Revision Notes for Class 11 Chemistry
Chapter 2 – Structure of Atom
1. INTRODUCTION
This chapter delves into the mysterious and surprising world of atoms on the inside. Atoms
and their structures underpin all of chemistry. We'll also look at how electrons behave and the
repercussions of their actions.
1.1 Discovery of fundamental particles
Dalton's atomic theory was successful in explaining the laws of conservation of mass, constant
composition, and multiple proportion, but it failed to explain the results of numerous tests,
such as the fact that solids like glass or ebonite create electricity when touched with silk or fur.
1.1.1 Electron discovery Of 1879, William Crookes investigated the electrical discharge in
cathode ray discharge tubes, which are partially evacuated tubes. A discharge tube is a glass
tube that is roughly 60cm long and has two thin metal bits called electrodes that are sealed
inside. Crooke's tube is the name for this type of tube. The negative electrode is referred to as
the cathode, whereas the positive electrode is referred to as the anode.
Invisible rays emanating from the cathode and producing a greenish light behind the perforated
anode on the glass wall coated with phosphorescent substance ZnS are detected when a gas is
confined at low pressure (10-4 atm) in a discharge tube is subjected to a high voltage
(10,000V).
Cathode Ray Experiment
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,Cathode rays were the name given to these beams.
1.1.2 Characteristics
1. They cast a sharp shadow on solid objects in their route, implying that they are travelling in
a straight line.
2. They are negatively charged because they are redirected towards the positive plate in an
electric field. Stoney gave them the name electrons.
3. They can make a light paddle wheel that rotates in front of them. They have kinetic energy
and are material particles, which means they have kinetic energy.
4. Their charge-to-mass ratio is 1.75882 1011C/kg.
5. They ionise the gases they pass through.
6. When they collide with a metallic target, they emit X-rays.
7. The material of the electrodes and the nature of the gas present in the cathode ray tube have
no effect on the characteristics of cathode rays (electrons).
As a result, we can conclude that electrons are the fundamental building blocks of all matter.
1.1.3 Charge to mass ratio of electron
J.J. Thomson Experiment-Charge to Mass Ratio of Electron
Class XI Chemistry www.vedantu.com 2
,J.J. Thomson used a cathode ray tube to measure the e/m ratio of electrons in 1897, employing
electric and magnetic fields that were perpendicular to each other and to the path of electrons.
In the presence of an electric or magnetic field, the extent to which electrons deviate from their
path is determined by:
(a) the charge on the electron,
(b) the mass of the particle, and
(c) the strength of the electric or magnetic field.
The electrons are deflected to point A when merely an electric field is applied. Electrons are
deflected to point C when merely a magnetic field is provided. The electrons can hit the screen
at point B by balancing the intensities of electric and magnetic fields.
1.1.4 Charge on the electron
To establish the charge on the electrons, R.A Millikan invented the oil drop experiment.
Millikan Invented the Oil Drop Experiment
Oil droplets in the form of mist were permitted to enter through a tiny hole in the upper plate
of the electrical condenser in this manner. Through a microscope with a micrometre eye piece,
the downward motion of these droplets was observed. Millikan was able to determine the mass
of oil droplets by monitoring the velocity at which they fell. By passing an X-ray beam through
Class XI Chemistry www.vedantu.com 3
, the air inside the chamber, it was ionised. Collisions with gaseous ions gave these oil droplets
their electrical charge.
Depending on the charge on the droplets and the polarity and strength of the voltage given to
the plate, the fall of these charged oil droplets can be slowed, accelerated, or made immobile.
Millikan discovered that the magnitude of electrical charge, q, on oil droplets is always an
integral multiple of the electrical charge, e, by carefully monitoring the effects of electrical
field intensity on the motion of oil droplets, q = n e, where n = 1, 2, 3…
The electron's charge is measured to be –1.6022 10–19C.
As a result, the mass of an electron was computed as
e 1.6022 10−19 C
m= = 11
= 9.1094 10−31 kg
e/m 1.758820 10 C/ kg
1.1.5 Where do cathode rays come from?
Because the gas molecules are bombarded by the high-speed electrons discharged initially
from the cathode, cathode rays are produced first from the cathode material and subsequently
from the gas inside the discharge tube.
1.2.1 Proton discovery
Because the atom is electrically neutral as a whole and the presence of negatively charged
particles has been proved, it was assumed that some positively charged particles must also be
present. As a result, during his research with cathode rays, physicist Goldstein devised a unique
discharge tube. Canal rays are a new type of ray that he found.
Canal rays get their name from the fact that they move in the opposite direction of cathode
rays in a vacuum tube, passing through and emerging from a canal or hole in the cathode.
Anode rays are another name for them.
1.2.2 Properties:
They travel in straight lines for the most part.
