zigya tab
Name the parameters that characterize a unit cell.

Unit cell is the smallest portion of a crystal lattice.
A unit cell is characterized by:
(i) its dimensions along the three edges a, b and c. These edges may or may not be mutually perpendicular.
(ii) angles between the edges, α (between b and c), β (between a and c),γ (between a, and b). Thus, a unit cell is characterised by six parameters, a, b, c, α, β and γ.

367 Views

Explain how much portion of an atom located at (i) corner and (ii) body-centre of a cubic unit cell is part of its neighbouring unit cell.

(i) Primitive cubic unit cell has atoms only at its corner. Each atom at a corner is shared between eight adjacent unit cells . four unit cells in the same layer and four unit cells of the upper (or lower) layer Therefore only 1/8th of an atom (molecule or ion) actually belongs to a particular unit cell.
since each cubic unit cell has 8 atoms on its corners, the total number of atoms in one unit cell is 8×1/8 = 1 atom.
(ii) The atom at the body centre wholly belongs to the unit cell in which it is present. Thus in a body-centered cubic (bcc) unit cell:
(i) 8 corners×1/8 per corner atom =8×1/8 = 1 atom 
(ii) 1 body centre atom =1×1=1
   therefore total number of atom per unit cell =2 atom.

366 Views

Advertisement

Distinguish between Face-centred and end-centred unit cells.


(ii) Face centered unit cell

End-centered unit cell

It contains one particle present at the centre of its each face, besides the ones that are at its corners.

In this unit cell, one constituent particle is present at the centre of any two opposite faces besides the ones present at its corners.

1270 Views

Advertisement
What is the two-dimensional coordination number of a molecule in square close packed layer ?

The co-ordination number is 4.
185 Views

Distinguish between Hexagonal and monoclinic unit cells.

Ans. (i)                                        Hexagonal Unit cell                      Monoclinic unit cell

(1) Axial distances or edge
lengths                                              a = b ≠ c                                       a ≠ b ≠ c
(2) 
Axial angles                                α = β = 90°                                    α = γ=90° β ≠ 90°

(3) Examples:                                   Graphite, ZnO, CdS                       Monoclinic Sulphur, PbCrO2
                                                                                                                     Na2SO4.10H2O
                                                                                                                      2 4 2

332 Views

Advertisement