Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Isotopic shift

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The isotopic shift (also called isotope shift) is the shift in atomic spectra which occurs when one nuclear isotope is replaced by another. If the spectra have also hyperfine structure the shift refers to the center of gravity of the spectra. There are two effects which contribute to this shift:

  • The mass difference. This difference induces a change in the reduced electronic mass. This purely kinematical effect, studied theoretically by Hughes and Eckart[1] is important for light elements.
  • The volume difference. This difference induces a change in the electric charge distribution of the nucleus. This effect is important in heavy elements and its first theory was formulated by Pauli and Peierls.[2][3][4]


  1. D. J. Hughes, C. Eckart (1930). "The Effect of the Motion of the Nucleus on the Spectra of Li I and Li II". Phys. Rev. 36: 694. doi:10.1103/PhysRev.36.694. 
  2. W. Pauli, R. E. Peierls, Phys. Z. 32 (1931) 670
  3. P. Brix, H. Kopfermann, Festschrift Göttinger Akad. Springer, 17 (1951)
  4. H. Kopfermann, Nuclear Moments, Academic Press 1958