The trap: Students predict that 5d transition metals (Hf, Ta, W) should be significantly larger than their 4d counterparts (Zr, Nb, Mo) because they sit one period lower. NEET exploits this by asking why 4d and 5d pairs show nearly identical atomic radii and similar chemistry.
What lanthanoid contraction actually is: The 14 lanthanoid elements (Ce to Lu) are filled across the 4f subshell. The 4f electrons shield nuclear charge poorly — their diffuse orbital shape means each added proton is only partially screened. Consequently, effective nuclear charge increases steadily from La (Z = 57) to Lu (Z = 71), shrinking atomic and ionic radii progressively across the series (NCERT Class 12 Chemistry Chapter 4, page 24).
The downstream consequence for 5d elements: By the time you reach Hf (Z = 72), the cumulative contraction across the 14 lanthanoids has offset the expected size increase from adding a new shell. Result: Zr (4d) and Hf (5d) have nearly identical atomic radii (~160 pm). This extends to Nb/Ta and Mo/W pairs — similar size means similar chemistry (lattice energies, bond strengths, coordination behaviour).
Key facts for NEET:
Watch out: Questions may frame the contraction as affecting only lanthanoids themselves. Remember the consequence extends to ALL subsequent elements — the 5d series and even post-lanthanoid p-block elements are affected.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
What is the primary cause of lanthanoid contraction?
Which is the most common and stable oxidation state exhibited by lanthanoid elements?
Ce⁴⁺ is stable because it achieves which electronic configuration?
Which pair of elements from the 4d and 5d series has nearly identical atomic radii due to lanthanoid contraction?
Eu²⁺ is unusually stable among lanthanoid divalent ions. What is the electronic configuration of Eu²⁺ that explains this stability?
As a consequence of lanthanoid contraction, which property of 4d and 5d transition metals in the same group becomes nearly equal?
A student claims: "Since Hf is one full period below Zr, it must have significantly larger atomic radius." What concept directly refutes this prediction?
Among the lanthanoid ions Sm³⁺ (4f⁵), Eu³⁺ (4f⁶), Gd³⁺ (4f⁷), and Tb³⁺ (4f⁸), which ion would be LEAST likely to deviate from the +3 oxidation state and show a +2 or +4 state?
Pattern: Lanthanoid contraction — effect on 4d/5d element properties (P.CHE.U11.LANTHANOID_CONTRACTION, observed in PYQ 2022, 2025).
Given
The following atomic radii are provided: - Zr (4d, Group 4): 160 pm - Hf (5d, Group 4): 159 pm - Ti (3d, Group 4): 147 pm
Required
Explain why Hf, despite being one period below Zr, has nearly the same atomic radius. Predict whether Hf and Zr would show similar or different chemical properties.
Concept
Lanthanoid contraction: across Ce (Z = 58) to Lu (Z = 71), poor shielding by 4f electrons causes a cumulative decrease in atomic radius (~12–15 pm total). This contraction offsets the expected increase from adding a new electron shell.
Formula/Principle
No quantitative formula — this is a qualitative comparison. The principle: if two elements in the same group have nearly identical atomic/ionic radii, they will exhibit similar lattice energies, bond strengths, and overall chemical behaviour.
Substitution/Application
- Expected: Hf should be ~10–15 pm larger than Zr (one extra shell, analogous to Ti→Zr increase of 13 pm). - Actual: Hf is 159 pm ≈ 160 pm (Zr). The "missing" expansion is absorbed by lanthanoid contraction across the 14 lanthanoid elements.
Calculation
Radius difference: |160 − 159| = 1 pm ≈ negligible. Compare with Ti→Zr difference: 160 − 147 = 13 pm (a full shell increase without intervening f-block).
Final answer
Lanthanoid contraction explains why Hf and Zr have nearly identical radii (difference ~1 pm vs expected ~13 pm). Consequence: Hf and Zr exhibit remarkably similar chemical properties — they form similar compounds, have similar coordination numbers, and are difficult to separate.
Common trap
Students predict Hf should be "much larger" because it's in period 6. They forget the 14-element lanthanoid contraction that intervenes between the 4d and 5d series. This trap appears as a distractor: "5d elements are always larger than 4d elements in the same group."
Similar NEET-style question
"Nb and Ta have very similar atomic radii. Which of the following best explains this observation? (A) d-orbital expansion (B) Actinoid contraction (C) Lanthanoid contraction (D) Relativistic effects" Answer: (C). The same principle applies to all 4d/5d pairs in Groups 4–7. ---
Steady decrease in atomic/ionic size across lanthanide series due to imperfect shielding by f-electrons. Causes 4d ≈ 5d size; explains close properties of Zr/Hf, Nb/Ta, Mo/W (4d/5d pairs).
-- NCERT Class 12 Chemistry, Ch. 4, p. 24Magnetic moment from n unpaired electrons. 1 unpaired: 1.73 BM; 5: 5.92 BM.
| Symbol | Quantity | SI Unit |
|---|---|---|
| n | unpaired electrons | - |
| mu | magnetic moment | Bohr magneton |
Predicts paramagnetic moment of d-block ion. n unpaired electrons in d-orbitals.
| Symbol | Quantity | SI Unit |
|---|---|---|
| n | unpaired electrons | - |
| mu | magnetic moment | BM |
Catalogues common stable oxidation states across first-row transition metals.
| Symbol | Quantity | SI Unit |
|---|---|---|
| OS | oxidation state | - |
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Inorganic Exception
Student assumes Mn²⁺ is the product regardless of medium. Acidic: → Mn²⁺ (5e⁻). Neutral/weakly basic: → MnO₂ (3e⁻). Strongly basic: → MnO₄²⁻ (1e⁻).
Question gives KMnO4 oxidation in unspecified or specific medium.
Always check medium. In acidic: Mn(+7) → Mn(+2). In neutral: → Mn(+4) (MnO₂). In basic: → Mn(+6) (manganate). The number of electrons (n) in Nernst calculations depends accordingly.
Root cause: concept gap
Acidic: → Mn²⁺ (5e⁻). Neutral/weakly basic: → MnO₂ (3e⁻). Strongly basic: → MnO₄²⁻ (1e⁻).
Root cause: concept gap
Lanthanoid contraction (imperfect 4f shielding) makes 5d elements similar in size to 4d. Zr/Hf, Nb/Ta, Mo/W have nearly identical chemistry.
10 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
The correct order of decreasing basic strength of the given amines is:
The pair of lanthanoid ions which are diamagnetic is
Which one of the following statements is correct?
Gadolinium has a low value of third ionisation enthalpy because of
Tritium, a radioactive isotope of hydrogen, emits which of the following particles? (β–)
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
uses acidic formula in basic medium
Assumes Mn²⁺ regardless of medium
predicts large difference due to period shift
Expects 5d to be much larger than 4d
misses d0 d10 stability
Doesn't account for closed-shell stability
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