The experimentally determined ionic radii of and are \AA and \AA, reflecting a typical group trend. — d and f Block Elements Chemistry Question
Question
The experimentally determined ionic radii of $Ti^{4+}$ and $Zr^{4+}$ are $0.68$ \AA and $0.74$ \AA, reflecting a typical group trend. However, the ionic radius of $Hf^{4+}$ is measured at $0.75$ \AA, making it nearly indistinguishable from $Zr^{4+}$. This precise structural anomaly is exclusively attributed to:
💡 Solution & Explanation
Moving from $Ti$ ($3d$) to $Zr$ ($4d$), the radius increases due to a new electron shell. However, before $Hf$ ($5d$) is filled, the $14$ lanthanide elements intervene, filling the $4f$ subshell. The highly ineffective shielding of these $4f$ electrons allows the massive $+14$ increase in nuclear charge to pull the valence shell inward tightly (Lanthanoid Contraction). This entirely offsets the size increase expected from adding a new shell, rendering $Zr^{4+}$ and $Hf^{4+}$ almost equal in size.