General Chemistry of ‘f’ Block Element – Chemistry Optional PYQs (2000–2026)

Here are all 53 General Chemistry of ‘f’ Block Element questions asked in UPSC Chemistry Optional Paper I from 2000 to 2026, arranged under the 2 topics of the official syllabus. Within each topic the latest questions come first.

Each question ends with a code such as [2024/15m/3b] – the year, the marks, the word limit (where UPSC set one) and the question number in the paper.

15.1] Lanthanides and actinides

41 questions

1. Why do $E{u}^{3+}$ and $S{m}^{3+}$ not follow the trend of magnetic moment of lanthanides at 300 K? Explain. [2026/5m/8c(i)]

2. How are lanthanides separated by a complex formation method? Give description. [2026/5m/8c(iii)]

3. Why do $C{e}^{3+}$ and $Y{b}^{3+}$ not absorb in the visible region, but show sharp absorption in the ultraviolet region? [2026/5m/8c(iv)]

4. The magnetic moment of d-block elements arises mainly from the contribution of spin motion of the electrons but in case of f-block elements, it is not valid. Justify the statement. [2025/5m/5h]

5. How many lanthanides can be easily separated from the lanthanide mixture by using valency change method? Justify your answer. [2025/10m/8c]

6. Write electronic configuration (outer) of the following lanthanide ions and calculate the magnetic moment in BM from L-S coupling. (i) $P{r}^{3+}$ (g = 4/5) (ii) $T{b}^{3+}$ (g = 3/2) [2025/10m/8d]

7. Briefly explain the principles involved in the following methods of separation of the lanthanides: (i) Repeated fractional crystallisation (ii) Solvent extraction (iii) Fractional precipitation (iv) Change of oxidation state. [2024/20m/8c]

8. Explain three main types of electronic transitions observed in UV-visible absorption spectra of actinide ions. [2023/10m/5d]

9. Write the ground-state electronic configuration of lanthanides mentioned below: (i) ${}_{59}Pr$ (Praseodymium) (ii) ${}_{63}Eu$ (Europium) (iii) ${}_{64}Gd$ (Gadolinium) Calculate the predicted effective magnetic moment (${\mu }_{S+L}$) for the metal ions in +3 oxidation state in the units of Bohr magneton. [2023/10m/7b]

10. Why are $4f$ metal ions (lanthanide ions) generally pale in colour? Why do they show line like electronic spectra? [2022/10m/6b]

11. The observed magnetic moments of lanthanide ions in general differ from observed magnetic moments of first row transition metal ions. Explain by giving reason(s). Identify the lanthanide ions having magnetic moments corresponding to spin-only value, and those which are diamagnetic. [2021/20m/8b]

12. Cerium (III) ion with $4f^1$ electronic configuration is colorless while cerium (IV) ion having $4f^0$ electronic configuration is red. Explain. [2020/10m/3b]

13. Calculate the magnetic moment of gadolinium (III) ion (in BM units). [2020/10m/8e]

14. (i) Suggest a method for the separation of heavy actinides like Am, Cm, Bk, Cf, Es, Fm and Md with appropriate sketch. (ii) Write the structure of $Th{\left(C_8{H}_8\right)}_2$ and that of $U{\left(C_8{H}_8\right)}_2$. [2019/10m/3c]

15. Find out the ground state term symbol of $H{o}^{3+}$ (Z = 67). [2018/10m/5e]

16. The commonly observed oxidation state of lanthanoids in aqueous solution is +3. – Comment. [2018/5m/7c(i)]

17. Calculate the effective magnetic moment of $C{e}^{3+}$ (Z = 58) at room temperature. [2018/10m/7c(ii)]

18. “Magnetic moment of Lanthanide (III) ions is generally different from the value predicted by spin state.” Explain. [2017/10m/5e]

19. “High oxidation states are more common in early actinides than lanthanides but the +2 oxidation state for americium and nobelium are more stable relative to those of other actinides.” Explain. [2017/10m/8b]

20. In general, the principal oxidation state of lanthanide elements is +3, but some elements also exhibit other oxidation states. Explain. [2016/10m/5e]

21. Why do lanthanoid ions have similar chemical properties even with variation of atomic number and have higher lifetime in excited states than transition metal ions? [2015/10m/5c]

22. (i) What property/properties of lanthanide $\beta$-diketone complexes make(s) them useful as shift reagents in NMR spectroscopy? (ii) What change in the position of iron in haemoglobin occurs in binding to oxygen and why? [2015/10m/8c]

