Structure of Atom - Study Material (Chemistry, Class IX) Class 9 Notes | EduRev

Class 9 : Structure of Atom - Study Material (Chemistry, Class IX) Class 9 Notes | EduRev

 Page 1


Grade-9                       CHEMISTRY - STUDY MATERIAL 
                                          CHAPTER-4  STRUCTURE OF ATOM- 
                                     
 
 
 
Discovery of Electron 
 
A discharge tube is a glass tube. It has two electrode, a source of electric current and a vacuum pump. 
 
 
(Diagram) 
 
Sir William Crooks (1895 performed experiments by passing electric current through gas in the discharge tube at 
very low pressure. He observed that at 10-4 (-4 is power to 10) atmosphere pressure, shining rays are emitted 
from cathode. These rays were named cathode rays. Cathode rays are material particles as they have mass and 
momentum. 
 
Properties of Cathode Rays 
 
The properties of these particles are given below: 
1. These particles are emitted from cathode surface and move in straight line. 
 
2. The temperature of the object rises on which they fall. 
Page 2


Grade-9                       CHEMISTRY - STUDY MATERIAL 
                                          CHAPTER-4  STRUCTURE OF ATOM- 
                                     
 
 
 
Discovery of Electron 
 
A discharge tube is a glass tube. It has two electrode, a source of electric current and a vacuum pump. 
 
 
(Diagram) 
 
Sir William Crooks (1895 performed experiments by passing electric current through gas in the discharge tube at 
very low pressure. He observed that at 10-4 (-4 is power to 10) atmosphere pressure, shining rays are emitted 
from cathode. These rays were named cathode rays. Cathode rays are material particles as they have mass and 
momentum. 
 
Properties of Cathode Rays 
 
The properties of these particles are given below: 
1. These particles are emitted from cathode surface and move in straight line. 
 
2. The temperature of the object rises on which they fall. 
 
3. They produce shadow of opaque object placed in their path. 
 
4. These particles are deflected in electric and magnetic fields. 
 
5. These particles are deflected towards positive plate of electric field. 
 
Discovery of Proton 
 
Gold Stein (1886) observed that in addition to the cathode rays, another type of rays were present in the discharge 
tube. These rays travel in a direction opposite to cathode rays. These rays were named positive rays. By using 
perforated cathode in the discharge tube the properties of these rays can be studied. Positive rays are 
also composed of metered particles. The positive rays are not emitted from anode. They are produced by the 
ionization of residual gas molecules in the discharge tube. When cathode rays strike with gas molecule, electrons 
are removed and positive particles are produced. 
 
Properties of Positive Rays 
 
1. They are deflected towards negative plate of electric field. Therefore these rays carry positive charge. 
 
2. The mass of positive rays is equal to the mass of the gas enclosed in the discharge tube. 
 
3. The minimum mass of positive particles is equal to the mass of hydrogen ion (H+). These positive ions are 
called Protons. 
 
4. The charge on proton is equal to +1.602×10^-19 Coulomb. (-19 is power of 10) 
 
 
Rutherford Experiment and Discovery of Nucleus 
 
Lord Rutherford (1911) and his coworkers performed an experiment. They bombarded a very thin, gold fail with 
Alpha particles from a radioactive source. They observed that most of the particles passed straight through the foil 
undeflected. But a few particles were deflected at different angles. One out of 4000 Alpha particles was deflected 
at an angle greater than 150. 
Alpha Rays 
 
Page 3


Grade-9                       CHEMISTRY - STUDY MATERIAL 
                                          CHAPTER-4  STRUCTURE OF ATOM- 
                                     
 
 
 
Discovery of Electron 
 
A discharge tube is a glass tube. It has two electrode, a source of electric current and a vacuum pump. 
 
 
(Diagram) 
 
Sir William Crooks (1895 performed experiments by passing electric current through gas in the discharge tube at 
very low pressure. He observed that at 10-4 (-4 is power to 10) atmosphere pressure, shining rays are emitted 
from cathode. These rays were named cathode rays. Cathode rays are material particles as they have mass and 
momentum. 
 
Properties of Cathode Rays 
 
The properties of these particles are given below: 
1. These particles are emitted from cathode surface and move in straight line. 
 
2. The temperature of the object rises on which they fall. 
 
3. They produce shadow of opaque object placed in their path. 
 
4. These particles are deflected in electric and magnetic fields. 
 
5. These particles are deflected towards positive plate of electric field. 
 
Discovery of Proton 
 
Gold Stein (1886) observed that in addition to the cathode rays, another type of rays were present in the discharge 
tube. These rays travel in a direction opposite to cathode rays. These rays were named positive rays. By using 
perforated cathode in the discharge tube the properties of these rays can be studied. Positive rays are 
also composed of metered particles. The positive rays are not emitted from anode. They are produced by the 
ionization of residual gas molecules in the discharge tube. When cathode rays strike with gas molecule, electrons 
are removed and positive particles are produced. 
 
