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Structure of the Atom / Inside the Atom – Summary

Latest CBSE syllabus topics –                                                                                                         

Structure of atom
Electrons, protons and neutrons ; Isotopes and Isobars.

Latest Maharashtra SSC Board syllabus topics
Inside the atom
Historical background ; Rutherford’s scattering experiment ; Distribution of electrons in orbits ; Electronic configuration of elements ; Isotopes and their applications.


  • E. Goldstein discovered positively charged sub-atomic particle ‘proton’.               
  • J.J.Thomson discovered negatively charged sub-atomic particle ‘electron’.               
  • Thomson’s model of an atom  (plum pudding theory) – The electrons are embedded in a positive sphere.
  • Rutherford’s model of an atom – Its aparticle scattering experiment led to the discovery of the atomic nucleus. A very tiny nucleus is present inside the atom and electrons revolve around it. It could not explain the stability of atom.
  • Bohr’s model of an atom – Electrons are distributed in different orbits/shells/energy levels
    (K, L, M, N,….) with discrete energy around the nucleus. If the atomic shells are complete,
    then the atom will be stable and less reactive. The electron moves in its orbits without loss of energy.
  • J.Chadwick discovered neutron which had no charge and a mass nearly equal to that of a proton. Maximum number of electrons present in a shell = 2n² (2, 8, 18, 32,..).  Electrons are filled in the shells in a step-wise manner.
  • Atomic number (Z) – It is the number of protons of an atom.
  • Mass number – The sum of the total number of proton and neutron present in the nucleus
    of an atom.
  • Isotopes – Atoms of the same element, having the same atomic number but different mass number.
  • Isobars – Atoms of different elements with different atomic numbers which have the same mass number.


  • The word atom originally meant a smallest possible particle of matter, not further divisible.
  • Maharshi Kanad (Indian philosopher, 6th century B.C.)  postulated that if one goes on dividing matter (Padarth), he would get smaller and smaller particles and a limit will come when he will come across smallest particles beyond which further division will not be possible.  He (Kanad) named the particles Parmanu.
  • More or less during the same period Greek philosophers, Leuappus and Democritus suggested similar ideas –  “by convention bitter, by convention sweet, but in reality atoms and void”. He coined the word ‘atomos’ or atom (means  indivisible) for the smallest particles of matter This idea was not accepted at that time but it remained alive.  All this was based on philosophical considerations and not much experimental work to validate these ideas could be done until Lavoisier (1789) gave his law –  Law of conservation of mass  and  law of constant proportions.
  • John Dalton (1766- 1844) Proposed an “atomic theory” with spherical solid atoms based upon measurable properties of mass.

Early theories of the structure of matter were not based upon experiments. As scientists began to study the relationship between several physical phenomenon such as electricity, and magnetism they began to develop different models about atomic structure.

Next –

Charged Particles in Matter

2 thoughts on “Structure of the Atom / Inside the Atom – Summary”

  1. Pingback: DISCOVERY OF CHARGED PARTICLES IN MATTER – Electron, Proton and Neutron – Freakgenie

  2. Pingback: Thomson’s Model of an Atom – Freakgenie

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