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The lanthanide contraction refers to the steady decrease in atomic radii and ionic radii of elements along the lanthanide series (elements with atomic numbers 57 to 71). This occurs due to poor shielding of the 4f electrons, which results in an increased effective nuclear charge and increased attraction between the electrons and protons in the nucleus. Consequently, the atomic radii decrease despite the increase in the principal quantum number as one moves across the lanthanide series.

The lanthanide contraction has an effect on the transition metals located in periods 5 and 6 of the periodic table as well. The elements in period 6 are located after the lanthanides, and because the lanthanides show poor shielding of 4f electrons, their contraction is also experienced by the elements in period 6. This means that the increase in atomic radius that would typically be expected as one moves down the periodic table (due to an increase in the principal quantum number) is offset by the lanthanide contraction in the case of period 6 transition metals.

Due to the influence of the lanthanide contraction, the atomic radii of the transition metals in period 6 are not significantly larger than those in period 5, as one might expect based on their position in the periodic table. Instead, their radii are nearly identical within each group because the increase in atomic size resulting from an additional electron shell is counteracted by the higher effective nuclear charge and strong electron-proton attraction caused by poor shielding of the 4f electrons in the lanthanides. In conclusion, the transition metals in periods 5 and 6 have nearly identical radii in each group due to the lanthanide contraction, which diminishes the expected increase in atomic radius as one moves down the periodic table. The poor shielding of 4f electrons in the lanthanides leads to a higher effective nuclear charge and stronger electron-proton attraction, ultimately causing similar atomic radii for transition metals in periods 5 and 6.