The Solid State

• Question 13

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

Solution
 (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.
Question 14

## 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.

Solution

(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  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 =
(ii) 1 body centre atom =${1}{×}{1}{=}{1}$
therefore total number of atom per unit cell =2 atom.

Question 15

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

Solution
The co-ordination number is 4.
Question 16

## A compound forms hexagonal closed-packed structure. What is the total number of voids in 0.5 mol of it? How many of these are tetrahedral voids?

Solution
Number of closed packed particeles                          =
therefore number of octhahedral voids                    =   3.011$×{10}^{23}$
and number oftetrahedral voids =$2×3.011×{10}^{23}=6.022×{10}^{23}$
therefore total number of voids=.