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Chapter 1

Matter in Our Surroundings
Matter constitutes the Observable Universe : Air, Glass, Planets, People — anything that has mass &
volume
...
Matter is made up of particles
...
Particles of matter have space
between them, due to which they can intermix with each other on their own
...
These forces are responsible to keep particles together
...

Physical properties are those that a substance shows by itself, without changing into or interacting with
another substance
...
Hence a physical
change occurs when a substance alters its physical form not its composition
...
e
...
, when ice melts several physical properties have changed, such as hardness,
density and ability to flow
...


Physical Change
 Water (liquid form)
Water (solid form) 
Chemical properties are the properties which a substance shows as it changes into or interacts with
another substance
...
g
...

A chemical change also called a chemical reaction, occurs when a substance is converted into different
substance
...
g
...
Physical and chemical properties of H2 and O2 are different from each other and
from water
...

Chemical change:
electric
Water 
Hydrogen + Oxygen
current

The Three States of Matter
Matter occurs commonly in three physical forms called States : Solid, Liquid and Gas
...
A liquid has fixed volume but takes up the shape of the container
...

Inter molecular space between the constituent units varies in the order
...

Solid > Liquid > Gas
Gases can be compressed due to large inter molecular space
...

1

Matter in Our Surroundings

Gas (disorder)

Class IX - Chemistry

Liquid (short range order)

Solid (long range order)

The Solid State
In Solid State, force of attraction dominates over the repulsive forces
...
Solids have negligible compressibility
...
e
...

In solid state :



The particle are very closely packed in some definite geometric pattern with very small voids
...




Particle motion is restricted to vibratory motion only
...
The temperature at which
it melts is known as its Melting Point
...
16 Kelvin) at
1 atm
...


b)

Pressure : On decreasing pressure, some solids can directly convert into gas
...
without undergoing liquid state
...


Amorphous Solids : Solids whose constituents do not posses the orderly arrangement over the long
range are called amorphous solids
...

They undergo irregular cleavage (cut)
...
e
...


2
...
They possess very sharp melting points
...


Classification of Solids : Depending upon the nature of interparticle forces the solids are classified into four
types :
a)

Ionic Solids
...
g
...


b)

Metallic Solids
...
g
...


2

Matter in Our Surroundings

Class IX - Chemistry

c)

Covalent Solids
...
These
solids are very hard because of strong interparticle covalent bonds e
...
, quartz, (SiO2)
...


d)

Molecular Solids
...
g
...


The Liquid State
In Liquid State, forces of attraction are almost comparable to that of repulsive forces
...
The particles of liquid move freely and have greater space between each other as
compared to particles in the solid state
...
Evaporation
occurs at all temperatures
...
In contrast to evaporation, boiling takes place only at
a definite temperature and it involves bubble formation below the surface
...

The rate of Evaporation increases with :
(a)

An increase of surface area
...


(c)

A decrease in humidity
...


ii)

Vapour Pressure : It is the pressure exerted by the vapours of the liquid in equilibrium with its liquid
at a given temperature
...
Liquids
having weak intermolecular attractive forces have a high tendency to evaporate and, therefore, have
high vapour pressure
...
For example, boiling point of water is 100ºC i
...
, 373
Kelvin
...
Liquids having weak
attractive forces have low boiling points and vice versa
...


iv)

Surface Tension : The force that acts at right angles to an imaginary line on unit length at the surface
of the liquid at rest is known as surface tension
...


The Gaseous State
In gaseous state repulsive forces are dominant over the forces of attraction
...
Gaseous particles occupy the volume
of the container
...

Depending on the temperature and pressure of the surroundings, many substances can exist in each of the 3
physical states and undergo changes in state as well
...
Similarly with decreasing temperature, water vapour condenses to liquid
water, and with further cooling, the liquid freezes to ice
...


ii)

All gases have simple molecular formula and, therefore have low molecular mass
...


iv)

All gases expand infinitely and uniformly to fill the whole, available space
...


v)

Gases are compressible, volume of a gas decreases on applying pressure on it
...


vii)

Gases exert pressure on their surroundings and in turn pressure must be exerted on them to confine
...


Physical Change of State of Matter

A physical change leads to a different form of the same substance (same composition), whereas a chemical
change leads to a different substance (different composition)
...

Sublimation of an element or substance is a conversion of the solid into gas phase with no intermediate liquid
stage
...
In these cases the transition from the solid to the gaseous state requires an intermediate liquid
state
...
Note that the pressure referred here is the vapour pressure of the
substance, not the total pressure of the entire system
...
The formation of frost is an example of meteorological
deposition
...
Naphthalene,
a common ingredient in mothballs, also sublimes slowly
...

Boiling Point
Boiling point of a liquid is the temperature at which vapour pressure of the liquid becomes equal to atmospheric
pressure
...

When atmospheric pressure is increased more heating of the liquid is required to make its vapour pressure
equal to atmospheric pressure and so boiling point of liquid is increased
...


In gaseous state
(a) forces of attraction are more than repulsive forces
(b) forces of attraction are lesser than repulsive forces
(c) forces of attraction are equal to repulsive forces
(d) none of these

2
...


Which one of the following statement is true for gases ?
(a) occupy the volume of a container (b) condenses to form solid
(c) undergo sublimation to form solid
(d) always exhibit pressure of one atmosphere

4
...


For the below interconversion of three states of matter, Y and X processes respectively are
Liquid
p
Va

Y

on

sa
en

ti
sa

X

ori

nd
Co

n
tio

Solid

Sublimation

Gas

Deposition

(a) vaporisation, solidification
(c) fusion, solidification

(b) solidification, fusion
(d) solidification, condensation

6
...


The process in which solid changes directly into gas is known as
(a) evaporation
(b) condensation
(c) deposition

(d) sublimation
5

Matter in Our Surroundings

8
...


Gases can be liquefied by
(a) decrease in temperature
(b) increase in pressure
(c) both (a) and (b)
(d) liquefication of gas do not depend on temperature and pressure

10
...


When humidity of air is low, then
(a) rate of evaporation is low
(c) rate of evaporation is high

(b) wet clothes take long time to dry
(d) the amount of vapours in air is large

12
...


The smell of food being cooked reaches us even from a considerable distance, due to
(a) Evaporation
(b) Condensation
(c) Diffusion
(d) sublimation

14
...


In which state the substance have highest compressibility
(a) Gaseous
(b) Solid
(c) Liquid

(d) Plasma

Rate of evaporation is affected by which of the following factor ?
(a) Humidity
(b) Wind speed
(c) Surface area

(d) All of these

16
...


The zig-zag movement of small particles suspended in a liquid is known as
(a) Tyndall effect

18
...
Which one of the
following properties is responsible for this observation ?
(a) Osmosis
(b) Dialysis
(c) Electrodialysis (d) Coagulation

Matter in Our Surroundings

19
...


