The lanthanoid contraction occurs when 4f orbitals are filled before the 5d series begins, resulting in a regular decrease in atomic radii. This phenomenon compensates for the expected increase in atomic size with increasing atomic number and causes the third (5d) series to have virtually the same radii as the corresponding members of the second (4d) series.
What is the significance of the lanthanoid contraction in the third (5d) series of elements, and how does it affect atomic radii?
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The lanthanoid contraction is significant in the third (5d) series of elements as it compensates for the expected increase in atomic size with increasing atomic number. This phenomenon is associated with the intervention of the 4f orbitals, which must be filled before the 5d series begins. The filling of 4f before 5d results in a regular decrease in atomic radii, known as lanthanoid contraction. Despite the increase in atomic mass, the lanthanoid contraction leads to virtually the same atomic radii for the third (5d) series as those of the corresponding members of the second (4d) series. This regular decrease contributes to the similarity in physical and chemical properties between the two series.