Lanthanide Contraction Effect

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Lanthanide contraction chiefly causes which pair of 4d/5d elements to have similar radii?

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1. **Lanthanide Contraction**: The steady decrease in atomic and ionic radii along the Lanthanide series ($Z=58$ to $71$).
2. **Reason**: Poor shielding by $4f$ electrons. The nuclear charge increases by $14$ units, but $4f$ electrons do not block it effectively.
3. **Impact on 5d elements**: Elements following the Lanthanides (like $Hf, Ta, W$) have an unexpectedly small radius.
4. **Observation**: This effectively cancels out the normal size increase expected when moving from $4d$ shells (period 5) to $5d$ shells (period 6).
5. **Pairs**: **Zirconium ($Zr$, radius $1.60$ Å)** and **Hafnium ($Hf$, radius $1.59$ Å)** end up with nearly identical atomic radii.

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Lanthanide Contraction Effect

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