Class IX - Chemistry

(b) Sodium chloride

(c) Camphor

(d) Ammonium chloride

Heat required to convert 1 kilogram of a solid at its melting point to liquid without any change in
temperature is called
(a) Heat capacity

(b) Specific heat capacity

(c) Latent heat of fusion

(d) Heat of vaporization

True and False
1
...


2
...


3
...


4
...


5
...


Fill in the blanks
1
...


2
...


3
...


4
...


5
...


Subjective type questions
1
...


Give the factors affecting rate of evaporation
...


What happens when solid carbon dioxide is kept under pressure lower than 1 atmosphere
...


In summer, why cotton clothes are mostly preferred ?

5
...

7

Matter in Our Surroundings

Class IX - Chemistry

ANSWERS
Multiple Choice Questions
1
...
(a)

3
...
(a)

5
...
(b)

7
...
(d)

9
...
(d)

11
...
(d)

13
...
(d)

15
...
(d)

17
...
(a)

19
...
(c)

2
...
True

4
...
True

2
...
gas, liquid

4
...
True
Fill in the blanks
1
...
melting point



8

Matter in Our Surroundings

Class IX - Chemistry

Chapter 2

Is Matter Around us Pure
The Components of Matter
On the basis of chemical constitution, matter can be mainly classified as
(i) Elements

(ii) Compounds

(iii) Mixtures

Elements
In 17th century, the great English scientist Robert Boyle gave the idea of an element which later established
experimentally by Lavoisier’s work
...
It consists of only one kind of atom
...
An element is a kind of pure substance whose composition
is fixed
...
g
...
Hence each element is unique because its atomic properties are different
...
While others
have been prepared in the laboratory
...


9

Is Matter around us Pure

Class IX - Chemistry

The clearest distinction among the elements is their classification as metals, non-metals and metalloids
...
They are generally shiny solids at room temperature that
conduct heat and electricity well and can be cooled into sheets [malleable] and wires [ductile]
...

e
...
, F2, Cl2, Metalloids have properties between those of metals and non-metals
...
g
...


Compounds
A compound is a form of matter consisting of two or more different elements that are chemically bonded
together
...
g
...

One defining feature of a compound is that the elements are present in fixed proportion by mass
...
e
...
, one molecule of carbon dioxide always contains one atom of carbon and 2 atoms
of oxygen
...
e
...
, soft, silvery sodium metal and yellow green chlorine gas have very different properties from
the compound they form white crystalline sodium chloride
...
e
...
, an electric current
breaks down molten, sodium chloride into metallic sodium and chlorine gas
...

Property

Sodium

Chlorine

Sodium Chloride

Melting point

97
...
4ºC

–34ºC

1413ºC

Colour

Silvery

Yellow-green
3

3

Colourless (white)

Density

0
...
0032 g/cm

2
...
This can be in
two general ways :
1
...

2
...
These processes
generate chemical bonds the forces that hold the atoms together in a compound
...
They can possess variable properties and can be separated into constituent components
by some suitable physical means
...

Homogeneous mixtures are mixtures which have same composition throughout, that is, it is uniform
...
g
...

Heterogeneous mixtures are that mixtures which have different composition in different parts that is nonuniform composition
...
g
...
The individual properties of the components are observable
...

Types of Mixture
i)
10

Solution

ii) Colloidal solution iii)

Suspension

Is Matter around us Pure

Class IX - Chemistry

Properties of Mixture
Property

Suspension
(Coarse dispersion)

1
...


Nature
Particle size*

3
...


Separation by
(i) Ordinary filtration
(ii)Ultra-filtration
Settling of particles

5
...


Tyndall effect

Shows

Shows

Do not show

7
...


Diffusion of particles
Brownian movement

Does not diffuse
May show

Diffuses slowly
Show

Diffuses rapidly
Negligible

Particles in colloids show brownian movement in which colloidal particle show zig-zag motion
...
The solute particles remain
suspended in the given medium
...
When the suspension is left undisturbed the solute
particles settle down at the bottom
...

Solutions
A solution is a homogeneous mixture of two or more substances
...
The substance which is dissolved is called solute, while the substance in which the
solute is dissolved is termed as solvent
...

When a substance is dissolved in water as solvent it is known as aqueous solution and in any other solvent
like alcohol etc
...


Types of Solutions
Solute
Gas
Liquid
Solid
Gas
Liquid
Solid
Gas
Liquid
Solid

Solvent
Gas
Gas
Gas
Liquid
Liquid
Liquid
Solid
Solid
Solid

Example
Air
Water in Oxygen
Iodine vapour in air
Carbonated water
Alcohol in water
Aqueous sodium chloride
Hydrogen in palladium
Mineral oil in paraffin
Gold/silver amalgam

Note : We mostly deal with solid solute in liquid solvent
...


Mass percent or percent by mass : It is defined as the mass of solute in grams per 100 g of the
solution
...
Mathematically, mass percent of a solute in solution can be expressed as,

Mass % of the solute =

Mass of solute
 100
Mass of solution

Both, the mass of solute and that of the solution must be expressed in the same mass units, viz
...

If

w = mass of solute dissolved in grams, and
W = mass of solvent in grams

2
...
For example, a 5% (v/v) solution
of ethyl alcohol contains 5 cm3 of alcohol in 100 cm3 of the solution
...
, both should
either be in cm3 or in litre (or dm3)
...

Example: Calculate the amount of NaCl present in 200 gms of its 5% (w/w) aqueous solution
...


200 gms solution contains

200  5
= 10 gms NaCl
100

Colloidal Solution
Colloidal solution is a heterogenous mixture but due to the relatively small size of particles as compared to
that of suspension the mixture appears to be homogenous
...

Types of Colloidal solution : Depending upon the phase of the Dispersed phase (solute) and Dispersion
medium (solvent), they are catagorised as :
12

Is Matter around us Pure

Class IX - Chemistry

Different Colloidal Systems
Dispersed Phase

Dispersion medium

Types

Examples

Liquid

Gas

Aerosol

Clouds, mists, fogs

Solid

Gas

Aerosol

Smoke, volcanic dust

Gas

Liquid

Foam

Liquid

Liquid

Emulsion

Solid

Liquid

Sol

Gas

Solid

Foam

Liquid

Solid

Gel

Solid

Solid

Solid Sol

Foams, whipped cream
Emulsions (milk, cod-liver oil
Starch, proteins, arsenic sulphide, gold
Adsorbed or occluded gases
Jellies, gels, cheese
Coloured precious stones, rock-salt

Colloids can be classified into 3 types on the basis of the size of colloidal particles :
1
...
Examples : Gold atoms, sulphur molecules
...


Macromolecular colloids : Particles are of bigger size
...


3
...


Properties of Colloidal System
1
...
They consist of two phases – the dispersed phase and the dispersion medium
...


