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ChemistryMediumClass 11

Group 13 — Boron family, properties, anomalous behavior of B

p-block Elements (Class 11 — Groups 13 & 14)

10

JEE Qs

8%

Hard

75

min

Focus on understanding the underlying reasons for observed trends and anomalous behavior, especially the role of d-orbitals and the inert pair effect, as conceptual questions are frequent.

🧮 Key Formulas

General electronic configuration: [Noble gas] ns²np¹
Oxidation States: +3 (common for all), +1 (stability increases down the group, e.g., Tl¹⁺ > Tl³⁺ due to inert pair effect)
Lewis Acidity of Boron Trihalides: BF₃ < BCl₃ < BBr₃ < BI₃ (due to backbonding and size of halogen)
Amphoteric reactions of Al: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O; Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]
Hydrolysis of AlCl₃: [Al(H₂O)₆]³⁺ + H₂O ⇌ [Al(H₂O)₅(OH)]²⁺ + H₃O⁺ (acidic solution)

✅ Key Points for JEE

  • 1Boron (B) exhibits anomalous behavior due to its small size, high ionization energy, high electronegativity, and absence of d-orbitals, restricting its maximum covalency to four.
  • 2The stability of the +1 oxidation state increases significantly down the group from Ga to Tl due to the 'inert pair effect', making Tl predominantly exist as Tl¹⁺.
  • 3Boric acid (H₃BO₃) is a weak monobasic Lewis acid, not a protic acid, accepting an electron pair from water to release H⁺: B(OH)₃ + H₂O ⇌ [B(OH)₄]⁻ + H⁺.
  • 4Aluminum and Gallium are amphoteric, meaning their oxides and hydroxides react with both acids and bases; Boron is acidic, while Indium and Thallium are basic.
  • 5Diborane (B₂H₆) is an electron-deficient compound with 'banana bonds' (3-center 2-electron bonds) and reacts readily with Lewis bases to form adducts.

⚠️ Common Mistakes

  • ✕Confusing the trend of Lewis acid strength of BX₃: Students often incorrectly assume BF₃ is the strongest due to high electronegativity, ignoring back-bonding.
  • ✕Not applying the inert pair effect correctly: Failing to recognize the increasing stability of the +1 oxidation state for heavier elements like Tl.
  • ✕Incorrectly stating boric acid as a protic acid: It's a Lewis acid, accepting hydroxide ions from water.
  • ✕Overlooking the unique properties of boron due to its diagonal relationship with silicon, such as forming covalent compounds and possessing metalloid character.

NCERT Chapters

  • Class 11 Chemistry Part 2 Ch 11: The p-Block Elements