CONCEPT OF Hydrogen Spectrum By-N.K.D. Sir Please -Rate review and share this lesson

When energy is supplied to a sample of hydrogen gas, the atoms present in the sample, absorb energy and are excited from the ground state to different higher energy states, depending upon the amount of energy absorbed. When these excited electrons return to the normal state, energy is emitted in the form of radiations of different wave lengths which appear in the form of different lines in the emission spectrum. The wave length (or frequency) of the lines depends upon the amount of energy emitted. Greater the amount of energy liberated, shorter will be the wave length of radiation emitted. The hydrogen spectrum contains a large number of closely spaced lines, hence, it is a line spectrum. Although, hydrogen atom contains only one electron in its atom, yet it gives a large number of lines in its spectrum. Its explanation is that a sample of hydrogen gas contains a very large number of atoms. The electrons in these atoms are excited to different energy states on absorption of energy, which give very large number of lines in the spectrum on de-excitation. Its lines in the spectrum are grouped into five series each of which is named after the name of its discoverer. The various series of lines of the hydrogen spectrum are

PFUND SERIES BRACKETT RADIATION OF SERIESENERGY WITH LONG WAVE LENGTH RADIATION OF ENERGY WITH INTERMEDIATE WAVE LENGTH PASCHEN SERIES 2 BALMER SERIES RADIATION OF ENERGY WITH SHORT WAVE LENGTH LYMAN SERIES

Series of Lines of Hydrogen Spectrum: (a) Lyman Series: Lines of this series are obtained when electrons fall from 2, 3, 4... etc. orbit to first orbit (ground state) For Lyman series ni- 1, n2-2, 3, 4, 5, 6, 7, The wave length of the lines in Lyman series can be calculated by using equation (13). For the first line of Lyman series, -= 1096781 2-22 - 109678x -c 4 2-1215.7A

For second line of Lyman series: n, = 3, n,-1 109678 1-cm = 1025.7A The wave length of the second line of Lyman series is less than that of the first line. Thus, the wave length of the other lines decreases continuously. The wave length of the last (limiting) line of the Lyman series is obtained when n20 = 1096781 -- .iows-G . 09678 m-911.7

The lines of the Lyman series appear in the ultraviolet region of the spectrum. The intenstes o spectral ines decreases with Increase in the value o n. e.g., the Intensity of irst yman line 2 >1 is greater than second line (3->1) (b) Balmer Series: Lines of this series are obtained when electrons fall from outer orbits tosecond orbit. Thus, for the various lines of this series, n 2, n2 3, 4, 5, 6, 7, 8 The wave length of the first line in the Balmer series can be calculated by putting n 2 and n2-3 109678 = 109678| cm 22 3 = 109678 -cm - 109678xc 36 36 36 109678x5 548390 6.56463< 10., or 6564.64

The wave length of the last (limiting) line of Balmer series is: 109678 | Cin 109678 1 cm 4 4. = = 3.647 10-5cm 109678 = 3647 The lines of Balmer series fall in the visible region of the spectrum. In the Balmer series of hydrogen spectrum, the first line (n2 = 3 n = 2) is4line. The line from n o 2is called limiting line

(c) Paschen Series: This series is produced when the electrons fall from outer orbits to third orbit i.e. for the lines of this series, n 3, n2 4, 5, 6, 7. For the first line of this series, ni 3, and n2 4 then n. n = 109678 109678 x cm 9 16 ! 09678x-cm-1 144 144 144 or cim 7 x109678 767746 = 1.87562 104cm -18756.2A or 3 Lines of this series lie in the infra-red region of the spectrum

(d) Brackett series: When the electronic transition takes place from outer orbits to the fourth orbit, lines of this series are obtained, i.e., for this series, ni = 4, n,-5, 6, 7, 8, 9 The wave length of the first line of the series can be calculated by putting n in the equation 4 and n2 5 !- :/ X In n 09678 5 109678 L 16-25]din-i; = 405234 This series also falls in the infra-red region of the spectrum

(e) Pfund Series: The lines in this series of hydrogen spectrum are obtained when electrons are de- excited from higher orbits to fifth orbit, i.e., for this series, n,-5, n,-6, 7, 8, 9, 10. The wave length of the first line of the series can be calculated by putting, n 5 and n2 6 2 109678 25 36 This series also falls in the infra-red region of the spectrum * In the Rydberg formula whenn,-, the line produced is called the limiting line of that series

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Niranjay Kumar Dwivedi

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Complete Chemistry faculty for IIT~JEE, and NEET examination.
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Layba noor

9 months ago

sir I have a doubt in question 4

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