What is the Octet Rule?

The octet rule dictates that atoms are most stable when their valence shells are filled with eight electrons. It is based on the observation that the atoms of the main group elements have a tendency to participate in chemical bonding in such a way that each atom of the resulting molecule has eight electrons in the valence shell. The octet rule is only applicable to the main group elements.

The molecules of the halogens, oxygen, nitrogen, and carbon are known to obey the octet rule. In general, the elements that obey this rule include the s-block elements and the p-block elements (with the exception of hydrogen, helium, and lithium).

The octet rule can be observed in the bonding between the carbon and oxygen atoms in a carbon dioxide molecule, as illustrated via a Lewis dot structure below.

The Octet Rule

The shared electrons fulfil the valency requirements of both the bonded atoms. Thus, it can be noted that both the oxygen atoms and the carbon atom have an octet configuration in the CO₂ molecule.

Upon observing that the noble gases were chemically inert, the electronic theory of valency was proposed by the German physicist Walther Kossel and the American chemist Gilbert Lewis. It was based on the tendency of atoms to assume the most stable state possible.

Examples

A few examples detailing the chemical bonding of atoms in compliance with the octet rule can be found in this subsection.

NaCl (Sodium Chloride)

• This compound features an ionic bond between the sodium ion (Na⁺) and the electronegative chloride ion (Cl^–).
• The chlorine atom holds 7 electrons in its valence shell and can attain an octet configuration by gaining an electron.
• The outermost shell of sodium has one electron. If it loses this electron, the second shell would become the valence shell (which is already filled with 8 electrons). Thus, the Na⁺ ion is more stable than metallic sodium.
• The sodium cation and the chloride anion now form an ionic bond, and the resulting molecule features octet configurations for both the participating atoms.

MgO (Magnesium Oxide)

• The bond between magnesium and oxygen in magnesium oxide is ionic in nature.
• The magnesium atom readily loses two electrons to obtain the stable electronic configuration of neon. This leads to the formation of the Mg²⁺
• Similarly, oxygen gains two electrons to form the O^2-
• The electrostatic attraction that arises between these two ions leads to the formation of an ionic bond between them.
• Both the atoms in a molecule of MgO have stable octet configurations.

Exceptions to the Octet Rule

Not all elements and compounds follow the octet rule. Some of the exceptions to this rule are listed below.

• An ion, atom, or a molecule containing an unpaired valence electron is called a free radical. These species disobey the octet rule. However, they are very unstable and tend to spontaneously dimerize.
• Since the first shell can only accommodate two electrons, elements such as lithium, helium, and hydrogen obey the duet rule instead of the octet rule. For example, lithium can lose an electron to have a stable configuration in which the valence shell holds two electrons.
• Due to the presence of a d-orbital, the transition elements do not obey the octet rule. The valence shells of these atoms can hold 18 electrons.
• Aromatic compounds involve a delocalization of pi electrons. These electrons obey Huckel’s rule

It can be noted that some electron deficient molecules such as boranes and carboranes follow Wade’s rules in order to obtain stability. These molecules feature 3c-2e bonds (three centred bonds) in which two electrons are shared by three atoms.

The Expanded Octet – Hypervalency

Some main group elements have the ability to form hypervalent compounds. Examples include sulfur hexafluoride (SF₆) and phosphorus pentachloride (PCl₅). If all the phosphorus-chlorine bonds in a PCl₅ molecule are covalent, it would imply that the phosphorus molecule is violating the octet rule by holding a total of 10 valence electrons.

The formation of five bonds by the phosphorus molecules can be explained by the sp³d hybridization in PCl₅. Here, one ‘s’ orbital, three ‘p’ orbitals, and one ‘d’ orbital undergo hybridization to form an sp³d hybrid. This hybrid orbital forms five covalent bonds with the five chlorine atoms.

The structure of the hypervalent PCl₅ molecule is trigonal bipyramidal, as illustrated above.

How is the Octet Rule Useful?

The chemical behaviour of the main group elements can be predicted with the help of the octet rule. This is because the rule only involves ‘s’ and ‘p’ electrons, where the octet corresponds to an electron configuration ending with s²p⁶. These elements tend to form bonds in order to obtain stable obtain octet configurations.

Do the Oxygen Atoms in O₂ Molecules Have Octet Configurations?

Yes, each oxygen atom in the O₂ molecule is surrounded by a total of 8 valence electrons. Oxygen has a total of 6 electrons in the valence shell. In order to obtain a stable octet, the two oxygen atoms share a total of four electrons via a double bond.

List Four Elements that do not Obey the Octet Rule

Since ‘1p’ subshells do not exist, some elements find stability in 1s² configurations. On the other hand, some elements exhibit hypervalency and have the ability to form hypervalent molecules. Some elements that disobey the octet rule include:

• Hydrogen
• Lithium
• Phosphorus
• Sulfur

Thus, a brief explanation of the octet rule is provided in this article along with its exceptions. To learn more about concepts related to chemical bonding, such as hybridization, register with BYJU’S and download the mobile application on your smartphone.