Crystal Lattice
Regular threedimensional arrangement of points in space
There are 14 possible threedimensional lattices, known as Bravais lattices.
Characteristics of a crystal lattice:
Each point in a lattice is called lattice point or lattice site.
Each lattice point represents one constituent particle (atom, molecule or ion).
Lattice points are joined by straight lines to bring out the geometry of the lattice.
Unit Cell
Smallest portion of a crystal lattice which, when repeated in different directions, generates the entire lattice
Characterised by −
(i) Its dimensions along the three edges a, b and c
(ii) Angles between the edges α, β and γ
The unit cells can be classified as follows:
Seven Crystal Systems
There are seven types of primitive unit cells, as given in the following table.
The given table lists seven primitive unit cells and their possible variations as centered unit cells.
Crystal Class  Axial Distances  Axial Angles  Possible Types of Unit Cells  Examples 
1. Cubic  a = b = c  α = β =γ = 90°  Primitive, bodycentred, facecentred  KCl, NaCl 
2. Tetragonal  a = b ≠ c  α = β =γ = 90°  Primitive, bodycentred  SnO_{2}, TiO_{2} 
3. Orthorhombic  a ≠ b ≠ c  α = β =γ = 90°  Primitive, bodycentred, facecentred, endcentred  KNO_{3}, BaSO_{4} 
4. Hexagonal  a = b ≠ c  α = β = 90°; γ= 120°  Primitive  Mg, ZnO 
5. Trigonal orRhombohedral  a = b = c  α = β =γ ≠ 90°  Primitive  (CaCO_{3}) Calcite, HgS (Cinnabar) 
6. Monoclinic  a ≠ b ≠ c  α = γ = 90°; β≠ 90°  Primitive and endcentred  Monoclinicsulphur, Na_{2}SO_{4}.10H_{2}O 
7. Triclinic  a ≠ b ≠ c  α ≠ β≠γ≠90°  Primitive  K_{2}Cr_{2}O_{7}, H_{3}BO_{3} 
Unit cells of 14 types Bravais lattices:

Cubic lattices: All sides are of the same length, and the angles between the faces are 90° each
Tetragonal lattices: One side is different in length from the other two, and the angles between the faces are 90° each
Orthorhombic lattices: Unequal sides; angles between the faces are 90° each
Monoclinic lattices: Unequal sides; two faces have angles not equal to 90°
Hexagonal lattice: One side is different in length from the other two, and the marked angles on two faces are 60°
Rhombohedral lattice: All sides are of equal length, and the marked angles on two faces are less than 90°
Triclinic lattice: Unequal sides; unequal angles, with none equal to 90°
Questions asked in previous years’ board examinations 
Ques. Name the type of structure possessed by a unit cell of CsCl. (1 mark) −2004 CBSE Delhi Sol: A unit cell of CsCl possesses bodycentred cubic structure. 
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