## What is the coordination number of atoms in a body centred cubic (bcc) structure ?

In case of BCC (body centred cubic) structure coordination number is 8

## Define the term amorphous : Give few example of amorphous solids .

Amorphus solids :-

## What type of substances would make better permanent magnets, ferromagnetic or ferrimagnetic . Justify your answer.

In case of ferromagnetic substaces the direction of domains align themselves in the direction of external magnetic field appllied . This leads to the formation of strong magnetic field. This magnetic field persists even on removing the external magnetic field. so the ferromagnmetic substances can be used for making the permanent magnet.

## What type of defect can arise when a solid is heated ? Which physical property is affected by it and in what way ?

on supply of heat the lattice particles absorb energy and leave the lattice this may lead to the vacancy defect. In this case the density of the lattice decrease as the particles permanently leave the lattice. The lattice may become coloured .

## An element with molar mass 2.7 x 10-2 kg / mol forms a cubic unit cell with edge length 405 pm . If its density is 2.7 x 10 3 kg /m3 , what is the nature of the cubic unit cell ?

Molar mass M = 2.7 x 10-2 kg mol -1

Edge length of unit cell a = 405 pm = 4.05 x 103

Number of atom per unit cell Z = ?

by using the formula

lets put the values

## Which of the following lattices has the highest packing efficiency? (i) Simple cubic , (ii) Body centred , (iii) Hexagonal close packed lattice .

Hexagonal close packed lattice has the highest packing efficiency that of 74 % .

bcc has 68% and simple cubic 52% packing efficiency .

## A compound is formed by two elements M and N . The element N forms CCP and atoms of M occupy 1/3 rd of tetrahedral voids. What is the formula of the compound ?

Solution:-

The number of N atoms in CCP (fcc) = 4 per unit cell

Number of tetrahedral voids = 2 x number of atoms

= 2 x 4

= 8

Atoms of element M occupy 1/3 rd of tetrahedral voids of tetrahedral voids, so

number of M atoms = 8 x 1/3

ratio of M and N = 8/3 : 4

= 2 : 3

formula of the compound = M2 N3

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

Here is the solution

values given:-

0.5 mol

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

In square close packing a single particle is touched by four particles . Thus the co-ordination number is 4.

## Explain how much portion of an atom located at (i) corner and (ii) body centre of a cubic unit cell part of its neighboring unit cell

(i) At corner :- The particle present at the corner is shared by eight unit cells . hence 1/8 part of the particle is the share of a single unit cell.

(ii) At the body centre :- In case of body centre , the particle is present exactly at the centre of unit cell. hence it is not shared with any other unit cell , thus it belongs to the unit cell as 1 complete particle.

## Distinguish between face centred and end centred unit cells.

In case of face centred unit cell the particles are present on the corners of the unit cell as well as on the centre of every face.

In case of end centred unit cell the particles are present on the corners as well as on the centres of any two opposite faces.

## Distinguish between Hexagonal and Monoclinic unit cells.

Hexagonal unit cells have two sides equal and two angles right angle , third angle is 120 degrees.

The monoclinic unit cells have unequal cell edges and two right angles.

## Name the parameters that characterise a unit cell .

A unit cell is characterised by six parameters :-

1. Three edges denoted by – a , b , c.
2. Angles between the edges – α , β , γ

## Give the significance of a lattice point .

The constituent particles (ions , atoms ) of a crystalline solid are denoted by a points . These points are known as the lattice points.

## What type of solids are electrical conductors , malleable and ductile ?

The metallic solids show these kind of characters.

## Ionic solids conduct electricity in molten state but not in solid state. Explain.

In case of ionic solids , the ions are the units for the conduction of electricity. when they are in solid state the ions are bound to each other by strong electrostatic forces of attraction . But in the molten form the ions are free to move under the applied electric field thus they can conduct the electricity in molten form.

## Solid ‘A’ is very hard , electrical insulator in solid as well as in molten state and melts at extremely high temperature. what type of solid is it ?

The covalent (network) solid has these kind of characters. It is insulator and has high forces of attractions due to which it has high melting point.

## Refractive index of a solid is observed to have the same value along all directions . Comment on the nature of solid . Would it show cleavage property ?

The solid which has same value for refractive index in all directions , shows isotropic behaviour and such solids are only the amorphus solids.

As it is an amorphus solid so it will not show the clean cut due to the presence of rough surface.

## Why the solids are rigid?

The solids have strong interparticulate forces which bind the particles so close that they can only oscillate about their mean position only.

## Why the boiling point of carboxylic acids is higher than that of the corresponding alcohols ?

In case of carboxylic acids the OH bond is more polar due to the presence of carbonyl group.

The C=O group attracts more electrons towards itself which makes the -OH more polar. Due to higher polarity the magnitude of H-bonding also increase considerably in case of carboxylic acids as compared to alcohols. So the boiling point of carboxylic acids also increase.