What are the postulates of Bohr's model of hydrogen atom ? Discuss the importance of this model to explain various series of line spectra in hydrogen atom.

Explanation

Postulates of Bohr's theory :
(1) Electrons revolve around the uncleus in certain , definite circular paths , called orbits . They are also known as energy levels or shells .
(2) Each orbit is associated with a definite amount of energy . Hence these orbits are called energy orbits or energy levels or energy shells .
(3) As the value of n increases , the size and energy of the orbit increases .
(4) As long as the electrons revolve in the circular orbits , they neither emit nor absorb energy . Hence , these orbits are also called stationary orbits .
(5) More than one stationary orbit is possible for any electron and the angular momentum of the revolving electron is quantized . That is , the electrons revolve in those circular orbits in which the angular momentum of the revolving electron is an integral multiple of `h//2pir` .
This can be expressed as `"mvr"=nxx(h)/(2pi)`
where m = mass of the electron,
v = velocity of the electron
r = radius of the orbit ,
h = Planck's constant ,
n = Principal quantum number .
(6) The energy of electron changes when the electron jumps from one orbit to another orbit . When the electron jumps from a lower orbit to a higher orbit , energy is absorbed . Similarly when the electron jumps from a higher orbit to a lower orbit , erergy is emitted . The energy emitted or absorbed is given by the equation , `Delta=E_(2)-E_(1)=hupsilon`
where
`E_(1)` = Energy of lower orbits ,
`E_(2)` = Energy of higher orbits ,
`DeltaE` = Difference in energy ,
h = Planck's constant,
`upsilon` = Frequency.
(B) Explanation of formation of different lines in various series of Hydrogen atomic spectrum : Hydrogen atom has only one electron and it revolves around the uncleus in the first orbit . When certain amount of hydrogen gas containing a large number of electrons , is heated or exposed to light energy or is subjected to electric discharge the different electrons absorb different amount of energy and get excited to different higher orbits. But , they do not stay there for longer peridos . Hence , they try to come back to lower orbits (de-excitation ) . This de - excitation need not necessarily be the same for all the atoms . Some excited electrons may come back from any of the hight energy levels to lower energy level n = 1 . Then Lyman series is formed in the U.V. region . Some excited electrons may come back from any of the higher energy levels to lower energy level n = 2 . Then Balmer series is formed in the visible region . Similarly , when the excited electron comes back , from any of the higher energy levels to lower 3 , 4 & 5 , we get Paschen , Bracket and Pfund series respectively in the I.R . region.
The de-excitation process from higher energy level to lower energy level also may take place in a single step or in different steps . For example , the de-excitation process of an electron from 4th energy level to `1^(st)` energy level can be as shown below.
Direct step : From 4th level to 1st (i.e.,) `4rarr1`
Different steps : `4rarr3rarr2rarr1,4rarr3rarr1,4rarr2rarr1`.
For each electronic transition one line is formed in the spectrum . Thus we get a large number of lines in a given series .