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Limitations of the Octet Rule, with Examples

Learning Objective: To study the limitations of the octet rule, with examples.

Skill Level – Intermediate

 

Prerequisites: 

 

Related – 

 

Chapter: Bonding in Atoms

Author's Note:  In the previous section, we learnt about the octet rule and how most atoms bond using this rule.  However, such progress cannot be without imperfections and therefore exceptions.  In this section, we will learn about the three significant limitations of the octet rule, with specific examples.

Limitations of the Octet Rule, with Examples

The octet rule could satisfactorily explain how and why the atoms of the second period formed covalent bonds by sharing. However, it could not explain why certain atoms like Boron or Phosphorous did not follow the octet rule or why the Nitrogen atom in NO (nitrogen monoxide) has an unpaired electron.

 

Incomplete octet

Beryllium has two, and Boron, or Aluminium, has three valence electrons. In compounds like BeCl2 or BCl3, the central Boron atom doesn’t complete its octet yet form covalent bonds, which the octet rule couldn’t explain.

 

What are the limitations of the octet rule? Examples

 

So, the octet rule is unsatisfactory in explaining covalent bonding in atoms with less than four valence electrons.

 

Expanded Octet

Elements of the third period and above have d-orbitals when compared to the elements of the second row, which have only up to p-orbitals. These d-orbitals can also participate in bond-forming reactions, since an atom can now incorporate more orbitals to form more bonds.

Phosphorus can form five bonds as PF5, and Sulphur can form six bonds as SF6, thereby expanding the octet to ten and twelve.

 

Expanded and incomplete octet - limitation of octet rule

 

Odd Electron

According to the octet rule, eight electrons and a complete octet in the outermost shell are why atoms form covalent bonds. However, there exist molecules that have an odd electron in their shell. The odd electrons are unlike the lone pair or bond pairs and are commonly called free radicals (denoted with a dot . over the atom).

For example, nitric oxide (.NO) and nitrogen dioxide (.NO2) have odd electrons. 

 

odd electron drawback of octet rule

 

The odd electron-containing atoms are reactive species, unstable, and some may largely exist as dimers.

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Related Reading- The Octet Rule

 

Bonding in Atoms
Organic Chemistry - Drawing Structures, Concepts, and Examples

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What is Organic Chemistry?

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Atom

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  • Power of Protons
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Bonding In Atoms

  • Octet Rule - Introduction and Bonding
  • Limitations of Octet Rule
  • Ionic Bond- Introduction and Formation
  • Formation of Ionic Compound
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Covalent Bond

  • Theories on Covalent Bond Formation
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Electronic Displacement in a Covalent Bond

  • Electronegativity- Introduction
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  • Resonance - Introduction and Electron Delocalization
  • Partial Double Bond Character and Resonance Hybrid
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  • Introduction to Covalent Bond Polarity and Dipole Moment
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Common Types of Reactions

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Drawing Organic Structures

  • Introduction
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Functional Groups in Organic Chemistry

  • What are functional groups?
  • Chemical and Physical Properties affected by the Functional Groups
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  • Functional Group Categorization- Functional Groups with Carbon-Heteroatom Single Bond
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Structural Isomerism

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Intermolecular Forces

  • Ion-Dipole Interactions-Introduction and Occurrence
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  • Ion-Induced Dipole - Introduction, Strength and Occurrence
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Physical Properties

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Fundamentals of Organic Reactions

  • Types of Arrows Used in Chemistry
  • Curved Arrows in Organic Chemistry- with Examples
  • Electrophiles - Introduction, Identification and Reaction
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Reactive Intermediates

  • Carbocation - Introduction, Nature, and Types
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Stereoisomerism - Conformation and Configurational Isomerism

  • Conformations in Organic Chemistry - An Introduction
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  • Conformational Analysis
  • Factors affecting the stability of conformers - Stabilizing Interactions |Hyperconjugation
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  • Conformation in Compounds with Lone Pairs
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  • Impact of cis-trans isomerism on physical properties
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