unit cell.pptx

Crystalline State

1. Bravais Lattices And Crystal Systems

2. Crystal lattice The three dimensional regular arrangement of constituent particles( atom,ion,or molecule)within the crystal is known as crystal lattice. Space lattice The three dimensional regular arrangement of lattice points in space is called space lattice.

3. Unit cell The smallest building block of crystal consisting of atoms,ions or molecule whose geometric arrangement when repeated in three dimensions will generate the complete crystal.

4. Types of Unit Cells There are two type of Unit Cells

5. Primitive (simple)unit cell : P rimitive unit cells are those in which particles are present only at the corners of the unit cell . Non- primitive unit cells: Non-Primitive unit cells are those in which particles are present at the corners as well as some other sites .

6. TYPES OF NON- PRIMITIVE UNIT CELLS

7. Body centered cubic (B.C.C.): There is one particle at every corner plus one particle at the centre of the cube.The atom in the centre belongs exclusively to the unit cell and as seen earlier,one atom amongst the corner atoms belongs to the unit cell.Hence 1+1=2 atoms belong to the unit cell.The corner atom do not touch each other,but each corner atom touches the body centre atom along the body diagonal.

8. BODY-CENTRED CUBIC ∕∕

9. FACE CENTRED CUBE(F.C.C.): When particles are present at the centre of each face in addition to a particle at each corner.

10. End face centered(E.F.C.): When particles are present at the centre of the two opposite faces along one axis in addition to particles at the corners.

11. CRYSTAL SYSTEM : A number of 230 crystal forms are possible,all of which have observed.They may be grouped into 32 classes on the basis of symmetry.These are called 32 point groupes.These point groupes are clubbed together to form seven crystal system namely cubic ,rhombic,triclinic, tetragonal ,monoclinic ,hexagonal and rhombohedral.

12. BRAVAIS LATTICES: Bravais (french mathematician)stated that there are 14 fundamental and unique ways in which particles can be arranged in three dimensional point lattices. These are known as Bravais lattices after his name.

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