Diffusibility : The colloidal particles constituting the dispersed phase do not readily diffuse through
parchment or other fine membranes
...


3
...
This is because even the finest filter paper has pores bigger than the colloidal dimensions
...


Visibility : It is not possible to see colloidal particles even with the help of a powerful microscope
...
The reason for
the invisibility of colloidal particles has already been discussed
...
But, these rays make no impression on the retina of the eye
...
The electron microscope, for instance, makes use of a beam of
cathode rays and by combination of special types of lenses, images of colloidal particles can be obtained
on photographic plates
...


Colligative Properties : The magnitude of osmotic pressure, lowering of vapour pressure, depression in
freezing point and elevation in boiling point, depend upon the number of solute particles present in a
given mass of the solvent
...
These are physical
aggregations of molecules
...
Thus, for a given mass of arsenic sulphide, the number of particles in the sol will be
only

1
th of the number present in true solution
...

6
...
However, when the same
13

Is Matter around us Pure

Class IX - Chemistry

beam of light is passed through a colloidal dispersion, it becomes visible as a bright streak
...
This phenomenon is due to the scattering of light from the surface of colloidal particles
...

e
...
The visibility of dust particles in a semi-darkened room when a sun beam enters or when a light is
thrown from a light projector
...

7
...
This kinetic activity of particles suspended
in a liquid is called Brownian movement
...
g
...


Purification of Colloidal Solutions
i)

Dialysis

ii) Ultrafiltration

iii) Ultracentrifugation

1
...


2
...


3
...


Separating the Components of a Mixture
i)

The natural substances are not chemically pure
...


Evaporation : The separation of volatile solvent can be done from nonvolatile solute
...

Filtration : Solid component can be separated from liquid component by
filtration by using filter paper, provided solid particles should not be very
small
...
For
example kerosene oil and water can be separated by separating funnel
...

Distillation : The two miscible liquid components in a given mixture with great difference in boiling point are
separated by distillation
...

14

Is Matter around us Pure

Class IX - Chemistry

Distillation
Fractional Distillation : If the two components in a mixture have boiling point difference less than 25°C,
they can be separated by fractional distillation
...
Graham classified substances into two categories on the basis of their rate of diffusion
...


Crystalloids : Substances having higher rate of diffusion
...


Colloids : Substances having slower rate of diffusion and can not diffused from permanent paper
...


*

Milk is an emulsion in which fat is dispersed in water and casein is emulsifier
...

15

Is Matter around us Pure

Class IX - Chemistry

ASSIGNMENTS
Multiple Choice Questions
1
...
2% w/v NaOH is required for the preparation of 2 litre of solution
...
2 g
(b) 2
(c) 4
(d) 0
...


Mass by mass percentage of a solution is
(b)

Mass of solute
 1000
Mass of solution

(c) Mass of solute × 100

(d)

Mass of solute
 100
Mass of solution

The correct statement is
(a) Emulsion is liquid in gas
(c) Sol is solid in solid

(b) Solid sol is liquid in solid
(d) Gel is liquid in solid

The property of Tyndall effect is shown by
(a) True solution
(b) Air

(c) Colloidal solution

(a)

3
...

5
...


Mass of solvent
 100
Mass of solute

(d) All of these

The process of making colloidal solution from the precipitate is called
(a) Coagulation
(b) Vulcanisation
(c) Peptisation

(d) Flocculation

The effective technique to purify the sol containing ionic impurities is
(a) Dialysis
(b) Electro-osmosis
(c) Electro-dialysis

(d) Electrolysis

7
...


Dispersion of AgCl in water is called
(a) Hydrophilic sol
(c) Benzosol

(b) Emulsion
(d) Hydrophobic colloid

The number of phases in colloidal system are
(a) One
(b) Two

(c) Three

9
...

11
...

13
...

15
...


Fog is an example of
(a) Solid dispersed in gas
(c) Liquid dispersed in solid

(b) Liquid dispersed in gas
(d) Solid dispersed in liquid

Vanishing cream is an example of
(a) Solid emulsion
(b) Foam

(c) Lyophilic sol

Alcosol is colloidal system
(a) of a solid dispersed in a gas
(c) in which dispersion medium is benzene

(b) of a gas dispersed in water
(d) in which dispersion medium is alcohol

(d) Suspension

Which type of property is the Brownian movement of colloidal sol
(a) Electrical
(b) Optical
(c) Mechanical

(d) Colligative

The dispersion medium in Gel is
(a) Solid
(b) Liquid

(d) Gas

(c) Water

Is Matter around us Pure

16
...
are known as
(a) Aerosols
(b) Emulsions

(c) Foam

17
...


Cheese is a colloidal system of
(a) Liquid dispersed in solid
(c) Gas dispersed in solid

(d) Gels

(b) Solid dispersed in liquid
(d) Solid dispersed in gas

19
...


Which of the following can be included in the category of colloids ?
(a) Milk

(b) Blood

(c) Latex

(d) All

Fill in the blanks
1
...


2
...


3
...


4
...


5
...


True / False
1
...


In case of gel and foam dispersed phase is solid
...


The two miscible liquids with boiling point difference less than 25ºC can’t be seperated by fractional
distillation
...


Sky appears blue because of Tyndall effect
...


Alloy is the mixture of two or more metals and can be seperated into their components by physical
methods
...


Give any three characteristic properties of metals
...


Give the two distinguishing characters between mixture and compounds
...


Define colloids and name the colloids having liquid as dispersion medium
...


Name only techniques used to seperate
i)

Napthalene and salt

ii) Two miscible liquids acetone and water
iii) Petrol and water
...


How many grams of solute ‘A’ must be present in 100 ml of water if 20 g of ‘A’ is present in 500 ml
...

17

Is Matter around us Pure

Class IX - Chemistry

ANSWERS
Multiple Choice Questions :
1
...

11
...


(c)
(c)
(b)
(b)

2
...

12
...


(d)
(c)
(a)
(a)

3
...

13
...


(d)
(d)
(d)
...


4
...

14
...


(c)
(b)
(c)
(d)

5
...

15
...


(c)
(c)
(a)
(d)

Fill in the blanks
1
...
miscible

3
...
colloidal

5
...
False

3
...
False

5
...
True



18

Chapter 3

Atoms and Molecules
Atoms and Molecules
Avogadro, an Italian scientist, distinguished between two ultimate particles of matter which takes part in a
chemical reaction (atom) and that which has independent existence (molecules)
...
It may or may not
have independent existence
...

Molecules are formed by combination of atoms of same (homo atomic) or different (hetero atomic) elements
...


Atomic and Molecular Masses
Concept of Atomic weight or atomic mass : The weight or the mass of an atom is so negligible that it is
not possible to weigh an atom in a chemical balance
...
67 × 10–24 g
...
It is this relative weight which is known as atomic weight
of an element
...

or,

1
...


weight of 1 atom of the element
The atomic weight of an element = weight of 1 atom of a hydrogen
“The atomic weight of carbon = 12” – It means that 1 atom of carbon is 12 times heavier that 1 atom
of hydrogen
...


