What are the limitations of the octet rule?

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Detailed Solution

The octet rule states that atoms tend to gain, lose, or share electrons to achieve eight electrons in their outermost shell. However, it has several limitations.

Limitations

Incomplete octet: Some atoms, like hydrogen (H), lithium (Li), and beryllium (Be), have less than eight electrons and still remain stable.
Expanded octet: Elements in Period 3 or beyond, like phosphorus (P), sulfur (S), and chlorine (Cl), can have more than eight electrons due to available d-orbitals.
Odd-electron molecules: Molecules like nitric oxide (NO) and nitrogen dioxide (NO₂) have an odd number of electrons, so they cannot follow the rule.
Metallic bonding: The octet rule doesn’t explain the bonding in metals where electrons are delocalized.

Examples

Exception Type	Example	Explanation
Incomplete Octet	BeCl₂	Beryllium has only 4 valence electrons.
Expanded Octet	SF₆	Sulfur has 12 valence electrons.
Odd-Electron	NO	Contains an unpaired electron.

Thus, while useful, the octet rule cannot explain all molecular structures, especially those involving transition elements or complex bonding.

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