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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 ("d", "f", etc.) 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 ("p" subshell is filled), 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:
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