STRUCTURE OF ATOM
THEORY
1. INTRODUCTION 3. They can make a light paddle wheel to rotate placed in their
path. This means they possess kinetic energy and are
In this chapter, we explore the inside world of atoms which
material particles.
is full of mystery and surprises. Whole chemistry is based
on atoms and their structures. We will also study the 4. They have a charge to mass ratio = 1.75882 × 10 11C/kg
behaviour exhibited by the electrons and their 5. They ionise gases through which they travel.
consequences. 6. They produce X-rays when they strike a metallic target.
7. The characterstics of cathode rays (electrons) do not
1.1 Discovery of fundamental particles
depend on the material of electrodes and nature of the gas
Dalton’s atomic theory was able to explain the law of present in the cathode ray tube.
conservation of mass, law of constant composition and law Thus, we can conclude that electrons are basic constituents
of multiple proportion very successfully but it failed to of all matter.
explain the results of many experiments like it was known 1.1.3 Charge to mass ratio of electron
that substances like glass or ebonite when rubbed with silk +
or fur generate electricity 1.1.1 Discovery of electron
Cathode Anode A
William Crookes in 1879 studied the electrical discharge
in partially evacuated tubes known as cathode ray N
discharge tubes. B
A discharge tube is made of glass, about 60cm long S
containing two thin pieces of metals called electrodes, C
sealed in it. This is known as crooke’s tube. The negative
electrode is known as cathode and positive electrode is Fluorescent screen
known anode. –
Magnet
When a gas enclosed at low pressure( 10-4 atm) in
discharge tube is subjected to a high voltage ( 10,000V), In 1897 J.J. Thomson measured e/m ratio of electron by
invisible rays originating from the cathode and producing using cathode ray tube and applying electric and magnetic
a greenish glow behind the perforated anode on the glass field perpendicular to each other as well as to the path of
wall coated with phosphorescent material ZnS is observed. electrons. The extent of deviation of electrons from their
These rays were called cathode rays. path in the presence of electric and magnetic field depends
on:
(a) Charge on the electron
(b) Mass of the particle
(c) The strength of electric or magnetic field
When only electric field is applied, the electrons are
deflected to the point A. When only magnetic field is
applied the electrons are deflected to the point C. By
balancing the strengths of electric or magnetic fields, the
electrons are allowed to hit the screen at point B i.e. the
1.1.2 Properties point where electrons hit in the absence of electric and
1. They produce sharp shadow of the solid object in their path magnetic field. By measuring the amount of deflections
suggesting that they travel in straight line. Thomson was able to calculate the value of e/m as
2. They are deflected towards the positive plate in an electric 1.758820 × 1011C/kg.
field suggesting that they are negatively charged. They points were recorded which were emerging from -
were named as electrons by Stoney. particles. Polonium was used as the source of -particles.
THEORY
1. INTRODUCTION 3. They can make a light paddle wheel to rotate placed in their
path. This means they possess kinetic energy and are
In this chapter, we explore the inside world of atoms which
material particles.
is full of mystery and surprises. Whole chemistry is based
on atoms and their structures. We will also study the 4. They have a charge to mass ratio = 1.75882 × 10 11C/kg
behaviour exhibited by the electrons and their 5. They ionise gases through which they travel.
consequences. 6. They produce X-rays when they strike a metallic target.
7. The characterstics of cathode rays (electrons) do not
1.1 Discovery of fundamental particles
depend on the material of electrodes and nature of the gas
Dalton’s atomic theory was able to explain the law of present in the cathode ray tube.
conservation of mass, law of constant composition and law Thus, we can conclude that electrons are basic constituents
of multiple proportion very successfully but it failed to of all matter.
explain the results of many experiments like it was known 1.1.3 Charge to mass ratio of electron
that substances like glass or ebonite when rubbed with silk +
or fur generate electricity 1.1.1 Discovery of electron
Cathode Anode A
William Crookes in 1879 studied the electrical discharge
in partially evacuated tubes known as cathode ray N
discharge tubes. B
A discharge tube is made of glass, about 60cm long S
containing two thin pieces of metals called electrodes, C
sealed in it. This is known as crooke’s tube. The negative
electrode is known as cathode and positive electrode is Fluorescent screen
known anode. –
Magnet
When a gas enclosed at low pressure( 10-4 atm) in
discharge tube is subjected to a high voltage ( 10,000V), In 1897 J.J. Thomson measured e/m ratio of electron by
invisible rays originating from the cathode and producing using cathode ray tube and applying electric and magnetic
a greenish glow behind the perforated anode on the glass field perpendicular to each other as well as to the path of
wall coated with phosphorescent material ZnS is observed. electrons. The extent of deviation of electrons from their
These rays were called cathode rays. path in the presence of electric and magnetic field depends
on:
(a) Charge on the electron
(b) Mass of the particle
(c) The strength of electric or magnetic field
When only electric field is applied, the electrons are
deflected to the point A. When only magnetic field is
applied the electrons are deflected to the point C. By
balancing the strengths of electric or magnetic fields, the
electrons are allowed to hit the screen at point B i.e. the
1.1.2 Properties point where electrons hit in the absence of electric and
1. They produce sharp shadow of the solid object in their path magnetic field. By measuring the amount of deflections
suggesting that they travel in straight line. Thomson was able to calculate the value of e/m as
2. They are deflected towards the positive plate in an electric 1.758820 × 1011C/kg.
field suggesting that they are negatively charged. They points were recorded which were emerging from -
were named as electrons by Stoney. particles. Polonium was used as the source of -particles.
