ATOMIC SPECTRUM OF HYDROGEN
The emission line spectrum
of hydrogen can be obtained by passing electric discharge through
the gas contained in a discharge tube at low pressure. The light radiation emitted is then examined
with the help of a spectroscope. The bright lines recorded on the photographic plate constitute the
atomic spectrum of hydrogen
In 1884 J.J. Balmer observed that there were four prominent coloured lines in the visible hydrogen
spectrum :
(1) a red line with a wavelength of 6563 Å.
(2) a blue-green line with a wavelength 4861 Å.
(3) a blue line with a wavelength 4340 Å.
(4) a violet line with a wavelength 4102 Å.
Hydrogen
discharge tube
The examination of the atomic spectrum of hydrogen with a spectroscope.
The above series of four lines in the visible spectrum of hydrogen was named as the Balmer
Series. By carefully studying the wavelengths of the observed lines, Balmer was able empirically to
give an equation which related the wavelengths (λ) of the observed lines. The Balmer Equation is
1/λ=R( 1/2^2-1/n^2)
where R is a constant called the Rydberg Constant which has the value 109, 677 cm– 1 and n = 3, 4,
5, 6 etc. That is, if we substitute the values of 3, 4, 5 and 6 for n, we get, respectively, the wavelength
of the four lines of the hydrogen spectrum.
Blue-green Blue Violet
6563 4861 434
Balmer series in the Hydrogen spectrum.
In addition to Balmer Series, four other spectral series were discovered in the infrared and
ultraviolet regions of the hydrogen spectrum. These bear the names of the discoverers. Thus in all
we have Five Spectral Series in the atomic spectrum of hydrogen :
Name Region where located
(1) Lyman Series Ultraviolet ( 1) UV
(2) Balmer Series Visible (2) visible
(3) Paschen Series Infrared (3) Infrared
(4) Brackett Series Infrared (4) infrared
(5) Pfund Series (5)infrared
Balmer equation had no theoretical basis at all. Nobody had any idea how it worked so
accurately in finding the wavelengths of the spectral lines of hydrogen atom. However, in 1913 Bohr
put forward his theory which immediately explained the observed hydrogen atom spectrum. Before
we can understand Bohr theory of the atomic structure, it is necessary to acquaint ourselves with the
quantum theory of energy
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