Oxygen scale
The definition of atomic weight has now been replaced by one based on the oxygen scale where
1/16th part of the weight of an oxygen atom being taken as unit of comparison
...
0000
...

19

Atoms and Molecules

Atomic weight =

Class IX - Chemistry

1

weight of an atom of the element
weight of an atom of the element
 16
=
weight of an oxygen atom
/16 th part of the weight of an atom

‘The atomic weight of carbon is 12’ means that the atom of carbon is 12 times heavier than 1/16th
part of an oxygen atom
...


ii) Hydrogen is the lightest element, and hence the experimental errors in the determination of
atomic weight of an element are magnified in terms of hydrogen scale
...


3
...


weight of an atom of the element
Atomic weight = weight of an atom of C-12 isotope  12
According to this scale the atomic weight of hydrogen is 1
...

Atomic weight of an element does not signify the actual weight of an atom of the element
...

Reason for accepting C-12 as standard instead of oxygen
The oxygen 16
...
At
present C-12 isotope has been taken as standard instead of oxygen
...
After the discovery it was found that ordinary oxygen consists of a mixture of
three isotopes – O16, O17 and O18
...
So an attempt was made to select a single isotope of an element
which would remain constant and could be employed as standard
...


Gram atomic weight or gram atom
When the atomic weight of an element is expressed in grams, it is known as gram atomic weight or gram
atom
...

Scientifically, gram atom of an element is defined as the weight in grams that contains the same number of
atoms as 12 g of carbon-12 isotope
...
0
...
5 × 12 = 6 g of carbon
...
The actual weight of one
20

Atoms and Molecules

Class IX - Chemistry

molecule of hydrogen is 3
...
68 × 10–22 g
...
To overcome these difficulties comparative values for the
weights of molecules are used instead of their actual weights
...
Consequently the molecular
weights are only numbers and have no unit
...

Molecular weight =

ii)

Oxygen scale : The molecular weight of a substance (element or compound) is a number which
represents how many times a molecule of the substance is heavier than 1/16th part of the weight of an
oxygen atom
...

Molecular weight =

1 /12 th

weight of 1 molecule of the substance
part of the weight of an atom of C-12 isotope

Atomicity
The number of atoms in a molecule of an element is called its atomicity
...
) and metals like Li, Na, K, Cu etc
...


ii)

The molecules of some elements contain 2 atoms e
...
, (H2, O2, N2, Cl2 etc
...


iii) The ozone molecule contains 3 atoms and so its atomicity is 3
...

In general, the molecular weight is always the sum of the atomic weights of the atoms in the molecule
...


Element or compound
Hydrogen
Oxygen
Carbon di-oxide
Nitric acid

Sulphuric acid

Glucose

Molecular formula
H2 – two atoms of hydrogen
O2 – two atoms of oxygen
CO2 – 1 atom of carbon and 2
atoms of oxygen
HNO3 – 1 atom of hydrogen,
1 atom of nitrogen and 3
atoms of oxygen
H2SO4 – 2 atoms of
hydrogen, 1 atom of sulphur
and 4 atoms of oxygen
C6H12O6 – 6 atoms of carbon,
12 atoms of hydrogen and 6
atoms of oxygen

Molecular weight
2×1=2
2 × 16 = 32
12 × 1 + 2 × 16 = 44

1 × 1 + 14 × 1 + 3 × 16 = 63
2 × 1 + 1 × 32 + 4 × 16 = 98

6 × 12 + 1 × 12 + 6 × 16 = 180
21

Atoms and Molecules

Class IX - Chemistry

Gram molecule or Gram molecular weight or Gram mole
The gram-molecular weight of a substance (element or compound) is simply the molecular weight of the
substance expressed in grams
...

In other words, gram molecular weight of an element or compound is that weight in gram which contains the
same number of molecules as the number of atoms contained in 12 gram of carbon 12
...
Thus 32 g of oxygen represent
gram molecular weight of oxygen or 1 gram mole of oxygen, 64 g (2 × 32) of oxygen represent 2 gram moles
of O2 and 6 g (3 × 2) of H2 represent 3 gram moles of hydrogen
...
5 mole of H2SO4
...


Gram molecular volume (GMV)
The volume occupied by one gram molecular weight of any gas or vapour (element or compound) at a fixed
temperature and pressure is known as molar volume of that gas or vapour at that temperature and pressure
...
4 litre at
N
...
P
...
T
...
is known as the molar
volume of that gas at N
...
P
...

Thus 1 gram mole of H2 = 2 g of H2, 1 gram mole of CO2 = 44 g of CO2, 1 gram mole of NH3 = 17 g of NH3
...
T
...
) the volume of 2 g of H2 or 44 g of CO2 or 17
g of NH3 will be 22
...
Similarly 2 gram moles of any gas will occupy 2 × 22
...

Again, 22
...
T
...
and one gram mole of any
substance contains 6
...
Hence 6
...
4 litre at N
...
P
...
T
...
of some gaseous substances are given below
...
T
...


1
2
1
4
1
3
5
10

44 gram
56 ”
64 ”
136 ”
71 ”
84 ”
160 ”
20 ”

22
...
8 ”
22
...
6 ”
22
...
2 ”
112 ”
224 ”

Avogadro’s Number
According to Avogadro’s hypothesis under the same conditions of temperature and pressure equal volumes
of all gases contain the same number of molecules
...
4 litre at N
...
P
...

22

Atoms and Molecules

Class IX - Chemistry

The number of particles present in one mole of a substance (element or compound) is known as Avogadro’s
number
...
023 × 1023
...
Thus, in one gram molecule of
any substance (element or compound) the total number of molecules is 6
...
023 × 1023
...
4 litre at N
...
P
...
4 litre of any gas at N
...
P
...

According to this hypothesis under the similar conditions of temperature and pressure the number of molecules
present in 1 ml of any gas will be the same, whatever be the nature of the gas – whether oxygen or chlorine
or CO2
...


Mole Concept
1 gram mole or gram molecule represents the molecular weight of the substance expressed in grams
...
It is evident from Avogadro’s hypothesis that one mole or 1 gram
mole of any gas or vapour contains the same number of molecules
...
This
number is called Avogadro’s number which is 6
...
Again the gram molecular volume of all gases or
vapour at N
...
P
...
4 litre
...
4
litre of any gas or vapour at N
...
P
...
023 × 1023
...
It also represents the weight of the substance (element or compound) expresses in gram which
contains Avogadro number or 6
...


