**Unit Cell-**The metallic crystals can be considered as consisting of tiny blocks which are repeated in three pattern. The tiny blocks formed by the arrangement of small group of atom are called the unit cell. It is the volume of solid from which the entire crystal can be constructed by transnational in three dimensions

Here in fig 2- The three unit vectors, a, b, c can define a cell as shown by the shaded region in Fig.(a) This cell is known as unit cell (Fig. b) which when repeated

in the three dimensions generates the crystal structure.

**Lattice Point and lattice array-**If each atom in lattice is replaced by point, then each point is called lattice point and the arrangement of the points is referred to as the lattice array

**Space Lattice-**space lattice s defined as an array of points in three dimensions in which every point has surroundings identical to that every other point in the array

**Bravais Lattices-**The unit vectors a,b and c are lattice parameters.Based on their length equality or inequality and their orientation (the angles between them, α, β and ɣ) a total of 7 crystal systems can be defined. With the centering (face, base and body centering) added to these, 14 kinds of 3D lattices, known as Bravais lattices

**7 crystal system table**

1. cubic a=b=c α = β= γ=90

2. monoclinic a<>b<>c α = β= γ<>90

3. triclinic a<>b<>c α <> β<> γ<>90

4. Tetragonal a<>b<>c α <> β<> γ<>90

5. Orthogonal a<>b<>c α = β= γ=90

6. Rhombohedral a=b=c α = β= γ<>90

7. Hexagonal a=b<>c α = β= γ=120

Answered by Sunil

student, OCC (AMIE Online coaching Class)

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