chem 3

SIR SADIQ’S NOTES
CHAPTER NO. 3

CHEMISTRY IX

Atomic structure

betheonline.com

03463330647

PREPARED AND
COMPOSED BY

�SIR SADIQ’S NOTES
Q: Define Dalton’s atomic theory.
Ans:
DALTONS ATOMIC THEORY
Introduction:
In 1808 john Dalton an English school teacher and
chemist suggested the fundamental ideas of atomic
theory.
POSTULATES OF DALTON’S ATOMIC THEORY

1. Composition: All elements are made up of small
indivisible, indestructible particles called atoms.
2. Identicalness: All atoms of a given element identical in
all respects, having same size, mass and chemical
properties. But the atoms of one element differ from the
other elements.
3. Formation of compounds: Compounds are
formed when atoms of more than one element
combine in a simple whole no. ratio.
4. Chemical reactions: A chemical reaction is a
rearrangement of atoms, but atoms themselves are not
changed this means that atoms are neither created nor
destroyed in chemical reaction
Q: What is modern atomic theory also give drawbacks of
Dolton's atomic theory?
Ans:
MODERN ATOMIC THEORY
Dolton's atomic theory assumed that the atoms
are indivisible and no particles smaller than atoms
existed, but according to modern atomic theory,
atom is a complex organization, composed of

betheonline.com

03463330647

PREPARED AND
COMPOSED BY

�SIR SADIQ’S NOTES
even smaller particles called subatomic particles e.g. Electrons, protons
and neutrons.
Q: Explain the discovery of electron / explain discharge tube
experiment.
Ans:
DISCOVERY OF ELECTRON
INTRODUCTION:
Electron was discovered in 1897 by J.J.Thomson.

Apparatus:
The apparatus used for the discovery of electron is called discharge
tube. Which is consist of a glass tube, fitted with 2 metal electrodes
connected to a high volt. Source and a vacuum.
Figure:

PROCEDURE:
When the tube is evacuated and a current of high potential is passed
between the electrodes, streak of blueish light extending from cathode
to anode was observed and these rays are called cathode rays.

betheonline.com

03463330647

PREPARED AND
COMPOSED BY

�SIR SADIQ’S NOTES
PROPERTIES OF CATHODE RAYS
➢ NATURE:
Cathode rays are fast moving electrons
➢ PATH:
These rays travel in a straight line
➢ MATERIAL PARTICLES:
They cause the light paddle wheel to rotate
showing that they are material particles.
➢ EFFECT ON OBJECTS:
These rays produce fluorescence (glow) in some
objects.
➢ PHOTOGRAPHIC PLATE:
They can affect a photographic plate.
➢ Electric field:
These rays are deflected by electric field.
➢ MAGNETIC FIELD:
These rays are also deflected by magnetic field.
➢ X-RAYS:
These rays are used to produce x –rays
➢ KINETIC ENERGY:
They possess kinetic energy
➢ e / m RATIO:
The e / m (charge per mass) ratio of cathode
particles is 1.76 x 108 col / gm.

betheonline.com

03463330647

PREPARED AND
COMPOSED BY

�SIR SADIQ’S NOTES
➢ CHARGE:
They are deflected towards the anode showing that
they contain negative charge.
Q: Describe the discovery of protons.
Ans:
DISCOVERY OF ELECTRON
INTRODUCTION:
In 1886 a German physicist Goldstein performed an experiment to
discover proton. using discharge tube with perforated cathode.

➢
➢

➢

➢

PROPERTIES OF POSITIVE RAYS.
PATH:
These rays travel in a straight line.
CHARGE:
These rays travel towards cathode showing that they
contain positive charge.
e / m RATIO:
The e / m (charge per mass) ratio of positive particles is
much smaller than that of electron. (it varies with the
nature of gas in the tube).
Electric field:
These rays are deflected by electric field.

betheonline.com

03463330647

PREPARED AND
COMPOSED BY

�SIR SADIQ’S NOTES
➢ MAGNETIC FIELD:
These rays are also deflected by magnetic field.
Q: Explain the discovery of neutron:
Ans:
DISCOVERY OF NEUTRON
INTRODUCTION:
neutron was discovered by an English physicist James Chadwick in
1932.
PROCEDURE:
He used artificial radioactivity process:
He bombarded Be (beryllium) with particles, some neutral particles were
evolved. Due to they fact that they do not contain any charge the name
neutron was given to these particles.

