•    One of the early model of atomic structure was proposed by J.J Thomson according to this moded positive charges of the atom is uniformly distributed throughout the atom and the negatively charged electrons are embedded in it like seeds of watermelon.
•    Thomson’s model is also known as plum pudding model of the atom.
•    Through his famous gold foil experiment by Rutherford, in which he bombarded alpha particles on a gold foil and studied its scattering.


•    Rutherford observed and concluded that most of the space inside an atom is empty and there is positive charge inside the atom which carries almost the whole mass of the atom.
•    Rutherford proposed a new model of atom through this experiment but it still had some flaws.
•    Through his experiment Rutherford concluded, there is a very small, massive and positively charged nucleus at the centre of the atom. Nucleus is surrounded by negatively charged electrons revolving in their stable orbits. The required centripetal force is provided by electrostatic force of attraction between electrons and nucleus.
•    The drawbacks in the model of Rutherford were overcome by Neil bohr. He used the quantum hypothesis as a basis for these changes. He concluded that the electromagnetic theory cannot be used to explain the processes at atomic level and combined the classical mechanics with quantum concept to give three postulates to explain the structure of atom.  

•    These postulates help to explain the energy of an electron in orbit. For electron in orbit closest to the nucleus i.e., n = 1 or ground state, the energy is the least. However, for electron in orbit n=2, 3,... the absolute value of the energy E is smaller.
•    When transition of an electron takes place from a higher energy state to a lower energy state, the difference of energy is carried out by a photon.
•    The radiated photon indicates the corresponding transition states as light is radiated at various discrete frequencies.  This gives rise to spectral series of hydrogen atom. The discovered series were Lyman series, Balmer series, Paschen series, Brackett series and Pfund series.
•    De-Broglie gave the explanation to the second postulate of Bohr. He concluded that quantized energy states of an electron are due to wave nature of the electron and only resonant standing waves can persist. Bohr’s model was not applicable for the atoms with more than one electron


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