Chemistry Solids ETEA 2023
PMDC Verified Question 22 of 54
All of the following crystal systems have \( \beta = \gamma = 90^\circ \) except:
A
Cubic
B
Orthorhombic
C
Tetragonal
D
Rhombohedral
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: Rhombohedral
Concept:
Crystal systems are classified by their unit cell dimensions: edge lengths (\(a, b, c\)) and interfacial angles (\(\alpha, \beta, \gamma\)).

Solution:
  • In Cubic, Orthorhombic, and Tetragonal systems, all three interfacial angles are strictly 90 degrees: \( \alpha = \beta = \gamma = 90^\circ \).
  • However, in the Rhombohedral (or Trigonal) crystal system, all edge lengths are equal (\( a = b = c \)), and all angles are equal to each other, but they are not equal to 90 degrees: \( \alpha = \beta = \gamma \neq 90^\circ \).
  • Therefore, the rhombohedral system is the exception.


Why other options are incorrect:
  • A, B, C: These systems explicitly require all angles to be exactly 90 degrees.

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