For example : 1 mole of CO2 are represented as follows
...
It can be calculated from the formula of the compound by using the following
relation :
% (mass) of an element =

(No
...
mass of element)×100
Molar mass of substance

For example :
The % of iron (Fe), Sulphur (S) and Oxygen (O) in ferric sulphate can be calculated as follows :
The formula of the compound is Fe2(SO4)3
...


Emperical formula : The emperical formula of a compound is the simplest formula which expresses
the simple whole number ratio of the atoms of different elements present in the molecule
...

Calculation of emperical formula :
i)

The percentage of each element present in the compound is determined by actual experiment
...
This gives the relative number of
different kinds of atoms present in the molecule
...
C
...
So as to
give a simple atomic ratio
...
If necessary, they are further reduced to whole numbers by multiplying with a suitable factor
...


2
...

It may be the same as emperical formula or an exact multiple of it
...
Where n is a simple integer
...

n = emperical formula wt
...
by its emperical formula
weight
...

Example :
Percentage composition of an organic solution as determined by analysis was, Carbon 40%, Hydrogen
6
...
33%
...
Calculate its molecular
formula
...
Wt
...
of
atoms

Simple atomic
ratio

Carbon

40

12

40/12 = 3
...
33/3
...
67

1

6
...
67

6
...
33 = 2

Oxygen

53
...
33/16 = 3
...
33/3
...
Let the molecular formula of the compound
is (CH2O)n when n is whole number
...
= 180


12n + 2n + 16n = 180

or

n=6

The molecular formula of the compound is (CH2O)6 = C6H12O6
...
These laws formed the basis of Dalton’s
atomic theory of matter
...


Law of conservation of mass : This law was stated by French chemist Antoine Laurent Lavoisier
...

Lavoisier showed that when mercuric oxide was heated it produced free mercury and oxygen
...


 Mercury
Mercuric oxide 
100 g
2
...
6 g

+

Oxygen
7
...

25

Atoms and Molecules

Class IX - Chemistry

This means in a compound, the elements are present in a fixed ratio by mass
...
g
...
always contains hydrogen and oxygen combined
together in the ratio 1 : 8 by mass
...


Law of multiple proportions : This law was discovered by John Dalton and it states that :
“When 2 elements combine with each other to form two or more than two compounds, the masses of
one of the elements which combines with fixed mass of the other, bear a simple whole no
...

e
...
, carbon and oxygen combine with each other to form CO and CO2
...

In CO2, 12 parts by mass of C combines with 32 parts by mass of O
...


Law of Reciprocal proportion : This law was proposed by Richter (1792)
...

This law can be explained by concerning the following example :
Let us consider three elements carbon, sulphur and oxygen
...
They also combine with each
other to form carbon disulphide (CS2)
...
In
sulphur dioxide (SO2), 32 parts by mass of sulphur combine with 32 parts by mass of oxygen
...
12 parts by mass of carbon combine with 64 parts by mass of sulphur
...
In other words ratio of masses of carbon and sulphur
which combine with fixed mass of oxygen is whole number multiple of the ratio in which carbon and
sulphur combine with each other
...


Symbol and Formula
Symbol
It was Berzelius (1811) who first introduced the use of modern symbol in chemistry
...
In other words, the
short-hand representation of an atom of an element is known as symbol
...

i)
26

By using the initial letter of the English name of an element in capital
...
g
...


Atoms and Molecules

ii)

Class IX - Chemistry

When the names of two or more elements begin with the same letter they are symbolised by using the
first two letters or by using the initial and another which is prominently heard in pronouncing the word
...
e
...
, Calcium = Ca, Chromium = Cr, Bromine = Br, Barium = Ba,
Argon = Ar, Arsenic = As etc
...
g
...

Significance :A symbol has both qualitative and quantitative significance
...


ii)

It denotes an atom of the element
...
Similarly 2H and 4H
denotes two and four atoms of hydrogen respectively
...
Thus, C
represents 1 atom of carbon as well as 12 parts by weight of carbon
...

iv) It denotes one gram atom of an element
...
In other words, the short representation
of one molecule of an element or compound is known as formula
...


ii)

It denotes one molecule of a substance (element or compound) and indicates the number of atoms of the
constituent elements in the molecule
...

iv) It represents the relative weights of the element present in the molecular weight of the compound
...
This formula denotes that in sulphuric acid 2 parts by weight
of hydrogen, 32 parts by weight of sulphur and 64 parts by weight of oxygen are present per 98 parts by
weight of H2SO4
...
T
...
e
...
, one
gram molecule of any gas occupies 22
...
T
...
Thus, CO2 indicates 22
...
T
...
3
CO2 denotes 3 × 22
...
T
...


Valency
A chemical compound is formed by the union of two or more elements in a definite proportion by weight
...
It is observed that one atom of an element combines with
a definite number of atoms of other elements to form a stable compound
...

27

Atoms and Molecules

Class IX - Chemistry

Analysis of a number of hydrogen compounds shows that except hydrazoic acid, no compounds of hydrogen
are known where more than one atom of an element combines with one atom of hydrogen
...
Consequently hydrogen atom has been
chosen as the standard for measuring the valencies of other elements, i
...
, the valency of hydrogen atom is
taken as 1
...

Thus, 1 atom of bromine, 1 atom of oxygen, 1 atom of nitrogen and 1 atom of carbon combines with 1, 2, 3
and 4 atoms of hydrogen to form HBr, H2O, NH3 and CH4 , respectively
...


Compounds of
hydrogen
Hydrogen bromide
Water
Ammonia
Methane

Formula
HBr
H2O
NH3
CH4

No
...

The valency of such elements is equal to the number of hydrogen atoms displaced by 1 atom of the element
...

Mg + 2HCl  MgCl2 + 2H
1 atom of aluminium displaces 3 atoms of hydrogen from HCl
...

There are 3 types of radicals
...
g
...

b) Positively charged radical : There are some radicals which are positively charged
...
g
...

During electrolysis, such type of radicals are discharged at the cathode
...
g
...
Such type of radicals sometimes display variable valency e
...
, [Fe(CN)6]
...
In the former case it is tetravalent and in the latter it is
trivalent
...
There are some elements which have more than one valency
...
Strictly speaking the number of
unpaired electrons in the ground state of an atom indicates its valency but in the excited state if the paired
electrons become unpaired and promoted to higher energy level giving rise to different number of unpaired
electrons, the element exhibits more than one valency
...
For transition elements e
...
, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu
etc
...
Normally, the suffix ‘ous’ is used
to denote an element in lower valency and ‘ic’ for higher valency
...
Similarly Iron is bivalent (Ferrous)
and trivalent (Ferric) in Fe2O3, FeCl3, Fe2(SO4)3 etc
...

