Limitations of The Octet Rule

Understanding Octet Rule

The octet rule suggests that atoms bond in a way that enables them to achieve a full outer shell of eight electrons for each atom hence offering them stability. This is best applied to main group elements, particularly to the second and third periods of the periodic table. These are done by three main mechanisms: ionic bonding, wherein electrons are transferred from atom to atom; covalent bonding, where they will be shared; and coordinate covalent bonding, in which both electrons come from one of the participating atoms.
While the octet rule can be applied to explain a good number of chemical reactions, it is not universal. For example, certain elements are stable with fewer than eight electrons. Other atoms can hold more than an octet. Molecules such as these are called hypervalent. Examples include SF₆ and PF₅. The octet rule is a guiding principle but one that falls way short of describing the full complexity of chemical bonding and molecular stability.

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Exceptions to the Octet Rule

Although octet rule is able to explain the formation of a large number of compounds, there are some exceptions to this rule as follows:

1. Formation of compounds involving hydrogen. As hydrogen atom has only one shell containing one electron, it needs one more electron to complete its shell, i.e., to acquire the nearest noble gas configuration of helium. Hence, hydrogen needs to complete its duplet and not octet.
2. Formation of compounds like BeCl₂, BF₃, AlCl₃ etc. In each of these molecules, the central atom (Be, B or Al) has less than 8 electrons, i.e., these are electron deficient compounds as shown below: Thus, octet rule is violated.
3. Formation of Compounds like PCl₅, SF₆, IF₇, H₂SO₄ etc. In each of these molecules, the central atom has more than 8 electrons (expandes octet) as shown below: These compounds are called hypervalent compounds. Again the octet rule is violated in these molecules.
4. Formation of compounds of noble gases. Noble gases which have already complete octets (or duplet in case of helium) should not form compounds. However, their compounds like XeF₂, XeF₄, XeF₆, KrF₂ etc. have been actually prepared.
5. Odd electron bonds/Odd electron molecules. There are some molecules and ions in which the bonded atoms contain odd number of electrons (usually 3) between them. The bonds thus present are called odd electron bonds and the molevules are called odd electron molecules. Some common examples are given below: A three electron bond is considered to be equivalent to half covalent bond.

Supplement Your Knowledge

Sugden's concept of singlet linkage. Sugden put forward the view that octet rule is never violated. He suggested that in case of molecules like PCl5, SF6, etc., some atoms are linked to the central atom by covalent bonds while others are linked by singlet bonds. A singlet bond is formed by one sided sharing of only one electron between the two atoms and is, therefore, represented by half arrow ($
\rightharpoonup
$) pointing from donor to acceptor. Thus, we have

It can be seen that

No. of singlet bonds $=$ Total no. of bonds - No. of electrons required to complete the octet.|

Real-Life Applications and Importance

In real-life applications, the constraints of the octet rule realize themselves in several areas: from materials science to biochemistry and pharmacology. As one simple example, the development of pharmaceuticals is based on the idea of intermolecular interactions at the atomic level. For many biologically active compounds, the octet rule is not observed strictly; therefore, advanced theories like molecular orbital theory are needed to predict their behavior.
On the other side, new material creation in material science depends on manipulating atomic interactions. Properties in most modern materials, for example, semiconductors or catalysts, boil down indeed to electron-deficient/hypervalent species, which cannot be accounted for by the octet rule alone.

Related Topics:

• Formal Charge And Its Properties
• Ionic Bond Or Electrovalent Bond
• Lattice Energy

Recommended topic video on ( Limitations of octet rule)

Some Solved Examples

Example 1:

Octet rule cannot be applied to the non-metals after:

1) Carbon
2) Silicon
3) Oxygen
4) Nitrogen

Solution:

The octet rule cannot be applied to the non-metals after silicon. These elements can “expand their octet” and have more than eight valence electrons around the central atom.

Hence, the answer is the option (2).

Example 2:

Octet rule is based upon:

1) Shape of the molecules
2) Chemical inertness of noble gases
3) Energy of the molecule
4) None

Solution:

The Octet rule is based on the chemical inertness of noble gases. This rule is derived from the fact that noble gases have a stable electron configuration with a full outer shell, influencing the bonding behavior of other elements.

Hence, the answer is the option (2).

Example 3:

The group having species with complete octets in their ionic form is:

1)$\mathrm{O}^{2-}, \mathrm{F}^{-}, \mathrm{Na}, \mathrm{Mg}^{2+}$

2)$\mathrm{O}^{-}, \mathrm{F}^{-}, \mathrm{Na}^{+}, \mathrm{Mg}^{2+}$

3)$\mathrm{O}^{2-}, \mathrm{F}^{-}, \mathrm{Na}^{+}, \mathrm{Mg}^{2+}$

4)$\mathrm{O}^{-}, \mathrm{F}^{-}, \mathrm{Na}, \mathrm{Mg}^{+}$

Solution:

The group having species with a complete octet in their ionic form includes O^2-, F^-, Na⁺, and Mg²⁺. These ions achieve a noble gas configuration with a complete octet.

Hence, the answer is option (3).

Example 4:

Which of the following is an example of an odd-electron molecule?

1) NO₂
2) CO₂
3) N₂
4) O₂

Solution:

Nitric oxide (NO) is an example of an odd-electron molecule because it has an odd number of valence electrons (11 in total).

Hence, the answer is the option (1).

Example 5:

The central atom in which of the following compounds is electron-deficient?

1) BeH₂
2) BF₃
3) CH₄
4) NH₃

Solution:

In BF₃, the boron atom is electron-deficient because it only has six electrons around it instead of the full octet.

Hence, the answer is the option (2).

Example 6:

Number of molecules from the following which are exceptions to octet rule is_
$\mathrm{CO}_2, \mathrm{NO}_2, \mathrm{H}_2 \mathrm{SO}_4, \mathrm{BF}_3, \mathrm{CH}_4, \mathrm{SiF}_4, \mathrm{ClO}_2, \mathrm{PCl}_5$, $\mathrm{BeF}_2, \mathrm{C}_2 \mathrm{H}_6, \mathrm{CHCl}_3, \mathrm{CBr}_4$ [JEE Main 2024]

Solution:

Hence, the answer is (6).

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Summary

The octet rule, in layman's terms, is one of the basic tenets of chemistry, guiding the formation of bonds between atoms to attain stability with a full outer shell of eight electrons. This paper has managed to point out some major limitations of the universal application of the octet rule. We have covered the concept, defining its tenets and mechanisms of bonding, but also its exceptions among incomplete octets, odd electron molecules, and hypervalent compounds.

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