J.J Thomson proposed his model according to which the positive charge of the atom is uniformly distributed throughout the atom and the negatively charged electrons are embedded in it like seeds of watermelon. Thomson model was picturesquely called plum pudding model of the atom. Rutherford devised an experiment in which he bombarded alpha particles on a gold foil and studied its scattering. From his observations he 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. On this basis Rutherford proposed a new model of an atom but it still had some flaws. According to the Rutherford nuclear model of atom, 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 to the electrons is provided by electrostatic force of attraction between electrons and nucleus. Niels Bohr made certain changes in the model suggested by Rutherford. 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. The energy of an electron in orbit can be determined with the help of these postulates. 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. During a transition from a higher energy state to a lower energy state, the difference of energy is carried out by a photon. The radiated photo indicates the corresponding transition states as light is radiated in 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. The explanation to the second postulate of Bohr was given by De-Broglie. He concluded that quantised 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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