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The octet rule is a basic chemistry rule that allows easy memorization of certain atomic properties. According to this useful rule of thumb, many, if not most, atoms will try to lose or gain electrons to have a total of eight in the external shell. Scientists have found that an atom is most stable with eight electrons in the outer layer, and the atoms appear to try and move toward this equilibrium.
The octet rule popularity is generally attributed to Gilbert Lewis, a Massachusetts-born scientist and professor of the early 20th century. While teaching at Harvard University in 1902, Lewis drew on his own research as well as that of a contemporary, German chemist Richard Albegg, to create a model for the octet rule. The idea had been around for some time, though Lewis was the first to visualize the concept, theorizing that atoms had a concentric cubic structure that had eight corners, thus creating the desire for eight electrons. The term octet rule was popularized by another chemist working on the same concept, a American scientist named Irving Langmuir.
The stability and reactivity of an atom is usually related to the configuration of its electrons. Noble gases, such as neon, argon, krypton, and xenon, tend to have eight electrons on the outer energy layer. Helium is a major exception to the octet rule, having only two electrons. When an atom has eight electrons, it is generally considered stable and will not usually react with other elements. Atoms with fewer than eight electrons are often far more reactive, and will join up or create bonds with other atoms to try and reach the octet level.
Chemists and bewildered students are quick to point out that the octet rule should not really be considered a rule at all, as there are many exceptions to the behavior. This is hardly surprising; as elements are so widely variable in behavior in other cases, it would be extremely unusual for all to subscribe to this interesting rule. Hydrogen, for instance, has only one electron, which prevents it from having enough spaces for seven other electrons to latch on from other atoms. Beryllium and boron, have only two and three electrons, respectively, and similarly could never reach a full octet.
Some atoms, such as sulfur, can actually have more than eight electrons on the outer layer. Sulfur has six electrons, but ordinarily only two are available to bond. Sometimes, an energy absorbing process will occur, causing all six electrons to become excited and available for bonding, making a total of 12 electrons possible on the outer layer.