[Chemistry Class Notes] Limitations of Octet Rule Pdf for Exam

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 the Octet Rule

• The Octet rule which can explain 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

An atom’s stability is determined by the octet rule. According to this rule, an atom is stable if its outermost shell has a total of 8 electrons. In order for an atom to be stable, the outermost shell must be completely filled. Essentially, this is a special case of the atom stability rule. There is another case of the stability rule of atoms known as the duplet rule. This rule, although not universal, provides useful insight into the structure of most organic compounds. In the modern periodic table, the octet rule mostly applies to the elements of the second and third periods.

There are Mainly three limitations of the Octet Rule:

1. The Incomplete Octet of the Central Atom

In the case of molecules that have nearly equal ionic and covalent characteristics, the octet rule fails because no bond is completely ionic or completely covalent.

In some molecules, such as LiCl, BeH₂ and BCl₃, there are fewer than eight electrons surrounding the central atom. There is only 1 valence electron in Li, Be, and B. The situation is particularly challenging when it comes to elements with valence shells with fewer than four electrons. BF₃ and AlCl₃ are examples of similar compounds.

2. Odd-electron Molecules

As for molecules that do not obey the octet rule, such as nitric oxide, NO, or nitrogen dioxide, NO₂, they have odd numbers of electrons.

3. The Expanded Atom

Bonding is possible on 3d orbitals, which are not available on 3s and 3p orbitals. In fact, an atom is stable once the shell surrounding its outermost part is completely filled. The entire outermost shell in this case (4th period) can hold up to 18 e-. These elements have compounds with valence electrons around their central atoms greater than eight. These compounds are known as expanded octets. PF₅, SF₆, H₂SO₄, and many other coordination compounds are examples of such compounds.

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 gases (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.