Properties of Positive Rays 
 
1. They are deflected towards negative plate of electric field. Therefore these rays carry positive charge. 
 
2. The mass of positive rays is equal to the mass of the gas enclosed in the discharge tube. 
 
3. The minimum mass of positive particles is equal to the mass of hydrogen ion (H+). These positive ions are 
called Protons. 
 
4. The charge on proton is equal to +1.602×10^-19 Coulomb. (-19 is power of 10) 
 
 
Rutherford Experiment and Discovery of Nucleus 
 
Lord Rutherford (1911) and his coworkers performed an experiment. They bombarded a very thin, gold fail with 
Alpha particles from a radioactive source. They observed that most of the particles passed straight through the foil 
undeflected. But a few particles were deflected at different angles. One out of 4000 Alpha particles was deflected 
at an angle greater than 150. 
Alpha Rays 
 
 
1. They are helium nuclei. They are doubly positively charged, He2+. 
 
2. They move with speed equal to the 1/10th of the velocity of the light. 
 
3. They cannot pass through thick-metal foil. 
 
Following conclusions were drawn from the Rutherford’s Alpha Particles scattering experiment. 
 
1. The fact that majority of the particles went through the foil undeflected shows that most of the space occupied 
by an atom is empty. 
 
2. The deflection of a few particles over a wide angle of 150 degrees shows that these particles strike with heavy 
body having positive charge. 
 
3. The heavy positively charged central part of the atom is called nucleus. 
 
4. Nearly all of the mass of atom is concentrated in the nucleus. 
 
5. The size of the nucleus is very small as compared with the size of atom. 
 
Defects of Rutherford Model 
 
Rutherford model of an atom resembles our solar system. It has following defects: 
 
1. According to classical electromagnetic theory, electron being charged body will emit energy continuously. 
Thus the orbit of the revolving electron becomes smaller and smaller until it would fall into the nucleus 
and atomic structure would collapse. 
 
2. If revolving electron emits energy continuously then there should be a continuous spectrum but a line spectrum 
is obtained. 
 
Bohr’s Atomic Model 
 
Neil Bohr (1913) presented a model of atom which has removed the defects of Rutherford Model. This model was 
developed for hydrogen atom which has only proton in the nucleus and one electron is revolving around it. 
Page 4


Grade-9                       CHEMISTRY - STUDY MATERIAL 
                                          CHAPTER-4  STRUCTURE OF ATOM- 
                                     
 
 
 
Discovery of Electron 
 
A discharge tube is a glass tube. It has two electrode, a source of electric current and a vacuum pump. 
 
 
(Diagram) 
 
Sir William Crooks (1895 performed experiments by passing electric current through gas in the discharge tube at 
very low pressure. He observed that at 10-4 (-4 is power to 10) atmosphere pressure, shining rays are emitted 
from cathode. These rays were named cathode rays. Cathode rays are material particles as they have mass and 
momentum. 
 
Properties of Cathode Rays 
 
The properties of these particles are given below: 
1. These particles are emitted from cathode surface and move in straight line. 
 
2. The temperature of the object rises on which they fall. 
 
3. They produce shadow of opaque object placed in their path. 
 
4. These particles are deflected in electric and magnetic fields. 
 
5. These particles are deflected towards positive plate of electric field. 
 
Discovery of Proton 
 
Gold Stein (1886) observed that in addition to the cathode rays, another type of rays were present in the discharge 
tube. These rays travel in a direction opposite to cathode rays. These rays were named positive rays. By using 
perforated cathode in the discharge tube the properties of these rays can be studied. Positive rays are 
also composed of metered particles. The positive rays are not emitted from anode. They are produced by the 
ionization of residual gas molecules in the discharge tube. When cathode rays strike with gas molecule, electrons 
are removed and positive particles are produced. 
 
Properties of Positive Rays 
 
1. They are deflected towards negative plate of electric field. Therefore these rays carry positive charge. 
 
2. The mass of positive rays is equal to the mass of the gas enclosed in the discharge tube. 
 
3. The minimum mass of positive particles is equal to the mass of hydrogen ion (H+). These positive ions are 
called Protons. 
 
4. The charge on proton is equal to +1.602×10^-19 Coulomb. (-19 is power of 10) 
 
 
Rutherford Experiment and Discovery of Nucleus 
 
Lord Rutherford (1911) and his coworkers performed an experiment. They bombarded a very thin, gold fail with 
Alpha particles from a radioactive source. They observed that most of the particles passed straight through the foil 
undeflected. But a few particles were deflected at different angles. One out of 4000 Alpha particles was deflected 
at an angle greater than 150. 
Alpha Rays 
 
 
1. They are helium nuclei. They are doubly positively charged, He2+. 
 
2. They move with speed equal to the 1/10th of the velocity of the light. 
 
3. They cannot pass through thick-metal foil. 
 
Following conclusions were drawn from the Rutherford’s Alpha Particles scattering experiment. 
 
