Question : With the help of neat sketch explain different types of  crystal structure.

Answer: 

A. Crystal Structure : 

 Crystal Structure

1. The crystal structure is formed by associating every lattice point with an assembly of atoms or molecules or ions, which are identical in composition, arrangement and orientation.
2. Crystals have the three types of unit cells which are mostly being used: 

a .Body Centred Cubic Structure (BCC):

Body Centred Cubic Structure (BCC)

1. BCC structure has atoms at its each corner and one atom in its center. So, the BCC consists of net total of two atoms.   Total atoms in BCC = (8/1)*8+1 =2 atoms
2. The coordination number of BCC arrangement is 8 and packing factor is 0.68.
3. The BCC structure can be generally seen in Lithium, Potassium, Sodium etc.

b. Face Centered Cubic Structure (FCC):

1. It consists of atoms at its each corner and one atom at center of each face. Total atoms in FCC unit cell = (1/8)*8+(1/2)*6  = 4atoms
2.  The coordination number is 12 and packing factor is 0.74.
3. The FCC structure can be generally seen in Copper, Gold, Silver and Lead etc.

c. Hexagonal Close-Packed Structure (HCP):

Hexagonal Close-Packed Structure (HCP

1. The unit cell is like a hexagonal prism in HCP.
2. There are twelve corners, and each corner have an atom and one atom at the centre of each of the two hexagonal faces and three atoms in the body of the cell. In total, seventeen atoms take part in formation of a HCP unit cell. {Total atoms in HCP = 12*(1/6)+3+2x(1/2)= 6 atoms}
3. This is identical to FCC, having coordination number 12 and packing factor 0.74.
4. This HCP structure is generally seen in Zinc, Magnesium and Beryllium etc.