23. Comment on the spectroscopic and magnetic properties of 4f elements. [2014/10m/7c]

24. The absorption spectra of lanthanide ions are sharper than those of ions of the first transition series. Explain. [2013/10m/7b]

25. (i) Borazine is a close analogue of benzene. Explain on the basis of preparation, molecular structure and reactions of borazine. (ii) Discuss magnetic and spectral properties of lanthanides. [2011/20m/6b]

26. How do you explain the magnetic properties of transition metal ions, $L{n}^{3+}$ by the Crystal Field Theory? [2010/15m/5b]

27. Why are 5f elements important? [2010/5m/8a(ii)]

28. (i) Mention the most stable oxidation state of each of the metals Ce, Eu, Th, U and Pu. (ii) “Magnetic susceptibility measurements are of little use in the structural elucidation of lanthanide complexes.” Justify the statement. [2009/20m/5d]

29. “Actinides are more prone to complex formation than lanthanides.” Justify the statement. [2008/10m/5d(i)]

30. What are actinides? Explain the oxidation state and complex forming tendency of actinides. [2007/20m/7b]

31. Lanthanides are generally trivalent but $C{e}^{4+}$ and $E{u}^{2+}$ are more stable. Explain this observation. [2006/20m/1c]

32. Name all the lanthanides and show how its common oxidation state is +3 though some of these exhibit other oxidation states as well. Explain why ionic radii decreases from $L{a}^{3+}$ to $L{u}^{3+}$ even though atomic number increases by 14. [2005/30m/6a]

33. Discuss the magnetic and spectral properties of lanthanides. How are they different from those of d-block elements? [2004/20m/7a]

34. Describe the modern methods adopted for the separation of the lanthanides. Discuss the important oxidation states of lanthanides. [2004/20m/7b]

35. Write the general expression for filling of electrons in 4f block elements. Justify the statement that 3+ oxidation state dominates the chemistry of lanthanides. Do some members of this series exhibit other oxidation states as well? Explain giving reasons. [2003/20m/5d]

36. Magnetic moment of lanthanides cannot be calculated based only on spin quantum number but by using quantum number J. Explain. [2003/30m/8a]

37. Compare the electronic configurations, spectral and magnetic properties of actinides with those of lanthanides. [2002/20m/7c]

38. The $f^1$ ion $C{e}^{III}$ is colourless in aqueous solution whereas the $d^1$ ion $T{i}^{III}$ is coloured. Explain. [2001/20m/7b]

39. +3 is the normal oxidation state of lanthanides but $E{u}^{+2}$ and $Y{b}^{+2}$ exist as the most stable species. Explain. [2001/20m/8c]

40. Lanthanides have little tendency to form complexes but when they form they can attain high coordination number of 7 or 8 even. Comment. [2000/20m/5c]

41. Lanthanides have little tendency to form complexes but their compounds are generally brilliantly coloured. Any explanation? [2000/20m/5d]

15.2] Lanthanide contraction

12 questions

1. What is actinide contraction? Explain. [2026/5m/5j]

2. Why do the metallic radii decrease from La to Lu except at Eu and Yb? [2026/5m/8c(ii)]

3. Explain lanthanide contraction. [2024/5m/5e(i)]

4. Hf is placed below Zr in the periodic table. Justify the statement. [2024/5m/5e(ii)]

5. Consider aqueous solutions of $LaC{l}_3$ (Lanthanum trichloride) and $LuC{l}_3$ (Lutetium trichloride). Which solution shows lower pH? Explain. [2022/10m/7b]

6. What do you understand by “Lanthanide contraction”? How does it affect the chemistry of 4d and 5d block elements. [2016/10m/7a(ii)]

7. Explain ‘lanthanide contraction’ underlying its relevance in periods other than 4f. [2014/10m/5c]

8. (i) Explain what is lanthanide contraction. (ii) $CuC{l}_2$ is coloured but $ZnC{l}_2$ is colourless. Explain. [2012/16m/8b]

9. Give electronic configuration of lanthanides and on the basis of this explain lanthanide contraction. [2011/20m/8a]

10. Explain why zirconium and hafnium occur together in nature. [2008/10m/5d(ii)]

11. Name all the lanthanides and show how its common oxidation state is +3 though some of these exhibit other oxidation states as well. Explain why ionic radii decreases from $L{a}^{3+}$ to $L{u}^{3+}$ even though atomic number increases by 14. [2005/30m/6a]

12. Lanthanides are responsible for the extreme similarity in the chemistry of zirconium and hafnium and also for the fact that their separation from each other has been a difficult task. Comment. [2000/20m/5b]

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Last updated: 30 October 2026