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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper II covers physical chemistry.

UPSC Combined Geo-Scientist, Mains · 2021 · Mains Paper II

Q1(d). Crystal systems from the unit-cell constants

Question

The data on the unit cell are given below. Identify the crystal system in each case. (i) a = b ≠ c, α = β = γ = 90°; (ii) a ≠ b ≠ c, α = β = γ = 90°; (iii) a ≠ b ≠ c, α ≠ β ≠ γ ≠ 90°; (iv) a = b ≠ c, α = β = 90°, γ = 120°; (v) a = b = c, α = β = γ = 90°.

Answer in brief

(i) tetragonal (ii) orthorhombic (iii) triclinic (iv) hexagonal (v) cubic.

Key points

  • (i) Tetragonal: rutile TiO₂, cassiterite SnO₂, zircon ZrSiO₄, white tin.
  • (ii) Orthorhombic: rhombic sulfur, olivine Mg₂SiO₄, aragonite CaCO₃, barite BaSO₄, KNO₃.
  • (iii) Triclinic: CuSO₄·5H₂O, K₂Cr₂O₇; (iv) Hexagonal: graphite, ZnO, Mg; (v) Cubic: NaCl, CsCl, diamond, pyrite FeS₂.
  • Essential symmetry: cubic four 3-fold axes, tetragonal one 4-fold, orthorhombic three 2-fold, hexagonal one 6-fold, triclinic none (at most i).
  • The other two systems: monoclinic (a ≠ b ≠ c; α = γ = 90° ≠ β) and rhombohedral (a = b = c; α = β = γ ≠ 90°).

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

Model answer to Q1(d), 2021 Mains Paper II: solution pages 5 and 6
Solution pages 5 and 6 of 36
Read the whole 2021 solution2021 question paper

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

Coverscrystal systemsunit cell parametersBravais latticestetragonalorthorhombictriclinic

All parts of Question 1