Lanthanoid Contraction: Why Zr and Hf Have Almost the Same Radius
Chemistry · Periodic Classification Of Properties · NEET
Zirconium (Zr) and Hafnium (Hf) are in the same group, so Hf should be much bigger. But 14 lanthanoid elements come just before Hf. As electrons fill the 4f orbitals, they shield the nucleus poorly, so the pull on outer electrons grows and size shrinks. This "lanthanoid contraction" cancels the expected size increase, so Zr (160 pm) and Hf (159 pm) end up almost equal. Memory hook: "14 f-electrons steal Hf's size, so Zr = Hf."
Hf should be much larger than Zr (it is a full period lower). But the 14 lanthanoids in between fill 4f orbitals that shield poorly, shrinking the size step by step. This lanthanoid contraction cancels the expected increase, so Zr (160 pm) and Hf (159 pm) end up almost equal.
Your doubts, answered
Why should Hf be bigger than Zr in the first place?
Zr and Hf are in the same group (Group 4). Hf is one full period below Zr, with an extra shell of electrons. Going down a group, atomic radius normally increases because a new shell is added and it sits farther from the nucleus. So by the usual rule Hf should be clearly larger than Zr.
So why are Zr and Hf almost the same size instead?
Between Zr's row and Hf's row, 14 lanthanoid elements (Ce to Lu) are inserted. Across these 14 elements, electrons enter the 4f orbitals. 4f electrons shield the nucleus very poorly, so the effective nuclear charge felt by outer electrons keeps rising and the size keeps shrinking. This steady shrink (the lanthanoid contraction) almost exactly cancels the expected size increase, so Hf ends up nearly the same size as Zr. NCERT values: Zr 160 pm, Hf 159 pm.
Why do 4f electrons shield so poorly?
f-orbitals have a diffuse, spread-out and complicated shape. They do not sit neatly between the nucleus and the outer electrons, so they block (shield) the nuclear charge weakly. Because of poor shielding, each added proton pulls the outer electrons in more, and the atom gets a little smaller with every lanthanoid added.
What is the difference between lanthanoid contraction and normal d-block contraction?
In a d-block series the size also drops a bit because d-electrons shield poorly, but the drop is small. Lanthanoid contraction is a much bigger total shrink because it runs across 14 elements filling 4f. This large cumulative shrink is what makes the third transition series (like Hf) so similar in size to the second series (like Zr).
What are the main consequences of lanthanoid contraction for NEET?
1) Zr and Hf (and other 4d/5d pairs like Nb-Ta, Mo-W) have almost equal atomic and ionic radii. 2) These pairs have very similar chemical and physical properties and are hard to separate. 3) Across the lanthanoids themselves, basic strength of hydroxides decreases (from La(OH)3 to Lu(OH)3) as ionic size falls. Remember: same size leads to same properties.
⚠️ The NEET trap ✗ Zr and Hf have similar radii because they belong to the same group (or because they have similar chemical properties). ✓ They have similar radii because of the lanthanoid contraction. The 14 lanthanoids before Hf cause poor 4f shielding, which cancels the expected size increase down the group. 🧠 'Same group' is the trap answer. Same group predicts Hf should be BIGGER. Only lanthanoid contraction explains why they are EQUAL. The similar properties are a RESULT of equal size, not the cause.
Real NEET questions
NEET 2021
Zr (Z = 40) and Hf (Z = 72) have similar atomic and ionic radii because of:
A · lanthanoid contraction ✓
B · having similar chemical properties
C · belonging to same group
D · diagonal relationship
Solution: Hf comes right after the 14 lanthanoids. Across the lanthanoids, electrons enter 4f orbitals which shield the nucleus poorly. This raises the effective nuclear charge and steadily shrinks the size (lanthanoid contraction). The shrink cancels the size increase expected on moving down the group, so Zr and Hf have nearly equal atomic and ionic radii (Zr 160 pm, Hf 159 pm). Option C is the trap: same group predicts Hf larger, not equal. Option B is a result, not the cause. Answer: A.
NEET 2018
The correct order of atomic radii in group 13 elements is:
A · B < Ga < Al < Tl < In
B · B < Al < Ga < In < Tl
C · B < Al < In < Ga < Tl
D · B < Ga < Al < In < Tl ✓
Solution: Down group 13, radius should increase B < Al < Ga < In < Tl. But Ga is actually smaller than Al. Reason: before Ga the 3d subshell is filled, and 3d electrons shield poorly, so Ga feels a higher effective nuclear charge and shrinks. This is the same 'poor shielding of inner d/f electrons' idea behind lanthanoid contraction. Correct order: B < Ga < Al < In < Tl. Answer: D.
Solved Periodic Classification Of Properties NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
NCERT gives Zr = 160 pm and Hf = 159 pm. They are almost identical, which is the visible proof of lanthanoid contraction.
Does lanthanoid contraction only affect Zr and Hf?
No. It affects all Group pairs where the 5d element follows the lanthanoids, such as Nb-Ta and Mo-W. All these pairs have nearly equal sizes and very similar properties.
Is actinoid contraction the same thing?
It is similar but happens in the 5f series. NCERT notes the shrink from element to element is greater in actinoids because 5f electrons shield even more poorly than 4f electrons.
How is lanthanoid contraction likely to be asked in NEET?
Almost always as a one-line reason question, like the 2021 'Zr and Hf have similar radii because of ___'. The answer is lanthanoid contraction. Do not pick 'same group' or 'similar properties'.
Why does the basic strength of lanthanoid hydroxides decrease across the series?
As you move across the lanthanoids the ionic radius falls (contraction). Smaller ions hold the OH more tightly, so the hydroxides become less basic from La(OH)3 to Lu(OH)3.
