Write the significance and limitations of the octet rule.

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Detailed Solution

The octet rule explains why atoms form bonds to reach eight valence electrons, but it fails for odd-electron species, incomplete octets, expanded octets, transition metals, and many ionic–covalent borderline cases.

Significance

Simple predictor: Helps explain ionic and covalent bonding in main-group compounds.
Stability idea: Noble-gas configuration is associated with low energy and chemical stability.
Lewis structures: Guides drawing of electron-dot formulas, lone pairs, and shared pairs.
Periodic trends: Explains typical valencies, e.g., halogens form −1, alkali metals +1.

Limitations

Case	Example	What goes wrong
Odd-electron molecules	NO, NO₂	Cannot have all atoms with eight electrons
Incomplete octet	BF₃, BeCl₂, AlCl₃	Central atom stable with < 8 electrons
Expanded octet	SF₆, PCl₅, ClF₃	Central atom exceeds eight using d-orbitals (valence bond view)
Transition metals	Complex ions	d-orbital chemistry and variable oxidation states not captured
Resonance and polarity	NO₃⁻, CO₃²⁻	Octet structures exist but simple dots miss delocalization
Partial ionic/covalent mixing	AgCl, MgO small scale	Hard–soft interactions and polarization not explained