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ChemistryMediumClass 12

Group 17 — Halogens, trends, interhalogen compounds

p-block Elements (Class 12 — Groups 15, 16, 17, 18)

10

JEE Qs

8%

Hard

90

min

Prioritize understanding periodic trends, the anomalous behavior of fluorine, and the properties and structures of interhalogen compounds, as these are frequent testing points.

🧮 Key Formulas

General formula for interhalogen compounds: XY_n (where X is larger halogen, Y is smaller halogen, n = 1, 3, 5, 7)

✅ Key Points for JEE

  • 1Master the periodic trends for Group 17 elements (atomic size, ionization enthalpy, electronegativity, electron gain enthalpy, bond dissociation enthalpy, oxidizing power) and identify exceptions like F2 bond enthalpy.
  • 2Understand the anomalous behavior of fluorine due to its small size, high electronegativity, and absence of d-orbitals, affecting its reactivity and compound stability.
  • 3Learn the properties, acidic strength (HI > HBr > HCl > HF), and reducing power (HI > HBr > HCl > HF) of hydrogen halides (HX) and their preparation methods.
  • 4Grasp the structure, reactivity, and stability of interhalogen compounds (XY_n). They are generally more reactive than individual halogens (except F2) due to weaker X-Y bonds compared to X-X or Y-Y bonds.
  • 5Focus on important reactions of halogens with water, alkalis, other halogens, and their applications (e.g., Cl2 as bleaching agent, HF etching glass).

⚠️ Common Mistakes

  • ✕Confusing the order of bond dissociation enthalpies: F2 < Cl2 > Br2 > I2, often forgetting that F2 is lower than Cl2 due to lone pair-lone pair repulsions.
  • ✕Incorrectly comparing the oxidizing power of halogens (F2 > Cl2 > Br2 > I2) with the reducing power of halide ions (I- > Br- > Cl- > F-).
  • ✕Not applying VSEPR theory correctly for predicting shapes of interhalogen compounds (e.g., ClF3, IF5, IF7) and oxyacids of halogens.
  • ✕Overlooking the fact that interhalogen compounds are generally more reactive than halogens (except F2) because X-Y bonds are weaker than X-X or Y-Y bonds.

NCERT Chapters

  • Class 12 Chemistry Part 1 Ch 7: The p-Block Elements