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Hydrogen Spectrum and Spectral Series

When hydrogen atom is excited, it returns to its normal unexcited (or ground state) state by emitting the energy it had absorbed earlier. This energy is given out by the atom in the form of radiations of different wavelengths as the electron jumps down from a higher to a lower orbit. Transition from different orbits cause different wavelengths, these constitute spectral series which are characteristic of the atom emitting them. When observed through a spectroscope, these radiations are imaged as sharp and straight vertical lines of a single color.
 
102712.png
Fig. 8 Emission spectra
 
The spectral lines arising from the transition of electron forms a spectra series.
  • Mainly there are five series and each series is named after it’s discover as Lymen series, Balmer series, Paschen series, Bracket series, and Pfund series.
  • According to Bohr’s theory the wavelength of the radiations emitted from hydrogen atom is given by
102418.png
⇒ 102725.png
where n2 = outer orbit (electron jumps from this orbit), n1 = inner orbit (electron falls in this orbit)
 
102732.png
Fig. 9
  • First line of the series is called first member. For this line wavelength is maximum (λmax).
     
    For maximum wavelength, if n1 = n, then n2 = n + 1.
     
    So 102424.png
  • Last line of the series is called series limit. For this line wavelength is minimum (λmin)
     
    For minimum wavelength n2 = ∞, n1 = n.
     
    So 102430.png
  • The ratio of first member and series limit can be calculated as 102436.png.
Table 2: Different spectral series
 
Spectral series
Transition
λmax
λmin
102442.png
Region
1. Lymen series
n2 = 2, 3, 4, …, ∞
n1 = 1
102451.png
102458.png
102468.png
Ultraviolet region
2. Balmer series
n2 = 3, 4, 5, …, ∞
n1 = 2
102474.png
102480.png
102486.png
Visible region
3. Paschen series
n2 = 4, 5, 6, …, ∞
n1 = 3
102493.png
102499.png
102505.png
Infrared region
4. Bracket series
n2 = 5, 6, 7, …, ∞
n1 = 4
102515.png
102521.png
102531.png
Infrared region
5. Pfund series
n2 = 6, 7, 8, …, ∞
n1 = 5
102537.png
102543.png
102549.png
Infrared region





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