Wednesday, May 11, 2011

Atomic Spectrum of HYDROGEN

When an electric discharge is passed through the hydrogen gas filled in the discharge tube at low pressure, light radiations are emitted by hydrogen. When these light radiations are passed through the prism, a spectrum consisting of bright lines known as Atomic Spectrum of Hydrogen is obtained.
In 1884, J. J Balmer observed that there were four prominent coloured lines in the visible region of hydrogen spectrum. This series of four lines in the visible region was named as the Balmer Series. Later on, by careful observation four other spectral series were discovered in the infra-red and ultra-violet regions of hydrogen spectrum. These series were named after their discoverers. Thus, atomic spectrum of hydrogen have five spectral series.

Name
Region where located
1. Lyman Series
2. Balmer Series
3. Paschen Series
4. Brackett Series
5. Pfund Series
Ultra-violet
Visible
Infra-red
Infra-red
Infra-red

Rydberg in 1890, gave a formula to calculate the wavelength of the various lines present in the particular series. The equation is as follows;
1/λ = R [ 1/n12 - 1/ n22 ]
Where, R is called Rydberg constant and its value is 109,677cm-1, n1 and n2 are whole numbers and for particular series n1 has particular value and n2 varies. E.g.
For Lyman Series,   n1 = 1, & n2 = 2,3,4-----
For Balmer Series,   n1 = 2, & n2 = 3,4,5-----
For Paschen Series, n1 = 3, & n2 = 4,5,6-----
For Brackett Series, n1 = 4, & n2 = 5,6,7-----
For Brackett Series, n1 = 5, & n2 = 6,7,8-----

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