According to John Dalton : All matter was composed of small  particle called atom.  
    Atom is a Greek word and its meaning Indivisible i.e. an ultimate particles which cannot be further subdivided.

     According to Dalton's theory -
(1)   Atom is the smallest indivisible part of matter which takes part in chemical reaction.
(2)  Atom is neither be created nor be destroyed.
(3)  Atoms of the same element are similar in size, mass and characteristics ; however, atoms of different elements have different size, mass and characteristics.
l    An atom is made up of three subatomic particle electron, proton & neutrons. These three particles are called fundamental particles of matter. 

 DO YOU KNOW :  Mass of Proton is  2000 times more than electron 

    There are two parts of atom :
    (i) Nucleus                     (ii) Outer part

 Nucleus : The size of nucleus of atom is very small in which neutrons and protons are present so the almost entire mass of the atom is situated in nucleus.

 Protons & neutrons present in the nucleus are collectively termed as nucleus. Number of neutrons is called mass number element. Size of the nucleus
  in the atom is 10–15m. 
   Mass number (A) = Number of protons in the nucleus (p) + Number of Neutrons (n).
  Outer Part : In the outerpart, electrons move around the nucleus in fixed orbits. These orbits are called energy levels.


   (Discovery of e–) : 
    In 1859 "Julius Plucker" started the study of conduction of electricity through gases at low pressure in a discharge tube

  . When a high voltage of 10,000 volts or more was impressed across the electrodes, some sort of inversible rays moved from

    the negative electrodes to the positive electrode. Since the negative electrode is referred to as cathode. These rays were called cathode rays. 

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