Question: Q. 3. (i) Using Bohr’s postulates, derive the expression for the total energy of the electron in the stationary states of the hydrogen atom.

U] [Foreign, OD Comptt. I, II, III 2014;

Delhi I, II, III 2013]

(ii) Using Rydberg formula, calculate the wavelengths of the spectral lines of the first member of the Lyman series and of the Balmer series.

[Foreign 2014]

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Solution:

Ans. (i) } \quad mvr=nh2π mv2r=14πε0e2r2 r=e24πε0mv2 r=Ze24πε0m(nh2πmr)2 r=ε0n2h2πme2 $$

 Potential energy, U=14πε0e2r =me44ε0n2h2  K.E. =12mv2=12m(nh2πmr)21/2 =n2h2π2m2e48π2me02n4h4  K.E. =me48ε02n2h2  T.E. = K.E. + P.E.  =me48ε02n2h2

(ii) Rydberg formula : For first member of Lyman series

1λ=R(112122) λ=43R=43×912\AA =1216\AA

For first member of Balmer Series

1λ=R(122132) λ=365R =365×912\AA =65664\AA

[CBSE Marking Scheme 2014]



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