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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2025 · Paper I

Q2(c). Radius ratio and the structure of ZnS

Question

The radii of Zn²⁺ and S²⁻ ions are 0.74 Å and 1.84 Å respectively. Determine the most stable form of arrangement of ions in ZnS crystal lattice. Draw the CCP (Cubic Close Packing) structure of ZnS.

Answer in brief

r⁺/r⁻ = 0.402, so Zn²⁺ sits in tetrahedral holes: 4 : 4 coordination, the zinc blende structure.

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Topic notes: crystal systems, Bragg's law, X-ray diffraction and radius ratio, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversradius ratio rulezinc blende structuretetrahedral voidscubic close packingcoordination numberwurtzite

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