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CodeVID-C12-08-T3-01
Assignment — f-Block: Lanthanoids & Actinoids
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all working for Sections B, C and D. Only final answers are given at the end — for full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
Lanthanoids involve the filling of:
- A.$3d$ orbitals
- B.$4f$ orbitals
- C.$5f$ orbitals
- D.$5d$ orbitals
2.
The contraction in size across the lanthanoids is called:
- A.actinoid contraction
- B.lanthanoid contraction
- C.inert pair effect
- D.shielding
3.
Which ion is colourless?
- A.$\text{Pr}^{3+}$
- B.$\text{Nd}^{3+}$
- C.$\text{Lu}^{3+}$
- D.$\text{Sm}^{3+}$
4.
All actinoids are:
- A.colourless
- B.diamagnetic
- C.radioactive
- D.gases
5.
The stable +4 state of cerium corresponds to the configuration:
- A.$4f^1$
- B.$4f^0$
- C.$4f^7$
- D.$4f^{14}$
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Write the general electronic configuration of the actinoids.
7.
Why is $\text{Eu}^{2+}$ relatively stable?
8.
State why $\text{La}^{3+}$ is colourless.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
Explain lanthanoid contraction and its cause.
10.
List three consequences of lanthanoid contraction.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Compare lanthanoids and actinoids with respect to electronic configuration, oxidation states, colour/magnetism and radioactivity.
Answer Key
Section A — Multiple Choice Questions
- (B) $4f$ orbitals
- (B) lanthanoid contraction
- (C) $\text{Lu}^{3+}$
- (C) radioactive
- (B) $4f^0$
Section B — Short Answer (2 marks)
- $[\text{Rn}]5f^{1-14}\,6d^{0-1}\,7s^2$.
- Europium attains the half-filled $4f^7$ configuration in the +2 state, which is extra-stable.
- $\text{La}^{3+}$ is $4f^0$; with no f electrons, no f-f transition is possible.
Section C — Short Answer (3 marks)
- A steady decrease in atomic/ionic radii from La to Lu; caused because each added $4f$ electron shields the outer electrons poorly, so the effective nuclear charge increases and pulls them inward.
- Nearly equal radii of 4d and 5d elements (Zr ≈ Hf) making separation difficult; steady fall in basicity of $\text{Ln(OH)}_3$; high densities of third-row transition metals.
Section D — Long Answer (5 marks)
- Configuration: lanthanoids fill $4f$ ($[\text{Xe}]4f^{1-14}5d^{0-1}6s^2$), actinoids fill $5f$ ($[\text{Rn}]5f^{1-14}6d^{0-1}7s^2$). Oxidation states: lanthanoids mainly +3 (occasional +2/+4); actinoids show +3 to +6/+7 because $5f$ electrons are loosely held. Colour/magnetism: both give coloured, paramagnetic ions via f-f transitions (with $f^0$/$f^{14}$ colourless), but actinoid spectra are broader. Radioactivity: lanthanoids are essentially non-radioactive (except Pm) while all actinoids are radioactive, with transuranium elements being man-made.
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