Q: Define properties of following:

ATOM
➢ Smallest particle of an element which shows all properties
of element is called atom.
➢ Atom takes part in chemical reactions independently.
➢ Atom can be divided into a number of sub-atomic particles.
➢ Fundamental particles of atom are electron, proton and
neutron.

ELECTRON
➢ CHARGE:
It is a negatively charged particle.
➢ MAGNITUDE OF CHARGE:
Charge of electron is 1.6022 x 10-19 coulomb.
➢ MASS OF ELECTRON:
Mass of electron is 0.000548597 a.m.u. or 9.11 x 10 -31 kg.
➢ SYMBOL OF ELECTRON:
Electron is represented by "e".
➢ LOCATION IN THE ATOM:

betheonline.com

03463330647

PREPARED AND
COMPOSED BY

�SIR SADIQ’S NOTES
Electrons revolve around the nucleus of atom in different circular orbits.

PROTON
➢ CHARGE:
Proton is a positively charged particle.
➢ MAGNITUDE OF CHARGE:
Charge of proton is 1.6022 x 10-19 coulomb.
➢ MASS OF PROTON:
Mass of proton is 1.0072766 a.m.u. Or 1.6726 x 10-27 kg.
➢ COMPARATIVE MASS:
Proton is 1837 times heavier than an electron.
➢ POSITION IN ATOM:
Protons are present in the nucleus of atom.

NEUTRON
➢ CHARGE:
It is a neutral particle because it has no charge.
➢ MASS OF NEUTRON:
Mass of neutron is 1.0086654 a.m.u. Or 1.6749 x
10-27 kg.
➢ COMPARATIVE MASS:
Neutron is 1842 times heavier than an electron.
➢ LOCATION IN THE ATOM:
Neutrons are present in the nucleus of an atom
Q: Define radio activity:
Ans:

RADIOACTIVITY
the process by which an unstable atomic nucleus loses energy by
emitting radiation, is called radioactivity.
Or
All the elements having atomic number greater than 82 emit invisible
radiations all the time. The phenomenon of emission of these radiations
is called "Natural Radioactivity" or radioactivity and the elements that
emit such rays are called "Radio Active Elements".

betheonline.com

03463330647

PREPARED AND
COMPOSED BY

�SIR SADIQ’S NOTES
Or
The emission of invisible radiations from the nucleus of an atom is called
radioactivity and the elements that emit such radiations are called
"Radio Active Elements".
Examples Of Radioactive Elements
Uranium
Thorium
Radium
There are about 40 radioactive elements.
DISCOVERY OF RAYS
A British physicist Ernest Rutherford performed an
experiment in which he observed that radioactive
elements emits 3 types of rays.
-Rays
β-RAYS
-RAYS
EXPERIMENT
Sample of a radioactive substance was placed in a lead block between 2
opposite charged plates it was observed that radiations are resolved into
3 components.

• One component was deflected towards the negative plate showing
that it carries positive charge, these were named -Rays.
• second component was deflected towards the positive plate
showing that it carries negative charge, these were named β-Rays.

betheonline.com

03463330647

PREPARED AND
COMPOSED BY

�SIR SADIQ’S NOTES
• Third component was not deflected showing that it does not carry

any charge, these were named

-RAYS.

Q: Describe the properties of the radioactive rays.
PROPERTIES OF –RAYS
NATURE:
-rays consist of 2He4 nucleus.

CHARGE:
-particles carry positive charge.
MASS:
Mass of each -particle is 4 times that of a proton or H-atom.
IONIZATION:
Ionization power of –rays is very high.
PENETRATION POWER:
Penetration power of a - rays is very small.
FLUORESCENCE:
–rays produce fluorescence in different substances.
EFFECT ON HUMAN BODY:
–rays produce burn and source on human body.
ARTIFICIAL RADIO ACTIVITY:
–rays can produce artificial radioactivity is certain nuclei.
IONIZATION CAPABILITY:
They have strong ionizing power
(because they remove electrons
from the atoms of gas through which they pass.)
VELOCITY:
Their velocity range is 3 x 107 m/s to 3 x 106 m/s

