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SUBJECT-CHEMISTRY


                                           CHAPTER-ATOMIC STRUCTURE


               SUBTOPIC-limitation of Rutherford's atomic model, Bohr's atomic model.

               KEY NOTES:


                     SPECTRUM :
                       The phenomenon of splitting of a beam of light in to radiations of different
                       frequencies after passing through the prism is called dispersion and the pattern of
                       radiation obtained after dispersion of beam is called spectrum.
                     The  study of spectra is called spectroscopy.
                     TYPES OF SPECTRUM
                       (1) Emission spectrum :
                          The spectrum of radiation emitted by a substance that has absorbed energy is
                          calledan emission spectrum. Since the radiations in the spectrum are emitted due to
                          energy changes taking place in the atoms, this spectrum is also called atomic
                          spectrum.
                                Continuous spectrum :
                                 When white light from any source like sun, a bulb, any hot glowing body is
                                 analysed by passing through a prism, it is observed that it splits up into seven
                                 different wide bands of colours from violet to red. These colours are so
                                 continuous that each of them merged into the next.The spectrum is called
                                 continuous spectrum.
                                Line spectrum :
                                 When some volatile salt like sodium chloride is placed in the Bunsen flame or
                                 an electric discharge is passed through a gas at low pressure, light is emitted.
                                 The colour of light emitted depends on the nature of the substance. For
                                 example, sodium or its salts emit yellow light while potassium or its salts give
                                 out violet light.If this light is resolved in a spectroscope, it is found that no
                                 continuous spectrum is obtained but  some isolated coloured lines are
                                 obtained on the photographic plate separated from each other by dark
                                 spaces.This spectrum is called  line spectrum.
                                Atomic spectrum of an element can be used to identify the element and is
                                 sometimes called fingerprint of its atom.
                                Elements like rubidium (Rb), caesium (Cs) thallium (Tl), indium (In),
                                 gallium (Ga) and scandium (Sc) were discovered when their minerals were
                                 analysed by spectroscopic methods. The element helium (He) was
                                 discovered in the sun by spectroscopic method.
                       (2) Absorption spectrum :
                                When a beam of continuous light is passed through a tube containing
                                 vapours or solution of  the substance and the transmitted light is analysed
                                 by a spectrometer, it is observed that the spectrum obtained contains a
                                 number of dark lines in otherwise continuous spectrum.Thesedarklines
                                 appear due to the absorption of radiations of corresponding wavelengths
                                 by the substance. The dark lines in the absoption spectrum of a substance
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