An octet is a stable arrangement of eight electrons in the outermost shell of an atom; a duplet is the stable arrangement of two electrons in the first shell, as in helium.
An octet is a stable arrangement of eight electrons in the outermost shell of an atom; a duplet is the stable arrangement of two electrons in the first shell, as in helium. Malay: oktet · Chinese: 八隅体.
The octet is the target that drives almost all the chemistry in this chapter. The noble gases (except helium) already have eight electrons in their outer shell, and this full, stable arrangement is why they are so unreactive. Every other element reacts precisely because it does not yet have an octet: atoms lose, gain or share electrons in order to reach the same stable eight, so the octet is the reason reactions happen at all. Helium is the special case, its only shell fills at two electrons, so its stable arrangement is a duplet, not an octet.
Example. A sodium atom (2.8.1) loses its single outer electron to become Na⁺ with the stable arrangement 2.8, an octet in its outer shell. A chlorine atom (2.8.7) gains one electron to become Cl⁻ with the arrangement 2.8.8, also an octet. Both have reached the electron arrangement of the nearest noble gas, and that is exactly the goal each atom is reaching for when it reacts.
Confusion to avoid. Do not apply the octet rule to hydrogen and helium, whose first shell is full at two, that is a duplet. And an octet describes the number of electrons in the outer shell being eight; it does not mean the atom has only eight electrons in total.
In the Periodic Table of Elements chapter the octet explains the stability of the noble gases and the reactivity of everything else, and it is the idea that carries straight into the next chapter on chemical bonding.
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