Atomic model

Rutherford alpha particle scattering experiment and atomic structure: 

  1. JJ Thomson discovered the electron. He believed that the atom is a uniform sphere of positive charge with electrons (plums) embedded in it. This model explained the electrical neutrality of atom, but it failed to explain how these fundamental particles were arranged in the atom. His model was called ‘plum pudding’ model.


  1. Ernest Rutherford (1871-1937) used α-particles to make it fall on the thin gold foil. He found that most of the α-particles were undeflected, some were scattered by appreciable angle and a few about 1 in 8000 surprisingly bounced back. To explain these results, in 1911 Rutherford proposed a ‘nuclear model’ of atom:


  1. ‘Most of the space in an atom is empty.
  2. All the positive charge and most of the mass of an atom formed a central dense part called nucleus of very small size compare to the size of atom. Size of a nucleus is 10,000 times smaller than the size of atom.
  3. The negatively charged electrons surrounding the nucleus.
  4. Total amount of negative and positive charge in a neutral atom is equal.
  5.  The atomic or proton number Z of an atom is the number of protons in the nucleus. It is also the number of electrons in the atom when atom is neutral.
  6. N is the number of neutrons in an atom.
  7. In general A=Z+N where A is the nucleon number or mass number of an atom