Structure of the Atom Class 9 PPT

 Page 1


CHAPTER - 4
STRUCTURE OF THE ATOM
 
                              
                       
 
Page 2


CHAPTER - 4
STRUCTURE OF THE ATOM
 
                              
                       
 
Page 3


CHAPTER - 4
STRUCTURE OF THE ATOM
 
                              
                       
 
1a) Charged particles in matter :-
      Atoms have three types of sub atomic particles. They are electrons, 
protons and neutrons.  
       Electrons are negatively charged (e
-
), protons are positively 
charged (p
+
) and neutrons have no charge (n).
      The mass of an electron is 1/2000 the mass of a hydrogen atom. The 
mass of a proton is equal to the mass of a hydrogen atom and is taken 
as 1 unit. The mass of a neutron is equal to the mass of a hydrogen 
atom and is and is taken as 1 unit. 
b) Discovery of sub atomic particles :-
      In 1900, J.J.Thomson discovered the presence of the negatively 
charged particles called electrons in the atom.
      In 1886, E.Goldstein discovered new radiations in gas discharge and 
called them canal rays. These rays were positively charged. This later 
led to the discovery of the positively charged particles called protons in 
the atom.
      In 1932 Chadwick discovered the presence of particles having no 
charge in the atom called neutrons.
Page 4


CHAPTER - 4
STRUCTURE OF THE ATOM
 
                              
                       
 
1a) Charged particles in matter :-
      Atoms have three types of sub atomic particles. They are electrons, 
protons and neutrons.  
       Electrons are negatively charged (e
-
), protons are positively 
charged (p
+
) and neutrons have no charge (n).
      The mass of an electron is 1/2000 the mass of a hydrogen atom. The 
mass of a proton is equal to the mass of a hydrogen atom and is taken 
as 1 unit. The mass of a neutron is equal to the mass of a hydrogen 
atom and is and is taken as 1 unit. 
b) Discovery of sub atomic particles :-
      In 1900, J.J.Thomson discovered the presence of the negatively 
charged particles called electrons in the atom.
      In 1886, E.Goldstein discovered new radiations in gas discharge and 
called them canal rays. These rays were positively charged. This later 
led to the discovery of the positively charged particles called protons in 
the atom.
      In 1932 Chadwick discovered the presence of particles having no 
charge in the atom called neutrons.
2) Structure of an atom :-
 a) Thomson’s model of an atom :-
     According to Thomson an atom is similar to a Christmas pudding. 
The pudding had positive charge and the electrons having negative 
charge were like plums on the pudding.
    He proposed that :-
 i) An atom consists of a positively charged sphere and the electrons are 
    embedded in it.
ii) The negative and positive charges are equal in magnitude So the 
atom as a whole is electrically neutral.
 
Page 5


CHAPTER - 4
STRUCTURE OF THE ATOM
 
                              
                       
 
1a) Charged particles in matter :-
      Atoms have three types of sub atomic particles. They are electrons, 
protons and neutrons.  
       Electrons are negatively charged (e
-
), protons are positively 
charged (p
+
) and neutrons have no charge (n).
      The mass of an electron is 1/2000 the mass of a hydrogen atom. The 
mass of a proton is equal to the mass of a hydrogen atom and is taken 
as 1 unit. The mass of a neutron is equal to the mass of a hydrogen 
atom and is and is taken as 1 unit. 
b) Discovery of sub atomic particles :-
      In 1900, J.J.Thomson discovered the presence of the negatively 
charged particles called electrons in the atom.
      In 1886, E.Goldstein discovered new radiations in gas discharge and 
called them canal rays. These rays were positively charged. This later 
led to the discovery of the positively charged particles called protons in 
the atom.
      In 1932 Chadwick discovered the presence of particles having no 
charge in the atom called neutrons.
2) Structure of an atom :-
 a) Thomson’s model of an atom :-
     According to Thomson an atom is similar to a Christmas pudding. 
The pudding had positive charge and the electrons having negative 
charge were like plums on the pudding.
    He proposed that :-
 i) An atom consists of a positively charged sphere and the electrons are 
    embedded in it.
ii) The negative and positive charges are equal in magnitude So the 
atom as a whole is electrically neutral.
 
b) Rutherford’s model of an atom :-
    Rutherford’s alpha scattering experiment :-
     Rutherford allowed a beam of fast moving alpha particles ( a – 
particles) having positive charge to fall on a thin gold foil. He observed 
that :-
   i) Most of the a – particles passed straight through the gold foil.
  ii) Some of the a – particles were slightly deflected by small angles.
 iii) Very few a – particles appeared to rebound.