Molecules Following Octet Rule in Chemistry Sets

To determine which options contain at least three molecules that follow the octet rule, let us analyze the electronic configuration and octet status of each given molecule:

1. Definition of Octet Rule and Exceptions:
   • Octet Rule: Atoms gain, lose, or share electrons to achieve a valence shell containing eight electrons ($8\, e^-$).
   • Incomplete Octet (Electron-deficient species): Molecules where the central atom has fewer than 8 valence electrons (e.g., $\text{BCl}_3$, which has $6\, e^-$ around $\text{B}$).
   • Odd-Electron Molecules: Species with an odd number of total valence electrons, where at least one atom cannot achieve 8 valence electrons (e.g., $\text{NO}$ and $\text{NO}_2$, where $\text{N}$ has $7\, e^-$).
   • Expanded Octet (Hypervalent species): Molecules where the central atom expands its octet to hold more than 8 electrons (e.g., $\text{H}_2\text{SO}_4$, where $\text{S}$ has $12\, e^-$ in its valence shell).

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1. Analysis of Individual Molecules:

   • $\text{CO}_2$: The Lewis structure is $\text{O}=\text{C}=\text{O}$.

     • Central $\text{C}$ atom forms 2 double bonds $\implies 4 \times 2 = 8\, e^-$.
     • Each $\text{O}$ atom has 2 lone pairs and 1 double bond $\implies 8\, e^-$.
     • Follows octet rule.

   • $\text{C}_2\text{H}_4$: The Lewis structure is $\text{H}_2\text{C}=\text{CH}_2$.

     • Each $\text{C}$ atom forms 2 single bonds with $\text{H}$ and 1 double bond with $\text{C} \implies 8\, e^-$.
     • Follows octet rule.

   • $\text{HCl}$: The Lewis structure is $\text{H}-\text{Cl}$.

     • $\text{Cl}$ has 3 lone pairs and 1 single bond $\implies 8\, e^-$.
     • Follows octet rule.

   • $\text{O}_3$: The Lewis structure is a resonance hybrid ($\text{O}=\text{O}^+-\text{O}^-$).

     • Central $\text{O}$ has 1 double bond, 1 single bond, and 1 lone pair $\implies 8\, e^-$.
     • Terminal oxygens also complete their octets.
     • Follows octet rule.

   • $\text{NO}$: Total valence electrons = $5 (\text{N}) + 6 (\text{O}) = 11$ (odd electron species).

     • $\text{N}$ has only $7\, e^-$ around it.
     • Does NOT follow octet rule.

   • $\text{NO}_2$: Total valence electrons = $5 (\text{N}) + 2 \times 6 (\text{O}) = 17$ (odd electron species).

     • $\text{N}$ has only $7\, e^-$ around it.
     • Does NOT follow octet rule.

   • $\text{BCl}_3$: Central $\text{B}$ forms 3 single bonds with $\text{Cl}$ atoms.

     • $\text{B}$ has $3 \times 2 = 6\, e^-$ (incomplete octet).
     • Does NOT follow octet rule.

   • $\text{H}_2\text{SO}_4$: Central $\text{S}$ forms 2 single bonds ($\text{S}-\text{OH}$) and 2 double bonds ($\text{S}=\text{O}$).

     • $\text{S}$ has $6 \times 2 = 12\, e^-$ (expanded octet).
     • Does NOT follow octet rule.

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1. Evaluating the Options:

   • Option (A): $\text{CO}_2$, $\text{C}_2\text{H}_4$, $\text{NO}$ and $\text{HCl}$

     • $\text{CO}_2$ (Yes), $\text{C}_2\text{H}_4$ (Yes), $\text{NO}$ (No), $\text{HCl}$ (Yes)
     • Number of molecules following octet rule = $3$
     • Option (A) is correct.

   • Option (B): $\text{NO}_2$, $\text{O}_3$, $\text{HCl}$ and $\text{H}_2\text{SO}_4$

     • $\text{NO}_2$ (No), $\text{O}_3$ (Yes), $\text{HCl}$ (Yes), $\text{H}_2\text{SO}_4$ (No)
     • Number of molecules following octet rule = $2$
     • Option (B) is incorrect.

   • Option (C): $\text{BCl}_3$, $\text{NO}$, $\text{NO}_2$ and $\text{H}_2\text{SO}_4$

     • $\text{BCl}_3$ (No), $\text{NO}$ (No), $\text{NO}_2$ (No), $\text{H}_2\text{SO}_4$ (No)
     • Number of molecules following octet rule = $0$
     • Option (C) is incorrect.

   • Option (D): $\text{CO}_2$, $\text{BCl}_3$, $\text{O}_3$ and $\text{C}_2\text{H}_4$

     • $\text{CO}_2$ (Yes), $\text{BCl}_3$ (No), $\text{O}_3$ (Yes), $\text{C}_2\text{H}_4$ (Yes)
     • Number of molecules following octet rule = $3$
     • Option (D) is correct.

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Correct Options: (A) and (D)