Question

Define the following terms in relation to crystalline solids :
(i) Unit cell (ii) Coordination number Give one example in each case. (All India) 2011 ​

Answer 1

$\orange\maltese$Unit cell :

$\:\:\:\:\:\:\:\:$

• $\red\leadsto$The smallest three dimensional portion of a complete space lattice which when repeated over and again in different directions produces the complete space lattice is called the unit cell.

$\:\:\:\:\:\:\:\:$

• $\pink\mapsto$Example: Cubic unit cell, Hexagonal unit cell etc.

$\:\:\:\:\:\:\:\:$

$\green\maltese$ Coordination number :

$\:\:\:\:\:\:\:\:$

• $\blue\leadsto$The number of nearest spheres with which a particular sphere is in contact is called co-ordination number.

$\:\:\:\:\:\:\:\:$

• $\purple\mapsto$Example : Co-ordination number of hexagonal (hep) structures is 12.

$\:\:\:\:\:\:\:\:$

_____________________

Answer 2

Answer:

(I) unit cell:

A unit cell is the smallest portion of a crystal lattice which, when repeated in different direction, generates the entire lattice.

(ii) COORDINATION NUMBER:

(ii) Coordination number: The number of nearest neighbours of any constituent particle in a packing is called its coordination number. The coordination number of an atom in the bcc structure is 8

OR

i) Unit Cell:- Smallest element of a crystal lattice which on repetition gives the entire crystal lattice. Eg - cubic unit cell

ii) Coordination Number :- The number of neighbour atoms attached to a central atom . eg - In hcp the C.N is 6