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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