Question
State and explain the cause of lanthanoid contraction. What are its consequences on the chemistry of 5d (third-row transition) elements?
This is a 2-mark + 3-mark standard question in CBSE Class 12 and has appeared repeatedly in JEE Main — most recently in 2022 and 2020 January sessions. Knowing the cause separately from the consequences is how you get full marks.
Solution — Step by Step
As we move from La (Z=57) to Lu (Z=71) across the lanthanoid series, the atomic and ionic radii progressively decrease. This steady decrease is called lanthanoid contraction.
The total shrinkage across the series is about 10 pm.
The 14 electrons being added go into 4f orbitals, which have a diffuse, complex shape. The key point: 4f electrons shield nuclear charge very poorly compared to d or s electrons.
So as Z increases by 1 each step, the effective nuclear charge felt by the outermost electrons increases almost by a full unit — pulling the electron cloud inward. The result: a shrinkage with every element added.
Shielding effectiveness follows the order: . The 4f subshell has complex angular nodes that keep it from effectively screening the nucleus.
Each new 4f electron added barely compensates for the +1 increase in , so keeps rising steadily across La → Lu.
This is the big payoff for JEE. The lanthanoids sit between the 4d and 5d transition series. Because of lanthanoid contraction, the 5d elements are abnormally small — their radii are nearly identical to the 4d elements directly above them.
For example:
| Pair | 4d element (pm) | 5d element (pm) |
|---|---|---|
| Zr / Hf | 160 | 159 |
| Nb / Ta | 146 | 146 |
| Mo / W | 139 | 139 |
Without lanthanoid contraction, the 5d elements should be larger (as you go down any group, size normally increases).
Because Zr and Hf have almost identical sizes, their ionic radii, ionization energies, and electronegativity are nearly the same. This makes them the hardest pair of elements to separate in nature — they always occur together in ores.
Similarly, properties like electrode potential, complex formation tendency, and hardness are nearly matched for the 4d/5d pairs (Mo/W, Nb/Ta, etc.).
Why This Works
The normal trend in the periodic table is: as you go down a group, atomic radius increases because a new electron shell is added. This should make 5d elements significantly larger than 4d elements.
But the 14 lanthanoid elements interrupt this pattern. Their poor 4f shielding means rises across the entire series, compressing the ionic radius of Lu to nearly the same as La was expected to be. By the time we reach the 5d elements, this accumulated contraction exactly cancels the expected increase from adding a new shell.
The net result: size increase from shell addition ≈ size decrease from lanthanoid contraction, giving us near-identical radii for 4d/5d pairs.
Alternative Method (Answering in Exam Format)
For CBSE board exams, the examiner wants a structured answer. Use this format directly:
Lanthanoid Contraction:
- Definition: Steady decrease in atomic/ionic radius from La to Lu.
- Cause: Poor shielding by 4f electrons → increased → radii shrink.
Consequences:
- Similar atomic radii of 4d and 5d transition elements (e.g., Zr ≈ Hf, Nb ≈ Ta).
- Chemical similarity of these pairs — they co-occur in nature and are difficult to separate.
- Higher density and hardness of 5d elements compared to expected values.
In JEE Main MCQs, the question often asks “which pair is hardest to separate due to lanthanoid contraction?” — the answer is always Zr and Hf. This specific pair has appeared in 2022, 2020, and 2019 papers.
Common Mistake
Students write: “lanthanoid contraction occurs because electrons are added to the inner 4f subshell, which is completely shielded.”
This is the exact opposite of the correct explanation. The 4f electrons are not well-shielded — they are poor shielders. The existing 4f electrons do not block the nucleus effectively from the incoming 4f electrons. Don’t confuse inner subshell with well-shielded. The 4f subshell is inner, yes — but that’s precisely why its electrons have poor shielding ability (they avoid the nuclear region less than s/p electrons do).
In exams, this reversed phrasing is a classic trap in match-the-column questions.