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(Chemistry Ch-2) 4. Atomic Spectra


  • The spreading of a ray of white light into a series of coloured bands is called spectrum.
  • The spectrum of white light which ranges from violet at 7.50 × 1014 Hz to red at 4 × 1014 Hz is called continuous spectrum.
Emission and Absorption Spectra
  • Emission spectrum is the spectrum of radiation emitted by a substance that has absorbed energy.
  • Absorption spectrum is like the photographic negative of an emission spectrum.
  • Spectroscopy is the study of emission or absorption spectra.
  • Atoms in the gas phase do not show a continuous spread of wavelength from red to violet. They emit only at specific wavelengths with dark spaces between them. Such spectra are called line spectra or atomic spectra.
Line Spectrum of Hydrogen
  • If an electric discharge is passed through gaseous hydrogen, then the H2 molecules get dissociated to produce energetically excited hydrogen atoms. These atoms emit electromagnetic radiations of discrete frequencies.
  • A spectrum of hydrogen consists of several series of lines.
  • When the spectral lines are expressed in terms of wave number, the visible lines of the hydrogen spectra obey the following formula:
Where, n is an integer equal to or greater than 3 (i.e., n = 3, 4, 5, …)
  • The series of lines that can be described by the formula is called Balmer series. It is the only series of line in the hydrogen spectrum which appears in the visible region of the electromagnetic spectrum.
  • All the series of lines in the hydrogen spectrum can be described by the following expression:
Where, n1 = 1, 2, …
n2 = n1 + 1, n2 + 2, …
109,677 is called Rydberg constant for hydrogen (RH)
  • The first five series of lines that correspond to n1 = 1, 2, 3, 4, 5 are known as Lyman, Balmer, Paschen, Brackett and Pfund series.
Series
n1
n2
Spectral Region
Lyman
1
2,3…
Ultraviolet
Balmer
2
3,4…
Visible
Paschen
3
4,5,…
Infrared
Brackett
4
5,6…
Infrared
Pfund
5
6,7…
Infrared
  • The Lyman, Balmer and Paschen series of transition for hydrogen atom are shown in the figure.

2 comments:

  1. what is the type of spectra of hydrogen which we are studying. Is it emission spectra or absorbtion spectra .

    ReplyDelete

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