Octet rule

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Ionic bonding animation

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Graphical comparison of bonds

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The Octet Rule is a chemical rule of thumb that reflects a theory in chemistry stating that atoms of main-group elements tend to combine 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 applicable to the main-group elements, especially carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium or magnesium.

Overview

The octet rule is based on the principle that atoms are more stable, or energetically favorable, when they have eight electrons in their valence shell, the outermost electron shell in an atom. This rule helps to explain why atoms form certain kinds of ions and molecules. For example, a sodium atom (Na) will lose one electron to achieve a full outer shell, becoming a sodium ion (Na+), while a chlorine atom (Cl) will gain an electron to fill its valence shell, becoming a chloride ion (Cl−).

History

The concept of the octet rule dates back to the early 20th century, with significant contributions from Gilbert N. Lewis and Walther Kossel. Lewis introduced the idea of the "cubical atom" in 1902, which laid the groundwork for the octet rule by suggesting that atoms could achieve stable configurations by having eight electrons in their outermost shell.

Exceptions

While the octet rule is a useful guideline for understanding chemical bonding, there are notable exceptions. These include molecules with an odd number of electrons, molecules in which one or more atoms possess more or fewer than eight electrons, and molecules of elements in the third period of the periodic table and beyond, which can hold more than eight electrons due to the availability of d orbitals in their valence shells. Examples of such exceptions include phosphorus pentachloride (PCl₅) and sulfur hexafluoride (SF₆).

Application

The octet rule is fundamental in the teaching of chemistry and is used to predict and explain the structure of molecules in organic chemistry, inorganic chemistry, and biochemistry. It is particularly important in the prediction of the shapes of molecules and ions, the understanding of the valence of elements, and the explanation of the types of chemical bonds, including ionic bonds, covalent bonds, and metallic bonds.

Limitations

Despite its widespread use, the octet rule has limitations and does not apply universally. Its applicability is mostly limited to the s and p block elements of the periodic table. Transition metals and heavier elements often do not follow the octet rule, forming compounds that have more or fewer than eight electrons in their valence shells.

See Also

• Valence electron
• Chemical bond
• Lewis structure
• VSEPR theory