2. They have a positive charge on them.
Class XI Chemistry www.vedantu.com 4
Chapter 2 – Structure of Atom
1. INTRODUCTION
This chapter delves into the mysterious and surprising world of atoms on the inside. Atoms
and their structures underpin all of chemistry. We'll also look at how electrons behave and the
repercussions of their actions.
1.1 Discovery of fundamental particles
Dalton's atomic theory was successful in explaining the laws of conservation of mass, constant
composition, and multiple proportion, but it failed to explain the results of numerous tests,
such as the fact that solids like glass or ebonite create electricity when touched with silk or fur.
1.1.1 Electron discovery Of 1879, William Crookes investigated the electrical discharge in
cathode ray discharge tubes, which are partially evacuated tubes. A discharge tube is a glass
tube that is roughly 60cm long and has two thin metal bits called electrodes that are sealed
inside. Crooke's tube is the name for this type of tube. The negative electrode is referred to as
the cathode, whereas the positive electrode is referred to as the anode.
Invisible rays emanating from the cathode and producing a greenish light behind the perforated
anode on the glass wall coated with phosphorescent substance ZnS are detected when a gas is
confined at low pressure (10-4 atm) in a discharge tube is subjected to a high voltage
(10,000V).
Cathode Ray Experiment
Class XI Chemistry www.vedantu.com 1
,Cathode rays were the name given to these beams.
1.1.2 Characteristics
1. They cast a sharp shadow on solid objects in their route, implying that they are travelling in
a straight line.
2. They are negatively charged because they are redirected towards the positive plate in an
electric field. Stoney gave them the name electrons.
3. They can make a light paddle wheel that rotates in front of them. They have kinetic energy
and are material particles, which means they have kinetic energy.
4. Their charge-to-mass ratio is 1.75882 1011C/kg.
5. They ionise the gases they pass through.
6. When they collide with a metallic target, they emit X-rays.
7. The material of the electrodes and the nature of the gas present in the cathode ray tube have
no effect on the characteristics of cathode rays (electrons).
As a result, we can conclude that electrons are the fundamental building blocks of all matter.
1.1.3 Charge to mass ratio of electron
J.J. Thomson Experiment-Charge to Mass Ratio of Electron
Class XI Chemistry www.vedantu.com 2
,J.J. Thomson used a cathode ray tube to measure the e/m ratio of electrons in 1897, employing
electric and magnetic fields that were perpendicular to each other and to the path of electrons.
In the presence of an electric or magnetic field, the extent to which electrons deviate from their
path is determined by:
(a) the charge on the electron,
(b) the mass of the particle, and
(c) the strength of the electric or magnetic field.
The electrons are deflected to point A when merely an electric field is applied. Electrons are
deflected to point C when merely a magnetic field is provided. The electrons can hit the screen
at point B by balancing the intensities of electric and magnetic fields.
1.1.4 Charge on the electron
To establish the charge on the electrons, R.A Millikan invented the oil drop experiment.
Millikan Invented the Oil Drop Experiment
Oil droplets in the form of mist were permitted to enter through a tiny hole in the upper plate
of the electrical condenser in this manner. Through a microscope with a micrometre eye piece,
the downward motion of these droplets was observed. Millikan was able to determine the mass
of oil droplets by monitoring the velocity at which they fell. By passing an X-ray beam through
Class XI Chemistry www.vedantu.com 3
, the air inside the chamber, it was ionised. Collisions with gaseous ions gave these oil droplets
their electrical charge.
Depending on the charge on the droplets and the polarity and strength of the voltage given to
the plate, the fall of these charged oil droplets can be slowed, accelerated, or made immobile.
Millikan discovered that the magnitude of electrical charge, q, on oil droplets is always an
integral multiple of the electrical charge, e, by carefully monitoring the effects of electrical
field intensity on the motion of oil droplets, q = n e, where n = 1, 2, 3…
The electron's charge is measured to be –1.6022 10–19C.
As a result, the mass of an electron was computed as
e 1.6022 10−19 C
m= = 11
= 9.1094 10−31 kg
e/m 1.758820 10 C/ kg
1.1.5 Where do cathode rays come from?
Because the gas molecules are bombarded by the high-speed electrons discharged initially
from the cathode, cathode rays are produced first from the cathode material and subsequently
from the gas inside the discharge tube.
1.2.1 Proton discovery
Because the atom is electrically neutral as a whole and the presence of negatively charged
particles has been proved, it was assumed that some positively charged particles must also be
present. As a result, during his research with cathode rays, physicist Goldstein devised a unique
discharge tube. Canal rays are a new type of ray that he found.
Canal rays get their name from the fact that they move in the opposite direction of cathode
rays in a vacuum tube, passing through and emerging from a canal or hole in the cathode.
Anode rays are another name for them.
1.2.2 Properties:
They travel in straight lines for the most part.
2. They have a positive charge on them.
Class XI Chemistry www.vedantu.com 4
