Chemical Bonding – Bond Order & Resonance

Bond Parameters

  • Bond angle: Angle between two adjacent bonds.
  • Bond energy: Energy released on bond formation ≡ energy needed to break the bond.
  • Bond order (B.O.): Number of bonds between two atoms.

Bond Order – Key Facts

  • Simple diatomics: \text{H}2 = 1, \text{O}2 = 2, \text{N}_2 = 3.
  • Isoelectronic species share identical B.O.
    • \text{F}2 & \text{O}2^{2-} → 1.
    • \text{N}_2, \text{CO}, \text{NO}^+ → 3.
  • Relationship: \text{B.O.} \propto \text{Bond energy} \propto \dfrac{1}{\text{Bond length}}.
  • Higher B.O. ⇒ shorter & stronger bond (MCQ answer: option 1).

Resonance – Essentials

  • Introduced by Heisenberg; developed by Pauling & Ingold (1920s).
  • Some molecules cannot be described by a single Lewis structure.
    • Individual Lewis forms = canonical / resonating structures.
    • Actual molecule = resonance hybrid (intermediate of all forms).
  • Canonical forms differ only in electron distribution, not atom positions.
  • All canonical forms possess the same number of unpaired e⁻.

Calculating Bond Order in Polyatomic Species

  • General formula: \text{B.O.} = \dfrac{\text{Total number of bonds in all canonical forms}}{\text{Number of resonating structures}}.

Worked Examples

  • \text{O}_3: 2 canonical forms → B.O.(O–O) =\dfrac{3}{2}=1.5 (explains identical bond lengths).
  • \text{CO}_3^{2-}: 3 forms, 4 total C–O bonds → \dfrac{4}{3}=1.33.
  • \text{PO}_4^{3-}: 4 forms, 5 P–O bonds → \dfrac{5}{4}=1.25.
  • \text{ClO}_4^-: 4 forms, 7 Cl–O bonds → \dfrac{7}{4}=1.75.

Resonance – MCQ Highlights

  • Species with resonance: \text{CO}3^{2-}, \text{PO}4^{3-}, \text{SO}_3^{2-} → all of these.
  • Nitrate ion \text{NO}_3^- has 3 resonance structures.
  • Canonical forms differ only in electronic arrangement (statement 2 is false).
  • \text{H}3\text{PO}3 canonical debate: only structure with two acidic H (attached to O) is valid; other is not a resonance form.

Bond Energy Trend in Alkynes vs Alkenes/Alkanes

  • In \text{C}2\text{H}2 (ethyne) B.O.(C≡C)=3 ⇒ highest bond energy; thus C–H bonds associated with sp hybrid C show greatest bond energy.

Quick Recall

  • Higher B.O. → shorter, stronger bond.
  • Use fraction formula for B.O. in resonance cases.
  • Resonance hybrid is more stable than any single canonical form.
  • Canonical forms are hypothetical constructs; molecule never switches between them.