Limitations of Octet Rule

Introduction - Octet rule and its limitations

In 1916, Kössel and Lewis developed an important theory of chemical combination between atoms known as the electronic theory of chemical bonding. Accordingly, atoms can combine either by transferring valence electrons from one atom to another (gaining or losing) or by sharing valence electrons so as to have one octet in their valence shells. This law is known as the Octet rule.

Significance of Octet Rule

  • Octet rule which can explain the chemical bonding in different compounds.

  • Octet rules illustrate the different types of bonds, such as a covalent, electrovalent and coordinated bond.

Limitations of Octet Rule in Points

The octet rule is not universal although it is useful. It is quite useful for understanding the structural arrangement of most organic compounds and mainly applies to the elements of the second period of the periodic table.

There are three exceptions of octet rule

The Incomplete Octet of the Central Atom

In some compounds there are fewer than eight electrons surrounding the central atom. This is particularly the case for elements with fewer than four valence electrons

Examples: BCl3, LiCl and BeH

Compounds like Li, Be and B have 1,2 and 3 valence electrons. 

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Odd-Electron Molecules

The octet rule is not satisfied for all atoms in molecules with an odd number of electrons such as nitric oxide, NO and nitrogen dioxide, NO2.

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The Expanded Octet 

In addition to the 3s and 3p orbitals, elements in and beyond the third period of the periodic table also have 3d orbitals available for bonding. There are more than eight valence electrons around the central atom in a number of compounds of those elements. This is called the expanded octet. Obviously, in such cases the Octet rule does not apply

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Other Drawbacks of the Octet Theory

The rule of the octet is evidently based on the chemical inertness of the noble gases.

However, some noble gasses (e.g. xenon and krypton) also combine with oxygen and fluorine to form compounds such as XeF2, KrF2, XeOF2, etc.

The shape of the molecules does not account for this theory. 

• It does not clarify how relatively stable the molecules are completely silent about a molecule 's energy.

FAQ (Frequently Asked Questions)

1. Which elements do not follow octet rule?

Boron and aluminum are the two elements that most frequently fail to complete an octet; they all readily form compounds in which they have six valence electrons, rather than the normal eight expected by the octet rule.

2. What is the significance of octet rule?

Atom needs to have 8 electrons in the valence shell to achieve stable configuration according to the octet rule.

SignificanceIt explains the reason why separate atoms combine chemically to form ionic or covalent bonds. It need not complete the octet in order to achieve stability.

3. How is the octet rule used?

When atoms other than hydrogen form covalent bonds, sharing achieves an octet. The octet rule can be used to describe an atom form by the number of covalent bonds. This number normally equals the number of electrons in its outer shell that the atom needs to have a total of eight electrons.