Octet Rule: Definition, Exceptions and Limitations

Octet rule refers to the tendency of atoms to form compounds in order to have eight valence electrons in their outermost shell. Hence forming the electron configuration of a noble gas. There are two ways by which the Octet law can be followed, one is by sharing the electrons of one atom with the other and the second being transferring the valence electrons from one to another.

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Definition of the Octet Law

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An atom, three particles are present: electron, neutron, and proton. Electron is a negatively charged particle, proton is a positively charged particle and neutron is a neutral particle. Protons and neutrons are present in the nucleus of an atom, while electrons are revolving in the outermost shell of an atom. The electrons present in the outermost shell of an atom are responsible for chemical bonding and chemical combination. These outermost electrons are called valence electrons. In 1916, the theory was developed by Kossel and Lewis about the chemical combination between atoms which is known as the octet rule. The atom has a tendency to complete its outermost shell and achieve stable configuration or noble gas configuration and become chemically inert. The maximum possible electron in the outermost shell is 8. This is according to the octet rule that states that most atoms tend to achieve eight electrons in their valence shell to achieve a stable configuration.

Octet Rule 

The octet rule states that an atom is most stable when its outermost orbit has eight electrons. The atom tries to complete its octet by gaining an electron or by losing outermost shell electrons or by sharing the outermost electrons. In the process of sharing an electron, the same electron fulfills the requirements of both atoms. This rule is followed by most of the elements of s and p block except for a few elements that achieve stable configuration without having eight electrons in their outermost shell. Examples of these elements are Hydrogen, helium, and lithium.

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Examples: Few atoms that follow the octet rule are sodium chloride, magnesium oxide, carbon dioxide, etc.

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Sodium Chloride (NaCl)

A sodium chloride bond formed is an ionic bond or electrovalent bond. In the formation of an ionic bond, the electron is transferred from one atom to another. In the case of sodium chloride, sodium being an electropositive metal has one electron in the outermost orbit while chloride being electronegative in nature has seven electrons in the outermost orbit and needs only one electron to attain a stable configuration. Sodium-ion donates its outermost electron and chloride ion receives that one electron and forms an electrovalent bond.

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Magnesium Oxide (MgO)

Magnesium Oxide is an ionic bond in nature. The magnesium atom gives out two electrons to attain stability in the electronic configuration of neon and leads to form the Mg²⁺. In the similar manner Oxygen obtains two electrons to form O^2-. The ions face an electrostatic attraction which helps them in forming an ionic bond and both of the atoms in the magnesium oxide molecule attain stable octet configurations.

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Hypervalency

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A hypervalent compound refers to the compound containing one or more than one main group of elements having more than eight electrons in their valence shells. Some of the main group elements have the capacity to form hypervalent compounds. Examples of which includes sulphur hexafluoride (SF₆) and phosphorus pentachloride (PCl₅). The phosphorus would defy the Octet law if all the molecules of the phosphorus-chlorine bond are covalent containing 10 valence electrons. 

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The PCl₅ formation through the hybridization of sp³d leads to the formation of five bonds by phosphorus molecules. Here a single ‘s’ orbital, one ‘d’ orbital and three ‘p’ orbital goes through the hybridization process to form an sp³d hybrid. This hybrid orbital leads to the formation of five covalent bonds along with five chlorine atoms. 

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Few More Exceptions of the Octet Rule

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1. The free radicals are considered to be the exception of the octet rule. Free radicals are referred to as the atoms or molecules that have an unpaired valence electron. These species are unstable and quickly dimerize.
2. Elements that are present on the right side of the periodic table are called the transition elements. These elements have empty d-orbital and the valence shell of these atoms can hold up to 18 electrons.
3. Aromatic compounds follow ‘Huckel’s Rule.

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Limitations of Octet Rule

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There are three types of limitations to the octet rule:

1. For compounds like LiCl, BeH₂, and BCl₃, the central atom is surrounded by less than eight electrons.
2. The octet rule does not satisfy all the atoms of nitric oxide and Nitrogen dioxide which have an odd number of electrons.
3. The octet rule is not followed by the elements in and beyond the third period. The number of compounds formed by these period elements can accommodate more than eight electrons in their outermost shell which is termed as an expanded octet. So here again octet rule is not applied. Few examples of such compounds are PF₅, SF₆, and many coordination compounds.

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Drawbacks of the Octet rule

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The drawbacks of the Octet rule are:

1. No explanation was given regarding the shape of the molecule.
2. Fails to explain the relative stability and energy of the molecule.
3. No explanation was given about the noble gas compounds such as XeF₂, KrF₂, etc.

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Things to Remember

• Octet rule refers to the tendency of atoms to form compounds in order to have eight valence electrons in their outermost shell.
• An atom, three particles are present: electron, neutron, and proton.
• Electron is a negatively charged particle, proton is a positively charged particle and neutron is a neutral particle.
• Protons and neutrons are present in the nucleus of an atom, while electrons are revolving in the outermost shell of an atom.
• The octet rule states that an atom is most stable when its outermost orbit has eight electrons.
• The atom tries to complete its octet by gaining an electron or by losing outermost shell electrons or by sharing the outermost electrons.
• The octet rule does not satisfy all the atoms of nitric oxide and Nitrogen dioxide which have an odd number of electrons.
• No explanation was given regarding the shape of the molecule.

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