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GATE 2025 CY – Question 23

Inorganic Chemistry · Solids: Crystal systems and lattices, Miller planes, crystal packing, crystal defects, Bragg’s law, ionic crystals, structure of AX, AX2, ABX3 type compounds, spinels, band theory, metals and semiconductors, Zeolites and their applications. · 1 mark · Multiple select

The essential symmetry(ies) for a monoclinic crystal system is/are the presence of

  1. one $C_3$ axis
  2. one $C_2$ axis
  3. one $C_4$ axis
  4. one $C_6$ axis

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Show answer and explanation

Correct answer: (B) one $C_2$ axis

Explanation

The seven crystal systems are defined by the **minimum symmetry** that a crystal of that system must have:

SystemEssential symmetry
Triclinicnone (only a centre or nothing)
**Monoclinic****one $C_2$ axis** (or one mirror plane)
Orthorhombicthree mutually perpendicular $C_2$ axes
Tetragonalone $C_4$ axis
Trigonalone $C_3$ axis
Hexagonalone $C_6$ axis
Cubicfour $C_3$ axes

The essential symmetry of the monoclinic system is **one $C_2$ axis** (option B). The $C_3$, $C_4$ and $C_6$ axes belong to the trigonal, tetragonal and hexagonal systems.