The atomic radii of elements of the and transition series are nearly identical (e.g., the radius of ). Which of the following is the primary physical reason for this phenomenon?

Question

The atomic radii of elements of the $4d$ and $5d$ transition series are nearly identical (e.g., the radius of $Zr \approx Hf$). Which of the following is the primary physical reason for this phenomenon?

Prashant Kotian · Accepted Answer

Correct answer: B. In the series, the orbitals are progressively filled before the orbitals. The -electrons have an extremely poor shielding effect. Thus, the continuous increase in nuclear charge strongly attracts the outermost valence shell, shrinking the atomic radius to virtually offset the expected size increase down the group. This is the lanthanoid contraction, making ( pm) and ( pm) almost identical in size.

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