Emission Spectra Classification

The nature of the vapour and the conditions under which it is excited determines the spectrum emitted by a glowing vapour. Every element when excited produces a spectrum characteristic to itself. This characteristic spectral property of an element helps in its detection in a mixture. There are three characteristic classes of emission spectra.

(i) Continuous spectrum: A continuous spectrum contains all the wavelengths from one end to the other. Continuous spectra are due to light from an incandescent solid or a dense luminious gas flame. The spectrum appears as an unbroken luminious band. The colour of the band changes from point to point. the intensity is maximum at a certain point and decreases on both sides. This point of maximum intensity shifts towards the violet region when the temperature of the solid is increased.

(ii) Band spectrum: A band spectrum is given by light emitted by chemical compounds in the vapour state. A band spectrum consists of a large number of luminious bands. These bands are sharply defined at one edge and shade off gradually at the other edge. When a band spectrum is studied with a spectrometer of high resolving power, each band consists of a large number of lines. These lines are more crowded at the sharper edge of the band than the other. A band spectrum is a discontinuous spectrum.

(iii) Line spectrum: If the source of light is from a gas through which electric discharge is passed or a flame into which a volatile salt is introduced, then the spectrum is discontinuous. Only a few characteristic coloured images of the slit are observed depending on the nature of the gas or vapour excited. Such a spectrum is called a line spectrum. Each image of the slit corresponds to a definite wavelength and the wavelengths of the lines are characteristic of the element. Thus, hydrogen, iron, sodium etc. always give the same set of characteristic lines at the same positions. The spectral series due to any particular element are very often regularly spaced and they form a series. Bohr’s theory of the hydrogen spectrum and the corresponding spectra originate in the atom. The spectral lines of hydrogen, cadmium, mercury, sodium and lead are given in fig.

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