Variable valencies of some other elements are tabulated below :Element
M ercury

Valency
1
Hg 2 O

Lead

2
HgO
PbCl 2
PbO

4

SnCl 2
HCl

5

6

PbCl 4
PbO 2
PCl 3
PF 3

Phosphorus
Tin
Chlorine

3

HClO 2

PCl 5
PF 5
SnCl 4
ClO 2

KClO 3

KClO 4

The valency of a radical
The valency of a radical can be evaluated by finding the number of hydrogen or chlorine atom with which the
radical in question combines e
...
, (i) NO3– radical unites with 1 atom of H to form HNO3
...
(ii) Again NH4+ radical combines with 1 atom of Cl to form NH4Cl
...
(iii) Each of SO4 2– and CO32 – radicals combines with 2 atoms of hydrogen
...
(iv) Similarly, PO43– and BO33– form H3PO4 (orthophosphoric acid) and H3BO3
(orthoboric acid) respectively
...
(v) Each of the CrO4– – (Chromate)
and Cr2O7– – (Dichromate) radicals unites with 2 atoms of K, a monovalent metal, to form K2CrO4 and
K2Cr2O7 respectively
...


Formula of Compounds
Classification of elements and radicals in terms of valency has made the study of chemistry much easier
...
An ionic
compound consists of two parts, a positive part and a negative part
...

Hence the total positive charge contained in the metallic part or basic radical must be equal to the negative
charge contained in the non-metallic part or acid radical
...
Let their valencies be x and y respectively
...
e
...
The same rule
is applied when two radicals or one element and one radical combine together
...
This method has been illustrated below in finding out the
formula of aluminium sulphate
...


The above procedure will be more clarified in the following table :-

Compound
Sodium
chloride

Na

Calcium
nitride

Ca

Ferric
sulphide

Fe

Cl

N
S

Formula of the
compound

1
1

NaCl

2
3

Ca3N2

3
2

Fe2S3

Cuprous
sulphate

Cu2SO4

Potassium
carbonate

K2CO3

Ammonium
sulphate

NH4

Phosphorous
pentoxide

P

5

O

2

Sodium
metaborate

30

Valencies of the atom and radicals
in the compound

1
2

SO4

Na
BO2

Aluminium
oxide

Al

Bismuth
sulphate

Bi

O
SO 4

(NH4)2SO4
P2O5

1
1

NaBO2

3
2

Al2O3

3
2

Bi2(SO4)3

Atoms and Molecules

Class IX - Chemistry

Chemical equations
A chemical reaction results either from the decomposition of a compound or by the union of atoms or
molecules, i
...
, by the rearrangement of the compound leading to new products whose properties are different
from those of the reacting species
...
The chemical
species which participate in the chemical change are called Reactants and the new compounds formed
after the change are known as Products
...

A chemical equation is a shorthand representation of a chemical reaction expressing its qualitative and
quantitative aspects with the aid of symbols and formulae or a chemical equation is a symbolic expression of
a chemical transformation
...
Since the atoms of elements cannot
exist in the free state, they are to be written in their molecular formula
...
g
...
However
monoatomic molecules are expressed by their symbols, e
...
, Cu, Na, Mg etc
...
g
...


ii)

The formula of the reactants are written on the left hand side and the products on the right hand side with
an equal to sign (=) in between them
...

Zn + H2SO4  ZnSO4 + H2
iv) In case of reversible reaction, the symbol  is used instead of equal to sing (=)
...
Hence the total number of atoms of the left
hand side
...

vi) If necessary, the equation is balanced by placing smallest simple integral numbers before the formulae in
such a way so that the total number of atoms on the left side is equal to the total number of atoms on the
right side
...


Significance of a Chemical Equation
A chemical equation has both qualitative and quantitative significances
...


ii)

Quantitatively it denotes the number of atoms and molecules of reactants and products
...

iv) In case of a gaseous reaction the equation signifies the volumes of the reactants and the products if
gaseous
...

Examples :
2H2 + O2  2H2O
The above equation conveys the following informations :31

Atoms and Molecules

Class IX - Chemistry

i)

Hydrogen combines with oxygen to form water
...


iii) Thus total number of atoms of the reactants (4 + 2 = 6) and that of the products (2 × 2 + 2 = 6) are the
same
...

v) 2 gram moles of hydrogen combine with 1 gm mole of oxygen to produce 2 gm moles of water
...

vii) At N
...
P
...
4 litres of H2 combines with 22
...
4 litres of steam
...
Whenever one or more than one substance undergo chemical
reaction, the structure of their molecules undergo transformations leading to new molecules of different
orientations
...

Chemical reactions are believed to take place by the collisions among the reacting molecules
...
But all molecular collisions are not effective for the chemical
change
...
Hence in order to have an effective collision leading to chemical transformation, the colliding
molecules must not only come closer together but also their orientations must be proper at the time of
collision
...
Mechanism of a
chemical reaction depends not only on the orientation of the molecules but also on (a) Concentration of
reactants, (b) Temperature, (c) Presence of a catalyst, (d) Pressure of the reaction
...


Synthesis : When a chemical compound is formed by the direct combination of its constituents the
resulting reaction is called synthesis
...

2Mg + O2  2MgO
...


N2 + 3H2 ƒ 2NH3,

2H2 + O2 = 2H2O, N2 + O2 = 2NO

Decomposition : Decomposition or Analysis is the process of disruption of the molecule of a substance
into one or more new substances having new properties
...
Thus when water is electrolysed the molecule of water is decomposed to form
hydrogen and oxygen
...


Substitution : It involves the replacement of one or more atoms of a molecule of a compound by atoms
of more active element
...

Fe + CuSO4  FeSO4 + Cu

32

Atoms and Molecules

4
...
Thus when
a solution of silver nitrate is treated with a solution of potassium iodide a precipitate of silver iodide and
potassium nitrate solution are formed
...


Reversible reaction : A reaction is said to be reversible when the rate of the forward and backward
reaction is the same, i
...
, when the products, as soon as are formed, react to reform the reactants
...

N2 + 3H2 ƒ

6
...
As a result of this the
products are usually acidic or alkaline
...

PCl5 + 4H2O  H3PO4 + 5HCl

7
...


ASSIGNMENTS
Multiple Choice Questions
1
...
7 g of sodium chloride is
(a) 2
...
6 g
(c) 6
...
1 g

2
...
8% zinc and 52
...
The empirical formula is Znx Cly
where x and y can have the values
(a) 1 and 1
(b) 1 and 2 respectively
(c) 2 and 1 respectively
(d) 2 and 3 respectively

3
...
204 × 1024

(c) 3
...


What is the total number of atoms present in 25
...
89 × 1019
(b) 2
...
02 × 1020
(d) 2
...


Volume at STP of 0
...
01 g of hydrogen
(b) 0
...


One mole of CO2 contains
(a) 6
...
1 × 1023 molecules of CO2

7
...
02 × 1023 atoms of O
(d) 3 g atoms of CO2

The number of moles of H2 in 0
...
1
(c) 0
...
001
33

Atoms and Molecules

8
...
02
The number of atoms in 4
...
0 × 1023

9
...
02 × 1023

(c)

(d)

16
 1023
3
...