PROPERTIES OF β-RAYS
NATURE:
β-rays consist of fast-moving electrons.

betheonline.com

03463330647

PREPARED AND
COMPOSED BY

�SIR SADIQ’S NOTES
CHARGE:
β-rays have negative charge.
VELOCITY:
Velocity of β-rays is from 9 x 107 m/sec to 27 x 107 m/sec.
EFFECT ON PHOTO GRAPHIC PLATE:
β-rays affect the photo graphic plate.
IONIZTION POWER:
Ionization power of β-rays is very small.
KINETIC ENERGY:
Kinetic energy of β-rays is less than that of a - rays.
FLUORESCENCE:
β-rays produce fluorescence in different substance.

PROPERTIES OF -RAYS
NATURE:
-rays are electromagnetic radiations.
CHARGE:
-rays are no charge.
VELOCITY:
-rays travel with the velocity of light that is 3 x 108 m/sec.
PENETRATION POWER:
Penetration power of -rays is very large. It is about hundred times
larger than that of β-rays.
FLUORESCENCE:
-rays produce feeble fluorescence When incident on screen

Q: Explain Rutherford atomic model. / explain discovery of
nucleus.
Ans:
RUTHERFORD ATOMIC MODEL
Introduction:

betheonline.com

03463330647

PREPARED AND
COMPOSED BY

�SIR SADIQ’S NOTES
In 1911 Rutherford performed an experiment in which he passed beam
of alpha particles through a very thin gold foil.

Observations:
In this experiment he observed that
1. Most of the -particles passed through it without any deflection.
2. Some of them deflected at large angles.
3. Very few of them bounced back.
nucleus
Results:
1. Major portion of the atom is empty. (extra
nucleus part)
2. The whole mass of the atom is concentrated in
the center of atom called nucleus. (nucleus)
Extra
Defects In Rutherford Atomic Model:
nucleus
• According to electromagnetic theory, a
revolving electron must emit energy due to this
the orbit of electron will decrease. And electron
will fall into the nucleus.
• If the electrons emit energy continuously, they
should form continuous spectrum. But actually,
line spectrum is obtained
Q: Define the following.
1. ATOMIC MASS
The sum of no. of protons and no. of neutrons present in an atom is
known as atomic mass.
Or

betheonline.com

03463330647

PREPARED AND
COMPOSED BY

�SIR SADIQ’S NOTES
The sum of protons and neutrons present in the nucleus of an atom is
called atomic mass.
➢ Atomic mass is denoted by “A”
➢ Its unit is “a.m.u”
➢ Its formula is “A = P+N”
e.g.:
Atomic mass of carbon © is 12 a.m.u.
Atomic mass of oxygen (O) is 16 a.m.u.

ATOMIC NO.
The no of protons present in the nucleus of an atom is called atomic no.
➢ It is denoted by “Z”
➢ It is also equal to the no. of electrons in an atom
e.g.
the carbon atom contains 6 protons/electrons that means
atomic no. of carbon © is 6
Q: Define isotopes.
Ans:
ISOTOPES
Atoms of the same element having same atomic number but different
masses number are called isotopes.
OR
Atoms of same element having same number of proton but different
number of neutrons are called
isotopes.

Example:
35
37
17Cl ,
17Cl
13
6C12,
6C

betheonline.com

03463330647

PREPARED AND
COMPOSED BY

�SIR SADIQ’S NOTES
ISOTOPES OF HYDROGEN
There are three isotopes of hydrogen.
1. Protium
2. Deuterium
3. Tritium
PROTIUM
Ordinary hydrogen is known as protium.
It has one electron, one proton but it has no neutron.
Atomic mass= 1
Atomic no. = 1
Symbol: 1H1
DEUTERIUM
Heavy hydrogen is known as deuterium.
➢
➢
➢
➢

➢
➢
➢
➢

It has one electron one proton and one neutron.
Atomic mass = 2
Atomic number = 1
Symbol: 1H2 or
TRITIUM

➢
➢
➢
➢

It has one electron, one proton and two neutrons.
Atomic mass = 3
Atomic number = 1
Symbol: 1H3 or T

betheonline.com

03463330647

PREPARED AND
COMPOSED BY