Using Bohr’s postulates, derive the expression for the frequency of radiation emitted when electron in hydrogen atom undergoes transition from higher energy state (quantum number ni ) to the lower state, (nf ).
When electron in hydrogen atom jumps from energy state ni =4 to nf =3, 2, 1, identify the spectral series to which the emission lines belong.
According to Bohr’s postulates, in a hydrogen atom, a single electron revolves around a nucleus of charge +e. For an electron moving with a uniform speed in a circular orbit on a given radius, the centripetal force is provided by the Coulomb force of attraction between the electron and the nucleus.
Therefore,
So, Kinetic Energy, K.E =
Potential energy is given by, P.E =
Therefore, total energy is given by, E = K.E + P.E =
E =
For nth orbit, E can be written as En,
Putting this value of v in equation (1), we get
Now, putting value of rn in equation (2), we get
For hydrogen atom Z =1,
If ni and nf are the quantum numbers of initial and final states and Ei & Ef are energies of electron in H-atom in initial and final state, we have
That is, when electron jumps from ni = 4 to nf = 3.21 .
Radiation belongs to Paschen, Balmer and Lyman series.
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