Which of the following contains maximum number of atoms ?
(a) 6
...
4 L of CO2 at STP
(c) 0
...


7
...
6 litres of volume at STP
...


Number of atoms in 558
...
85 g mol–1) is
(a) twice that in 60 g carbon
(b)
6
...
5 × 6
...


Valency of a phosphate ion is
(a) – 1
(b) – 2

14
...


Which of the following is a displacement or substitution reaction?
(a) NaOH + HNO3  NaNO3 + H2O
(b) Cu + 2AgNO3  Cu(NO3)2 + 2Ag
(c) 2Hg + O2  2HgO
(d) FeCl3 + 3NaOH  3NaCl + Fe(OH)3

16
...
When the reaction was complete
1120 cm3 of CO2 was obtained at S
...
P
...


The moles of O2 required for reacting with 6
...
NH3 +
...
NO +
...
5
(c) 1

(d) 0
...


20 cc of CO2 are passed over red hot coke
...


What quantity of lime stone (CaCO3) on heating will give 56 kg of CaO ?
(a) 1000 kg
(b) 56 kg
(c) 44 kg
(d) 100 kg

20
...
5 mol of BaCl2 is mixed with 0
...
7
(b) 0
...
30
(d) 0
...


N2 + 3H2  2NH3 is _____________ type of reaction
...


The simplest formula which expresses the simple whole number ratio of the atoms is called
_____________ of the compound
...


11
...
T
...
contains ___________________ oxygen atoms
...


Equal volumes of different gases under similar conditions of temperature and pressure contain equal
number of ___________________
...


The number of hydrogen atoms in 80 a
...
u
...


True / False
1
...


2
...
T
...
occupy 22
...


3
...
0 g of oxygen contains 6
...


4
...


5
...


Subjective Type Questions
1
...

b) Give the valency of
i) S in Na2SO4,
ii) Cl in KClO3,
iii) Cr in K2Cr2O7,
iv) I in ICl3,
v) P in H3PO4

2
...
Write the formula of their oxides and chlorides

3
...


H2 + I2

How much sulphuric acid is required to dissolve completely 3g of MgCO3 ?
What is the weight of CO2 liberated ?
MgCO3 + H2SO4 
 MgSO4 + CO2 + H2O
How much vol
...


How much potassium nitrate must be heated to yield as much oxygen as would be obtained from 35
gm of KClO3 ?
2KClO3 
 2KCl + 3O2
2KNO3 
 2KNO2 + O2

ANSWERS
Multiple Choice Questions :
1
...

11
...


(b)
(a)
(a)
(b)

2
...

12
...


(b)
(c)
(a)
...

8
...

18
...

9
...

19
...

10
...

20
...

(b)
...
synthesis

2
...
6
...
molecules 5
...
True

2
...
True



36

4
...
True

Chapter 4

Structure of Atom
Introduction
In 1808, Sir John Dalton, first started the ball of atomic theory to roll on a scientific path, by putting forward
his atomic theory
...

In 1833, Faraday’s laws of electrolysis proved conclusively that matter was electrical in nature and electricity
contains some discrete particles
...
New discoveries regarding the atom from the middle of nineteenth century led to conclusion that
atoms consist of three fundamental particles electrons, protons and neutrons and more than 30 sub fundamental
particle like neutrino, antineutriano and meson etc
...


Stable particles : (a) Electron, (b) Proton and (c) Neutron
...


Unstable particles : (a) Positron, (b) Meson, (c) Neutrino and anti-neutrino and (d) Anti-proton
...


Composite : (a) Deuteron, (b) Alpha particle etc
...


Cathode ray – Discovery of electron :
Around 1879, Willian Crookes observed that when an electric discharge from high potential source is
passed through a gas at 0
...
This glow or fluorescent is formed due to the
bombardment on the glass by certain rays which are emitted from the cathode-surface and move towards
the anode with tremendous velocity
...
Irrespective of the nature of
the cathode or residual gas inside the discharge tube, the following properties of cathode rays are recorded
...


ii)

They are deflected from their original straight line path by the application of both electric and
magnetic fields and the direction of deflection indicates that they consist of negatively charged
particles
...


iv)

They produce a mechanical motion on small and very light paddle wheel when it is placed in the
path of cathode rays
...

37

Structure of Atom

Class IX-Chemistr y

v)

These rays affect a photographic plate which is used to detect and measure the cathode rays
...


vii)

They emit fluorescent radiation when they are allowed to fall on glass wall coated with zinc
sulphide or barium platinocyanide
...


The properties of cathode rays and their behaviour in electric and magnetic fields, prove that cathode
rays are made up of fast moving tiny material particles charged with negative electricity
...

a)

Charge of electron : R
...
Milliken in 1913 determined the charge of an electron by his classical oil
drop experiment and found the value to be 1
...
8 × 10–10 e
...
u
...
This is the minimum value
of negative charge
...
That is why the numerical magnitude of electronic charge represents the fundamental
unit of electricity
...
109 × 10–28 g
...
008 g
...
008
= 1
...

6
...
109  10 –28
1



–24
Mass of 1 atom of hydrogen 1
...
In other words the mass
1837
of one hydrogen atom is 1837 times greater than the mass of one electron
...
e
...


Positive ray – Discovery of protons :
An atom is electrically neutral and since it contains negatively charged electrons, it must also contain
equal number of positively charged particles
...

While carrying out experiments on the conduction of electricity through gases at low pressure in a
discharge tube, Eugene Goldstein in 1866 used a perforated metal disc as a cathode
...
The rays
were coming from the anode surface in straight lines opposite to that of the cathode rays
...
In 1907 these
rays were called positive rays by Sir J
...
Thomson
...
These experiments go to prove that positive rays are composed of
stream of very small positively charged particles
...
It is found to be 1836 times heavier
than an electron
...
6725 × 10–24 gms
...


iii)

3
...
Hence, the charge on a proton is 4
...
s
...
or
1
...
The charge carried by a proton has been accepted as the unit of positive
electricity as no positively charged particle is known which has charge less than that of a proton
...
Chadwick was successful in discovering a new fundamental particle by bombarding beryllium with
fast moving alpha particles
...
This particle was identified as neutron, the existence
of which was predicted earlier by Rutherford
...
The bombardment of the nuclei of light metals (Be, B etc
...


ii)

Except hydrogen atom, all atoms contain neutron, hence neutron is also a fundamental particle of
matter
...
00893 a
...
u
...
675 × 10–24 g
...
Rutherford (1911) which was based upon his
experiment of scattering of alpha particles by thin
metal foil
...
A
narrow beam of alpha particles escaped through a
small hole in the lead box and hit a thin metal foil
of gold which was placed in the beam of the alpha
particles
...
A flash of light was observed when the
alpha particles hit the screen after penetrating
through the thin foil
...
Very few -particles suffered minor
deflection
...

