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SUBJECT-CHEMISTRY


                                           CHAPTER-ATOMIC STRUCTURE


               SUBTOPIC-Rutherford's gold foil experiment,Rutherford's atomic model

               KEY NOTES:


                     Thomson atomic model is otherwise known as water melon model.
                     Observation of gold foilexperiment :

                            most of the – particles passed through the gold foil undeflected.
                            a small fraction of the –particles was deflected by small angles.
                            a very few – particles (1 in 20,000) bounced back, that is, were deflected
                              bynearly 180°.
                     Conclusion of gold foil experiment :
                            Most of the space in the atom is empty as most of the –particles passed
                              through the foil undeflected.
                            A few positively chargedparticles were deflected. The deflection must
                              be due to enormous repulsive force showing that the positive charge of
                              the atom is not spread throughout the atom as Thomson had presumed.
                              The positive charge has to be concentrated in a very small volume that
                              repelled and deflected the positivelycharged alpha particles.
                            Calculations by Rutherford showed thatthe volume occupied by the
                              nucleus isnegligibly small as compared to the totalvolume of the atom.
                              The radius of theatom is about 10–10 m, while that ofnucleus is 10–15 m.
                              One can appreciate.
                     Rutherford Atomic model :
                       (i)    The positive charge and most of the mass of the atom was densely
                              concentrated in extremely small region. This very small portion of the
                              atom was called nucleus
                                        by Rutherford.
                       (ii)   The nucleus is surrounded by electrons that move around the nucleus
                              with a very high speed in circular paths called orbits. Thus, Rutherford’s
                              model of atom resembles the solar system in which the nucleus plays the
                              role of sun and the electrons that of revolving planets.
                       (iii) Electrons and the nucleus are held together by electrostatic forces of
                       attraction.

                     Atomic Number and Mass Number
                     The number of protons present inthe nucleus is equal to atomic number (Z ).For
                       example, the number of protons in thehydrogen nucleus is 1, in sodium atom it is
                       11, therefore their atomic numbers are 1 and11 respectively.
               Atomic number (Z) = number of protons inthe nucleus of an atom
               = number of electronsin a neutral atom

                     protons and neutrons present in thenucleus are collectively known as nucleons.
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