Engineering Chemistry S Datta

486 ENGINEERING CHEMISTRY
Table 22.1 Crystal systems and their characteristics
System Axial Angles No. of Examples
characteristics
space lattices
1. Cubic a = b = c α = β = γ = 90° 3 NaCl, KCl, CsCl, ZnS
2. Tetragonal a = b ≠ c α = β = γ = 90° 2 TiO 2
, SnO 2
3. Orthorhombic a ≠ b ≠ c α = β = γ = 90° 4 KNO 3
, K 2
SO 4
, BaSO 4
,
AgBr
4. Monoclinic a ≠ b ≠ c α = γ = 90° 2 NaHCO 3
, Na 2
SO 4
. 10 H 2
O,
β ≠ 90°
monoclinic sulphur
5. Triclinic a ≠ b ≠ c α ≠ β ≠ γ ≠ 90° 1 CuSO 4
. 5H 2
O, K 2
Cr 2
O 7
,
H 3
PO 3
6. Hexagonal a = b ≠ c α = β = 90° 1 SiO 2
, HgS, BN, PbI 2
γ = 120°
7. Trigonal a = b = c α = β = γ ≠ 90° 1 Al 2
O 3
, CaCO 3
, graphite,
or Sb, NaNO 3
Rhombohedral
90°
90°
90°
c
a
c
90°
90°
90°
a
a
a
90°
90°
90°
a
a
a
Cubic Tetragonal Orthorhombic
c
90°
°
120°
90°
°
a
°
c
°
a
a
° a
90°
°
a
a
a
b
b
Rhombohedral Hexagonal Monoclinic Triclinic
c
Fig. 22.6 The seven crystal systems or crystalline forms.
This classification is based on the magnitude of the unit cell length and the angle of
inclination between them. The unit cell is the smallest building unit in the space of a crystal,
which when repeated over and over again in three dimensions, forms a space lattice of the
crystalline substance. Thus the unit cell is the essential feature of the crystal structure. All the
crystal systems are made up of 14 types of crystal lattice and these crystal lattices are known
as Bravais lattices, shown in Fig. 22.7.