Some -particles have their path reversed showing that they were deflected through 180º
...

39

Structure of Atom

Class IX-Chemistr y

Conclusions from the experiment
i)

Since most of the -particles pass through the metal foil without any deflection, the major part of an atom
is empty
...
Hence it has high kinetic energy
...
Consequently the alpha
particles can not be deflected even through small angles by the collision with electrons
...

Hence, Rutherford concluded that the mass of the atom is concentrated to a small region which contains
the whole positive charge of the atom – this is the nucleus of the atom
...

iv) The electrons reside outside the nucleus in different shells
...
This proves that the
major part of the atom is empty
...

Rutherford’s nuclear model
Based on the above classic experiment, Rutherford proposed a nuclear model of atom
...

i)

Nucleus – Centre of an atom :
a) All atoms contain a minute but massive positively charged body at its centre called nucleus
...

c) It is to be noted that when the nuclear model of Rutherford was proposed, neutron was not
discovered
...
Neutron is also present inside the nucleus and these two particles,
protons and neutrons are called nucleons
...
So Rutherford’s model is
analogous to solar system
...


Defects of Rutherford’s model of atom
a) Rutherford’s atomic model is extremely unstable because the electrons will gradually loose energy and
the radius of curvature of its path will steadily decrease, its orbit will become smaller and smaller i
...
, the
electron will follow a spiral path and finally fall into the nucleus
...

ii)

Bohr’s atomic model
In order to overcome the difficulties encountered in Rutherford’s atomic model Bohr made the
following revolutionary postulates :
a) Stationary state : The electrons always move in certain selected orbits having particular energies
...
These orbits or paths are called as stationary
states
...

mvr = nh/2, where n = 1, 2, 3
...

c) The third postulate of Bohr is that if energy is available an electron will absorb energy in quanta
and move to the higher energy level
...

From Bohr’s atomic model we can calculate the energy, radius, velocity of electron moving in
an orbital around the nuclei
...
e
...

Normally the term quantum number is used to indicate the various energy levels or orbits of an atom
...
Each electron in an atom
is confined to its own orbital
...

1
...


2
...


3
...


4
...


Types of Subshells and Orbitals
In a given shell the electrons are present in subshells s, p, d, f
...
Similarly, d-subshell is with five orbitals dxy, dyz, dxz, d x2 – y2 , d z 2
...
e
...

41

Structure of Atom

Class IX-Chemistr y

Filling of Electrons in Different Orbitals
1
...
The sequence of
filling up of the electrons in different available orbitals is determined by the (n + l)
...
Pauli’s Exclusion Principle
Pauli’s exclusion principle which states that, no two electrons in an atom can have the four identical
quantum numbers
...
Hund’s Rule of Maximum Multiplicity Rule :
This indicates the distribution of electrons in an orbital
...


Isotopes, Isobars, and Isotones
All the atoms of a particular element have same number of protons in their nuclei, however the number of
neutrons may be different
...
Thus, isotopes of an element are the atoms of the element with same
atomic number but different mass numbers
...
All the three isotopes have atomic number 1
...
The isotopes of other elements do not have special names, they are indicated by
giving mass number value on the symbol
...
The atoms of
different elements which have same mass number are called isobars
...


14
6

C and

14
7

N are

Sometimes atoms of different elements contain same number of neutrons
...

Thus, isotones may be defined as the atoms of different elements containing same number of neutrons
...
For isotones, the difference of mass number and atomic number is same
...


Calculation of Number of Electrons, Protons and Neutrons
From the knowledge of atomic number and mass number of an element it is possible to calculate number of
electrons, protons and neutrons in an atom of the element
...
Number of electrons, protons and neutrons in an atom of it can be
calculated as under :
Number of protons = Atomic number = 13
Number of electrons = Atomic number = 13
Number of neutrons = Mass number – Atomic number
= 27 – 13 = 14
...

2
...


Rutherford’s scattering experiment is related to
(a) neutrons
(b) atom
(c) electron

(d) nucleus

Size of the nucleus is of the order
(a) 10–12 m
(b) 10–8 m

(d) 10–10 m

(c) 10–15 m

Name of scientist associated with the discovery of Neutron is
(a) J
...
Thomson
(b) James Chadwick (c) Rutherford

(d) Yukawa

4
...
This
is called
(a) Hund’s rule
(b) Aufbau principle
(c) Uncertainty principle
(d) Pauli’s exclusion principle

5
...


Cathode rays have
(a) mass only
(c) no mass, no charge

(b) charge only
(d) mass and charge both

Electron was discovered by
(a) Rutherford
(b) J
...
Thomson

(c) Chadwick

7
...


Atomic number of an element is equal to the number of
(a) electrons
(b) protons
(c) neutrons
(d) electrons or protons

9
...


Bohr’s orbits are called stationary states because
(a) electrons in them are stationary
(b) their orbits have fixed radii
(c) the electrons in them have fixed energy
(d) the protons remain in the nuclei and are stationary

11
...


The idea that led to the discovery of neutron was that
(a) no
...
of electrons were less than they should be
(c) mass of the atom was less than they should be
(d) it was a chance discovery

13
...


Rutherford’s scattering experiments led to the discovery of
(a) nucleus
(b) presence of neutrons in the nucleus
(c) both (a) and (b)
(d) revolving nature of electrons around the nucleus

15
...


The total number of neutrons in dipositive zine ion with mass number 70 is
(a) 34
(b) 40
(c) 36
(d) 38

17
...


19
...


44

77
22

Ge

(b)

77
33

As

(c)

77
34

Se

(d)

81
36

Kr

The quantum number which specifies the location as well as energy is
(a) principal quantum number
(b) azimuthal quantum number
(c) spin quantum number
(d) magnetic quantum number

Fill in the blanks
1
...


2
...


3
...


4
...


5
...


True / False
1
...


2
...


3
...
(n)
...


Rutherford’s experiment showed the presence of nucleus
...


Two electrons in the same orbital can have parallel spins
...


Each atom has a positively charged nucleus
...


Atoms are electrically neutral though they contain electrically charged particles
...


Rutherford’s model of atom is unstable
...


Neutrons do not influence the chemical properties of an atom
...


Not more than two electrons can be accommodated in the same orbital
...

(d)
6
...

(a)
16
...

7
...

17
...

(d)

1
...
Twice

3
...

13
...


(b)
(B)
(c)
(c)

2
...
Azimuthal

4
...

14
...


(d)
(a)
(a)
(b)

5
...

15
...


(c)
...

(b)
(a)

3
...
False

2
...
True

4
...
False


45

Contents
Chapter 1

Matter in Our Surroundings
...
09

Chapter 3

Atoms and Molecules
...
37



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