Question

How many types of unit cells are present in different types of crystals? Explain with appropriate figures.

Answer 1

Seven types of unit cells are present in different types of crystals.

1. Cubic or isometric systems: In a cubic system, the edge lengths are equal, i.e., a = b = c, and the three edges are perpendicular to each other. Thus, the angles α = β = γ = 90°. This form of crystal includes NaCl, KCl, zinc blende, Cu, and other compounds.
   
2. Tetragonal systems: In a tetragonal system, the first two edges (a and b) are equal, but the third edge (c) has a different length (a = b ≠ c). The angles α, β, and γ are equal and equal to 90°. This class of systems includes white tin crystals, SnO₂, and so on.
     
3. Orthorhombic systems: In these systems, the three edges of the unit cell are of different lengths (i.e., a ≠ b ≠ c), yet all three angles are equal and equal to 90° (i.e., α = β = γ = 90°). This kind has rhombic sulphur crystals.
   
4. Rhombohedral or trigonal systems: In a rhombohedral system, the unit cell’s three edges are equal (a = b = c). All three angles (α, β, and γ) are equal but not equal to 90°. Calcite crystal is an example of such a system.
   
5. Monoclinic systems: In a monoclinic system, the unit cell’s three edges (a, b, and c) have distinct lengths. The first two angles, α and γ, are equal to 90°, whereas the third angle is not. Thus, α = γ = 90°, but β is not 90°. Monoclinic sulphur, CaSO₄.H₂O, and other compounds produce crystals of this kind.
   
6. Hexagonal systems: In a hexagonal system, the unit cell has two identical edges (a and b), while the third edge is of variable length (a = b ≠ c). Two angles, α and β, are 90° apiece, while the third angle, γ, is equal to 120°. Thus, α = β = 90°, γ = 120°. Graphite, AgI, HgS, zinc, and other materials contain this form of crystal.
   
7. Triclinic systems: In a triclinic system, the unit cell’s three edges (a, b, and c) have varying lengths. The angles α, β, and γ are not equal to 90°. This sort of system consists of potassium dichromate crystals (CuSO₄.5H₂O).