Octet Rule & Its Exceptions
Lewis-Kossel Theory | Chemical Bonding Class 11
1. The Octet Rule
Proposed by Kössel and Lewis (1916).
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.
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.
Such a great explanation.
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