Indian Forest Service (Mains), Chemistry optional · 2016 · Paper I
Q1(d)(i). Crystal systems with mutually perpendicular axes
Question
How many crystal systems are there? Name the crystal systems wherein the unit cell axes are mutually perpendicular. Give one example in each case.
Answer in brief
Seven crystal systems (14 Bravais lattices). Axes mutually perpendicular (α=β=γ=90°) in three: cubic (NaCl), tetragonal (SnO₂, rutile TiO₂) and orthorhombic (rhombic sulfur).
Key points
- There are seven crystal systems: cubic, tetragonal, orthorhombic, hexagonal, trigonal (rhombohedral), monoclinic and triclinic. With centring they give 14 Bravais lattices.
- Cubic: a=b=c, α=β=γ=90°. Example: NaCl (also KCl, Cu, diamond).
- Tetragonal: a=b≠ c, α=β=γ=90°. Example: SnO₂ (rutile TiO₂, white tin).
- Orthorhombic: a≠ b≠ c, α=β=γ=90°. Example: rhombic sulfur (KNO₃, BaSO₄).
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.
Coversseven crystal systemsNaClSnO₂rhombic sulfur
All parts of Question 1
- 1(a)(i) Bond angle of F₂O smaller than that of H₂O5 marks
- 1(a)(ii) Residual entropy of CO and N₂O at 0 K5 marks
- 1(b)(i) Activation energy from rate constants at two temperatures5 marks
- 1(b)(ii) Ionic strength of 0.01 m H₂SO₄ + 0.02 m NaCl5 marks
- 1(c) Heat of reaction at 120°C: Kirchhoff's law10 marks
- 1(d)(i) Crystal systems with mutually perpendicular axes5 marks
- 1(d)(ii) Spacing of the (110) planes, axes mutually perpendicular5 marks