron dot diagram as shown at right for carbon dioxide. The electrons shared by the two atoms in a covalent bond are counted twice. In carbon dioxide each oxygen shares four electrons with the central carbon, and these four electrons are counted in both the carbon octet and the oxygen octet.
Read more about Octet Rule: Example: Sodium Chloride, History, Explanation in Quantum Theory, Exceptions
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