Question

We expect the atomic radius to increase down a group in the periodic table. Can you suggest why the atomic radius of hafnium breaks this rule? (See the following data)

Element	Atomic Radius	Element	Atomic Radius
Sc	1.57	Ti	1.477
Y	1.693	Zr	1.593
La	1.915	Hf	1.476

Short answer

Zr and Hf have similar atomic radii because of lanthanoid contraction.

Step-by-step solution

Describe the general trend of atomic radii in the periodic table.

Atomic radii increase on moving down the group and decrease on moving left to right in a period.

Explain the concept of lanthanoid contraction.

So, Lanthanoid or lanthanide contraction is the steady decrease in the radii of atoms & ions of rare earth elements from La (at. No. 57) through Lu (at. No. 71).

It is caused by the poor shielding effect of the 4f electrons.

Use this concept to explain why Hf breaks the general trend of atomic radius.

Hf comes after the 4f series (lanthanoid). Because of the poor shielding effect of 4f electrons, the nuclear attraction on the outer shell electron increases.

Due to this size of Hf decreases and is similar to the size of Zr.