Asked in PMC Practice Test 10 (2021) 2021

In which system are all the three axes and three angles of unequal length and none of the angles is 90?

Correct answer: B. Triclinic system

  • A. Cubic system
  • B. Triclinic system
  • C. Tetragonal system
  • D. Monoclinic system

Explanation

Option A: In the cubic crystal system, all three axes (a, b, c) have equal lengths, and all angles (α, β, γ) are 90 degrees. This system exhibits a high degree of symmetry and is characterized by its cubic shape. Option B: In the triclinic crystal system, all three axes (a, b, c) have unequal lengths, and none of the angles (α, β, γ) are equal to 90 degrees. The angles can take on any value between 0 and 180 degrees. This system has the least symmetry among all the crystal systems. Option C: In the tetragonal crystal system, two of the axes (a, b) have equal lengths, while the third axis is of a different length. All the angles (α, β, γ) are 90 degrees. This system exhibits a four-fold rotational symmetry around the c-axis. Option D: In the monoclinic crystal system, all three axes have unequal lengths, and only one angle (β) is 90 degrees, while the other two angles (α, γ) are not equal to 90 degrees. This system exhibits a single mirror plane and lacks rotational symmetry.

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About Crystalline Solids

Crystalline solids contain particles arranged in a regular repeating lattice described by a unit cell. Their study includes coordination number, packing efficiency, density, anisotropy, cleavage, polymorphism and common crystal systems, along with ionic, molecular, covalent network and metallic crystals. Crystalline order differs from the random arrangement of amorphous solids.

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