Octet Rule & Exceptions | Chemical Bonding Class 11

Octet Rule & Its Exceptions

Lewis-Kossel Theory | Chemical Bonding Class 11

1. The Octet Rule

Proposed by Kössel and Lewis (1916).

Statement: Atoms tend to combine either by transfer of valence electrons or by sharing of valence electrons in order to have 8 electrons in their valence shells.

This configuration ($ns^2 np^6$) corresponds to the stable configuration of Noble Gases (e.g., Neon, Argon).
Exception: Hydrogen and Helium follow the Duplet Rule (2 electrons in K-shell).

2. Exception: Incomplete Octet

Molecules in which the central atom has less than 8 valence electrons. Also called Hypovalent or Electron-Deficient species.

Examples:

• $LiCl$: Li has 2 valence electrons (Duplet).
• $BeH_2$: Be forms 2 bonds. Total valence electrons = 4.
• $BCl_3$: B forms 3 bonds. Total valence electrons = 6.

These compounds often act as Lewis Acids (electron acceptors).

3. Exception: Expanded Octet

Molecules in which the central atom has more than 8 valence electrons. Also called Hypervalent or Super-Octet species.

Cause: Elements in Period 3 and beyond (P, S, Cl, etc.) have vacant d-orbitals available for bonding, allowing them to expand their valency beyond 4.

Examples:

Molecule	Central Atom	Valence Electrons
$PCl_5$	Phosphorus	$5 + 5(1) = 10 e^-$
$SF_6$	Sulfur	$6 + 6(1) = 12 e^-$
$IF_7$	Iodine	$7 + 7(1) = 14 e^-$
$H_2SO_4$	Sulfur	$12 e^-$ (bonded state)

4. Exception: Odd-Electron Molecules

Molecules where the total number of valence electrons is an odd number. It is impossible for all atoms to satisfy the octet rule in these cases.

Examples:

• Nitric Oxide ($NO$): Total valence $e^- = 5 + 6 = 11$. (Odd).
• Nitrogen Dioxide ($NO_2$): Total valence $e^- = 5 + 2(6) = 17$. (Odd).

These species are Paramagnetic in nature.

5. Other Limitations

• Noble Gas Compounds: The octet rule assumes noble gases are inert. However, Xenon and Krypton form compounds like $XeF_2, XeOF_4, KrF_2$, proving the octet isn't the ultimate stability limit.
• Shape of Molecules: The octet rule explains bonding but fails to predict the shape or geometry of molecules (explained later by VSEPR theory).

Practice Quiz

Identify the exceptions to the Octet Rule.

Your Score: 0 / 10

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