1. The fact that majority of the particles went through the foil undeflected shows that most of the space occupied 
by an atom is empty. 
 
2. The deflection of a few particles over a wide angle of 150 degrees shows that these particles strike with heavy 
body having positive charge. 
 
3. The heavy positively charged central part of the atom is called nucleus. 
 
4. Nearly all of the mass of atom is concentrated in the nucleus. 
 
5. The size of the nucleus is very small as compared with the size of atom. 
 
Defects of Rutherford Model 
 
Rutherford model of an atom resembles our solar system. It has following defects: 
 
1. According to classical electromagnetic theory, electron being charged body will emit energy continuously. 
Thus the orbit of the revolving electron becomes smaller and smaller until it would fall into the nucleus 
and atomic structure would collapse. 
 
2. If revolving electron emits energy continuously then there should be a continuous spectrum but a line spectrum 
is obtained. 
 
Bohr’s Atomic Model 
 
Neil Bohr (1913) presented a model of atom which has removed the defects of Rutherford Model. This model was 
developed for hydrogen atom which has only proton in the nucleus and one electron is revolving around it. 
 
Postulates of Bohr’s Atomic Model 
 
The main postulates of Bohr’s Model are given below: 
1. Electrons revolve around the nucleus in a fixed orbit. 
 
2. As long as electron revolves in a fixed orbit it does not emit and absorb energy. Hence energy of electron 
remains constant. 
 
3. The orbit nearest to the nucleus is the first orbit and has lowest energy. When an electron absorbs energy it 
jumps from lower energy orbit to higher energy orbit. Energy is emitted in the form of radiations, when an 
electron jumps from higher energy orbit to lower energy orbit. The unit of energy emitted in the form of radiations 
is called quantum. It explains the formation of atomic spectrum. 
 
4. The change in energy is related with the quantum of radiation by the equation : 
E2 – E1 = hv where, 
E1 = Energy of first orbit       
E2 = Energy of the second orbit       
 h = Planck’s constant 
v = Frequency of radiation 
 
Atomic Number 
 
The number of protons present in the nucleus of an atom is called atomic number or proton number.  
It is denoted by z. The proton in the nucleus of an atom is equal to number of electrons revolving around its 
nucleus. 
 
 
Mass Number  =       No of Protons  + No of neutrons  
                    A =               Z              +        N 
Isotopes 
 
The atoms of same elements which have same atomic number but different mas number are called Isotopes.  
The number of protons present in the nucleus of an atom remains the same but number of neutrons may differ. 
 
Isotopes of Different Elements 
Page 5


Grade-9                       CHEMISTRY - STUDY MATERIAL 
                                          CHAPTER-4  STRUCTURE OF ATOM- 
                                     
 
 
 
Discovery of Electron 
 
A discharge tube is a glass tube. It has two electrode, a source of electric current and a vacuum pump. 
 
 
(Diagram) 
 
Sir William Crooks (1895 performed experiments by passing electric current through gas in the discharge tube at 
very low pressure. He observed that at 10-4 (-4 is power to 10) atmosphere pressure, shining rays are emitted 
from cathode. These rays were named cathode rays. Cathode rays are material particles as they have mass and 
momentum. 
 
Properties of Cathode Rays 
 
The properties of these particles are given below: 
1. These particles are emitted from cathode surface and move in straight line. 
 
2. The temperature of the object rises on which they fall. 
 
3. They produce shadow of opaque object placed in their path. 
 
4. These particles are deflected in electric and magnetic fields. 
 
5. These particles are deflected towards positive plate of electric field. 
 
Discovery of Proton 
 
Gold Stein (1886) observed that in addition to the cathode rays, another type of rays were present in the discharge 
tube. These rays travel in a direction opposite to cathode rays. These rays were named positive rays. By using 
perforated cathode in the discharge tube the properties of these rays can be studied. Positive rays are 
also composed of metered particles. The positive rays are not emitted from anode. They are produced by the 
ionization of residual gas molecules in the discharge tube. When cathode rays strike with gas molecule, electrons 
are removed and positive particles are produced. 
 
Properties of Positive Rays 
 
1. They are deflected towards negative plate of electric field. Therefore these rays carry positive charge. 
 
2. The mass of positive rays is equal to the mass of the gas enclosed in the discharge tube. 
 
3. The minimum mass of positive particles is equal to the mass of hydrogen ion (H+). These positive ions are 
called Protons. 
 
