Deuterium was discovered in 1932 by Harold Urey by measuring the small change in wavelength for a particular transition in 1 \mathrm{H} and 2 \mathrm{H}. This is because the wavelength of transition depends to a certain extent on the nuclear mass. If nuclear motion is taken into account then the electrons and nucleus revolve around their common centre of mass. Such a system is equivalent to a single particle with a reduced mass \mu, revolving around the nucleus at a distance equal to the electron-nucleus separation. Here \mu=\mathrm{me} \mathrm{M} /(\mathrm{me}+\mathrm{M}) where \mathrm{M} is the nuclear mass and \mathrm{me} is the electronic mass. Estimate the percentage difference in wavelength for the 1 st line of the Lyman series in 1 \mathrm{H} and 2 \mathrm{H}. (Mass of 1 \mathrm{H} nucleus is 1.6725 \times 10^{-27} \mathrm{~kg}, Mass of 2 \mathrm{H} nucleus is 3.3374 \times 10^{-27} \mathrm{~kg}, Mass of electron =9.109 \times 10^{-31} kg.)
Deuterium was discovered in 1932 by Harold Urey by measuring the small change in wavelength for a particular transition in 1 \mathrm{H} and 2 \mathrm{H}. This is because the wavelength of transition depends to a certain extent on the nuclear mass. If nuclear motion is taken into account then the electrons and nucleus revolve around their common centre of mass. Such a system is equivalent to a single particle with a reduced mass \mu, revolving around the nucleus at a distance equal to the electron-nucleus separation. Here \mu=\mathrm{me} \mathrm{M} /(\mathrm{me}+\mathrm{M}) where \mathrm{M} is the nuclear mass and \mathrm{me} is the electronic mass. Estimate the percentage difference in wavelength for the 1 st line of the Lyman series in 1 \mathrm{H} and 2 \mathrm{H}. (Mass of 1 \mathrm{H} nucleus is 1.6725 \times 10^{-27} \mathrm{~kg}, Mass of 2 \mathrm{H} nucleus is 3.3374 \times 10^{-27} \mathrm{~kg}, Mass of electron =9.109 \times 10^{-31} kg.)

The energy of an electron in the nth state is given by the expression,

E_ n=-\mu Z^{2} \mathrm{e}^{4} / 8 \varepsilon_ 0^{2} h^{2}\left(1 / n^{2}\right)

For hydrogen atom we have,

\mu _{H}=\mathrm{m_eM_h} /(\mathrm{m_e}+\mathrm{M_h})

For deuterium atom we have,

\mu _{D}=\mathrm{m_eM_d} /(\mathrm{m_e}+\mathrm{M_d})

The wavelength of the transition for hydrogen can be calculated as,
\lambda_ H=3 / 4 \mu \mathrm{He}^{4} / 8 \varepsilon_ 0^{2} h^{2} c

The wavelength of the transition for deuterium can be calculated as, \lambda_ D=3 / 4 \mu D e^{4} / 8 \varepsilon_ 0^{2} h^{2} c

The difference between the wavelength can be calculated as, \lambda=\lambda_ H-\lambda_ D=2.714 \times 10^{-2} \%