CBSE 2025, Set 3

Question

While studying the periodic properties, Arti came across an abnormal behaviour in the atomic size of Hf. She found that, even though Hf is placed below Zr in the same group, both have almost similar atomic sizes.

(I) Which phenomenon is responsible for the above behaviour? Define it.

(II) Mention any other consequence of the above phenomenon.

Edvaya · Accepted Answer

The correct answer is Subjective. **(I) Phenomenon: Lanthanoid Contraction** **Definition:** The steady decrease in atomic and ionic radii of the lanthanoid elements (La to Lu) from lanthanum to lutetium due to the **poor shielding effect of 4f electrons**. The 4f electrons shield the nuclear charge ineffectively (diffuse, non-directional orbitals), so as the nuclear charge increases across the series, the outer electrons experience an increasing effective nuclear charge → progressive contraction of the atom. **Why Hf ≈ Zr in size:** After La (Z=57), the 14 lanthanoid elements (Ce to Lu) are filled. Due to lanthanoid contraction, the atoms shrink progressively. By the time Hf (Z=72) is reached, the contraction due to the intervening 14 lanthanoid elements exactly counteracts the expected increase in size from the fifth period to sixth period. Hence Hf and Zr have nearly identical atomic radii (~160 pm). **(II) Another consequence of lanthanoid contraction:** **Decrease in basic character of lanthanoid hydroxides** from left to right across the series (La(OH)$_3$ is more basic than Lu(OH)$_3$) due to decreasing ionic radius → increasing polarising power of the cation.

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Chemistry > Inorganic Chemistry > D And F Block Elements

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