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Going down a group, atoms usually get bigger. Then you reach hafnium, which sits below zirconium and is almost exactly the same size. That surprise is the lanthanoid contraction, and examiners like it because it links electron configuration, shielding, radii and the chemistry of the third transition series. This post explains the cause, works through the numbers, and lists the consequences you need for JEE and boards.
Where does it appear in the syllabus?
The JEE (Main) 2026 syllabus lists "Lanthanoids – Electronic configuration, oxidation states and Lanthanoid contraction", and the NEET (UG) 2026 syllabus uses almost the same line. The JEE (Advanced) 2026 brochure has "Lanthanoid and actinoid contractions", and the CBSE Class 12 syllabus for 2025–26 asks for "lanthanide contraction and its consequences". The explanation you need is in Unit 4 of NCERT Class 12 Chemistry Part I. Check the current year's syllabus before you plan revision.
What causes the lanthanoid contraction?
From lanthanum (Z = 57) to lutetium (Z = 71), each step adds one proton to the nucleus and one electron, mostly into the 4f subshell. NCERT puts the cause in one phrase: imperfect shielding of one electron by another in the same set of orbitals. The 4f orbitals are spread out in a way that makes them poor at screening the outer electrons from the extra nuclear charge. NCERT adds that the shielding of one 4f electron by another is even less than that of one d electron by another.
So, across the series:
Nuclear charge goes up by one at each step.
The added 4f electron cancels only a small part of that increase.
Effective nuclear charge felt by the outer electrons rises.
The whole electron cloud is pulled in, and the atom or ion shrinks.
The same thing happens on a smaller scale across a d series. The lanthanoid version matters more because it runs over fourteen elements and then sits between the 4d and 5d transition series.
How large is the contraction? A worked example
Compare M³⁺ ions with the same coordination number, so you are comparing like with like. In Shannon's 1976 table of effective ionic radii (searchable in the Imperial College database), six-coordinate La³⁺ is 103.2 pm and six-coordinate Lu³⁺ is 86.1 pm.
Total contraction: 103.2 − 86.1 = 17.1 pm.
As a fraction: 17.1 / 103.2 = 0.166, about 17%.
There are 14 steps from La to Lu, so the average is 17.1 / 14 ≈ 1.2 pm per element.
A shrink of about 1 pm per step looks small. Added up over fourteen elements, it cancels the size increase you would normally expect from adding a whole new shell.
NCERT also notes that the decrease is "not quite regular" for atomic (metallic) radii, but fairly regular for the M³⁺ ions. If a question shows a graph of metallic radii with bumps, that is expected. Our post on d- and f-block elements explains the configurations behind it.
What are the consequences?
1. The 4d and 5d series are nearly the same size. NCERT's example is zirconium at 160 pm and hafnium at 159 pm. Shannon's six-coordinate radii tell the same story for the ions: Zr⁴⁺ is 72 pm and Hf⁴⁺ is 71 pm.
2. Zr and Hf are found together and are hard to separate. NCERT links their almost identical radii to their occurring together in nature and the difficulty of separating them. With the same size and the same charge, their chemistry is almost the same.
3. Neighbouring lanthanoid ions are very alike. Next-door Ln³⁺ ions differ by only about a picometre on average, and they carry the same charge, so their chemistry changes only gradually along the series.
4. Some unexpected size matches. Yttrium sits above lanthanum, yet six-coordinate Y³⁺ (90.0 pm) is almost the same size as Ho³⁺ (90.1 pm) in Shannon's table.
5. A smooth change in chemistry across the series. Smaller ions with the same charge hold their surroundings more tightly. You may see questions that ask how basic character of the hydroxides changes from La(OH)₃ to Lu(OH)₃. Answer from your textbook and syllabus, and reason from the falling ionic size.
Common mistakes
"Hf is bigger because it is lower in the group." For the 4d and 5d pair, the contraction cancels the usual increase. Ti is smaller than Zr, but Zr and Hf are nearly equal.
Blaming the f electrons for being "added to the outside". The 4f electrons are not the outermost electrons. The effect comes from their poor shielding of the outer 5d and 6s electrons.
Mixing radii from different tables. NCERT, Shannon and other data sets give different numbers for the same ion. Keep to one source in any comparison.
Confusing it with the actinoid contraction. Similar cause, different series. JEE Advanced names both.
If you want a lesson that builds this up from shielding and effective nuclear charge, the Fermi Concept Library covers every concept in chemistry, each with its own lesson.
A short practice routine
Write the cause of the contraction in four numbered steps without notes, then compare with NCERT.
List five consequences, each with one piece of evidence (a radius or an example).
Rank Ti, Zr and Hf by atomic radius, and Zr⁴⁺, Hf⁴⁺ and Ti⁴⁺ by ionic radius, explaining each in one line.
Mix in two isoelectronic-ion questions so you practise choosing the right principle; our post on ranking isoelectronic ionic radii has some.
Tonight, write the La³⁺ to Lu³⁺ calculation from memory and check each number against the table.
