Lanthanide Contraction

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Lanthanide contraction results in:

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1. **Definition**: As atomic number increases in the Lanthanide series ($Ce$ to $Lu$), the atomic and ionic radii gradually decrease. This is the **Lanthanide Contraction**.
2. **Screening Effect**: It happens because $4f$ electrons have poor shielding ability, allowing the nucleus to pull the outer electrons closer as nuclear charge increases.
3. **Consequence on Basicity**: According to Fajan's rules, a smaller cation ($Lu^{3+}$) has higher polarizing power than a larger cation ($Ce^{3+}$).
4. **Covalent Nature**: Therefore, the $M\u2013OH$ bond becomes more covalent as we move from $Ce$ to $Lu$.
5. **Result**: The ability to release $OH^-$ ions decreases, meaning **basicity decreases** across the series ($Ce(OH)_3$ is most basic, $Lu(OH)_3$ is least basic).

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Lanthanide Contraction

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

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