Question

1.explain why there are more lines in the hydrogen emission spectrum than in the absorption spectrum?

2. indicate asborption, from a lower energy level to a higher energy level. Then indicate emission for the same transition as through various pathways, which would then result in many more emission peaks than there was in the original absorption (transition).

Answer 1

Ans. 1   When a beam of polychromatic light is passed through a prism ,it breaks into its constituent colours . This array of light is called Spectrum.Further spectrum is of two types :-

i) Emission Spectrum :- This type of spectrum is obtained when the light coming after passing through the prism is examined directly with a spectroscope.

ii) Absorption Spectrum :- When the light from a source emitting a continuous spectrum is first passed through an absorbing substance ,and then observed through a spectroscope , it will be found that certain colours are missing which leave dark lines or bands at their places . The observed spectrum is called absorption spectrum .

Although an atom of hydroger contains only one electron ,yet its atomic spectrum consists of a large number of lines which have been grouped into five series ,i.e. Lyman , Balmer,Paschen ,Brackett and Pfund . This may be explained as follows :

Any given sample of hydrogen contains a large number of molecules . When such a sample is heated to a high temperature or an electric discharge is passed , the hydrogen molecules split into hydrogen atoms . The electrons in different hydrogen atoms absord different amounts of energies and are excited to different energy levels. For example , the electrons in some atoms are excited to second energy level (L) , while in others they may be promoted to third (M) , fourth (N) , fifth (O) energy levels and so on . Since the lifetime of electrons in these excited states is very small , they return to some lower energy level or even to the ground state in one or more jumps . The different excited  electrons adopt different routes to return to various energy levels or the ground state. As a result , they emit different amounts of energies and thus produce a large number of lines in the atomis spectrum of hydrogen .

For example , when the electron jumps from energy levels higher than n=1, i.e. n=2,3,4,5,6.....to n=1 energy level , the group of lines produced is called Lyman series. These lines lie in the ultraviolet region .

But in case of absorption spectrum , when energy falls on a normal lower excited state of an atom , the electrons jump from an inner to an outer orbit , resulting in the absorption of definite quantities of energy . If this excitation energy is obtained from a beam of light , the lines with corresponding frequencies would be missing from the spectrum of light used. Infact , lines of darkness will be seen against a background of light in such a spectrum. These dark lines against a background of light are termed as the absorption spectrum of light absorbing element .