4. The charge on proton is equal to +1.602×10^-19 Coulomb. (-19 is power of 10) 
 
 
Rutherford Experiment and Discovery of Nucleus 
 
Lord Rutherford (1911) and his coworkers performed an experiment. They bombarded a very thin, gold fail with 
Alpha particles from a radioactive source. They observed that most of the particles passed straight through the foil 
undeflected. But a few particles were deflected at different angles. One out of 4000 Alpha particles was deflected 
at an angle greater than 150. 
Alpha Rays 
 
 
1. They are helium nuclei. They are doubly positively charged, He2+. 
 
2. They move with speed equal to the 1/10th of the velocity of the light. 
 
3. They cannot pass through thick-metal foil. 
 
Following conclusions were drawn from the Rutherford’s Alpha Particles scattering experiment. 
 
1. The fact that majority of the particles went through the foil undeflected shows that most of the space occupied 
by an atom is empty. 
 
2. The deflection of a few particles over a wide angle of 150 degrees shows that these particles strike with heavy 
body having positive charge. 
 
3. The heavy positively charged central part of the atom is called nucleus. 
 
4. Nearly all of the mass of atom is concentrated in the nucleus. 
 
5. The size of the nucleus is very small as compared with the size of atom. 
 
Defects of Rutherford Model 
 
Rutherford model of an atom resembles our solar system. It has following defects: 
 
1. According to classical electromagnetic theory, electron being charged body will emit energy continuously. 
Thus the orbit of the revolving electron becomes smaller and smaller until it would fall into the nucleus 
and atomic structure would collapse. 
 
2. If revolving electron emits energy continuously then there should be a continuous spectrum but a line spectrum 
is obtained. 
 
Bohr’s Atomic Model 
 
Neil Bohr (1913) presented a model of atom which has removed the defects of Rutherford Model. This model was 
developed for hydrogen atom which has only proton in the nucleus and one electron is revolving around it. 
 
Postulates of Bohr’s Atomic Model 
 
The main postulates of Bohr’s Model are given below: 
1. Electrons revolve around the nucleus in a fixed orbit. 
 
2. As long as electron revolves in a fixed orbit it does not emit and absorb energy. Hence energy of electron 
remains constant. 
 
3. The orbit nearest to the nucleus is the first orbit and has lowest energy. When an electron absorbs energy it 
jumps from lower energy orbit to higher energy orbit. Energy is emitted in the form of radiations, when an 
electron jumps from higher energy orbit to lower energy orbit. The unit of energy emitted in the form of radiations 
is called quantum. It explains the formation of atomic spectrum. 
 
4. The change in energy is related with the quantum of radiation by the equation : 
E2 – E1 = hv where, 
E1 = Energy of first orbit       
E2 = Energy of the second orbit       
 h = Planck’s constant 
v = Frequency of radiation 
 
Atomic Number 
 
The number of protons present in the nucleus of an atom is called atomic number or proton number.  
It is denoted by z. The proton in the nucleus of an atom is equal to number of electrons revolving around its 
nucleus. 
 
 
Mass Number  =       No of Protons  + No of neutrons  
                    A =               Z              +        N 
Isotopes 
 
The atoms of same elements which have same atomic number but different mas number are called Isotopes.  
The number of protons present in the nucleus of an atom remains the same but number of neutrons may differ. 
 
Isotopes of Different Elements 
 
Isotopes of Hydrogen 
Hydrogen has three isotopes: 
1. Ordinary Hydrogen or Protium, H. 
2. Heavy Hydrogen or Deutrium, D. 
3. Radioactive Hydrogen or Tritium, T. 
 
Protium 
Ordinary naturally occurring hydrogen contains the largest percentage of protium. It is denoted by symbol H. It 
has one proton in its nucleus and one electron revolve around the nucleus. 
Number of Protons = 1 
Number of Electrons = 1 
Number of Neutrons = 0 
Atomic Number = 1 
Mass Number = 1 
Deutrium 
Deutrium is called heavy hydrogen. The percentage of deutrium in naturally occuring hydrogen is about 0.0015%. 
It has one proton and one neutron in its nucleus. It has one electron revolving around its nucleus. It is denoted by 
symbol D. 
Number of Proton = 1 
Number of Electron = 1 
Number of Neutrons = 1 
Atomic Number = 1 
Mass Number = 2 
 
Tritium 
 
Radioactive hydrogen is called tritium. It is denoted by symbol T. The number of tritium isotope is one in ten 
millions. It has one proton and 2 neutrons in its nucleus. It has one electron revolving around its nucleus. 
Number of Proton = 1 
Number of Electron = 1 
Number of Neutron = 2 
Atomic Number = 1 
Mass Number = 3 
 
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