Solids have definite mass, volume and shape. Depending upon order, they are classified as crystalline solids and amorphous solids. Crystalline solids are further classified into molecular, ionic, metallic and covalent solids. Crystalline solids are made up of constituent particles which are arranged in a regular pattern, which is depicted in the form of three dimensional array of points in spaces called lattice points. Small characteristic repeating portion of lattice is called unit cell.
Unit cells can be primitive and also centred unit cells like face centred, body centred, end centred. Solids are not perfect. There are various types of defects (i) line defect (ii) point defect. Point defects are of three types : stoichiometric, impurity and non-stoichiometric defects. Solids show magnetic properties depending upon the orientation of electron.
Based on the paragraph Answer the following :
For a primitive unit cell, the edge lengths are a ≠ b ≠ c and axial angles α ≠ β ≠ γ ≠ 90°, then find the crystal system
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Triclinic
The seven crystal systems are distinguished by their edge lengths (a, b, c) and axial angles (α, β, γ). The triclinic system is the least symmetrical: all three edges are unequal and all three angles are unequal and none is 90°:
a ≠ b ≠ c; α ≠ β ≠ γ ≠ 90°
It has only the primitive unit cell. Examples: \( \ce{K2Cr2O7} \), \( \ce{CuSO4} \).\( \ce{5H2O} \), \( \ce{H3BO3} \).
Why the other options are incorrect
Option (1) Hexagonal — Incorrect. a = b ≠ c, with α = β = 90° and γ = 120°.
Option (2) Rhombohedral — Incorrect. a = b = c, with α = β = γ ≠ 90°.
Option (3) Tetragonal — Incorrect. a = b ≠ c, with all three angles 90°.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.