Questions & explanations
1. Which of the following correctly differentiates actinoids from lanthanoids?
- A) Actinoids are paramagnetic while lanthanoids are diamagnetic
- B) Actinoids show greater tendency to form complexes than lanthanoids
- C) Lanthanoids have higher ionization enthalpies than actinoids
- D) Lanthanoids exhibit more number of oxidation states than actinoids
Answer: B) Actinoids show greater tendency to form complexes than lanthanoids
Actinoids have a greater tendency to form complexes due to higher nuclear charge and smaller size of ions, as well as more availability of f, d, and s orbitals for bonding. Option A is false because both can be paramagnetic. Option C is not a consistent differentiation; actinoids generally have lower ionization enthalpies. Option D is false because actinoids exhibit more oxidation states.
2. Which factor primarily explains why actinoids exhibit a wider range of oxidation states compared to lanthanoids?
- Actinoids have larger atomic radii.
- The 5f orbitals are more effectively shielded than the 4f orbitals.
- The energy difference between the 5f, 6d, and 7s orbitals is smaller.
- Actinoids have a higher nuclear charge.
Answer: The energy difference between the 5f, 6d, and 7s orbitals is smaller.
In actinoids, the 5f, 6d, and 7s orbitals are closer in energy due to poorer shielding and relativistic effects. This allows electrons from multiple orbitals to participate in bonding, resulting in a greater number of oxidation states (e.g., +3, +4, +5, +6, +7) compared to lanthanoids, where the 4f orbitals are more deeply buried and the energy gap to the d and s orbitals is larger.
3. What is the special feature of actinoid contraction compared to lanthanoid contraction?
- It is less pronounced due to greater shielding of 5f electrons.
- It is greater in magnitude due to poor shielding of 5f electrons.
- It is uniform across the series.
- It is absent in the first half of the series.
Answer: It is greater in magnitude due to poor shielding of 5f electrons.
Actinoid contraction is the gradual decrease in atomic/ionic radii across the actinoid series. The 5f orbitals are less penetrating and provide poorer shielding of the nuclear charge than 4f orbitals, leading to a stronger effective nuclear pull on the outer electrons. Consequently, the contraction is more pronounced than the lanthanoid contraction.
4. Lanthanoid contraction refers to the steady decrease in atomic and ionic radii of lanthanoids with increasing atomic number. This phenomenon is primarily attributed to:
- A) Increase in nuclear charge with poor shielding of 4f electrons
- B) Increasing number of protons in the nucleus
- C) Regular decrease in the number of electrons
- D) Weak metallic bonding
Answer: A) Increase in nuclear charge with poor shielding of 4f electrons
As we move along the lanthanoid series, the nuclear charge increases. The 4f electrons are poorly shielding because of their diffuse shape, so the effective nuclear charge experienced by outer electrons increases, causing a contraction in size. Options B, C, D do not correctly explain the cause.
5. Which of the following is a direct consequence of lanthanoid contraction?
- A) Similar atomic radii of Zr (Z=40) and Hf (Z=72)
- B) Increased reactivity of lanthanoids
- C) Decrease in ionization enthalpy across the series
- D) Variable oxidation states of lanthanoids
Answer: A) Similar atomic radii of Zr (Z=40) and Hf (Z=72)
Lanthanoid contraction causes the radii of elements after the lanthanoid series to be similar to those of their earlier counterparts. Specifically, Hf has almost the same atomic radius as Zr due to the contraction. Options B, C, D are not consequences of lanthanoid contraction.
6. Among the following lanthanoid ions, the one that is colourless is:
- A) Pr^3+ (Z=59)
- B) Nd^3+ (Z=60)
- C) La^3+ (Z=57)
- D) Eu^3+ (Z=63)
Answer: C) La^3+ (Z=57)
Colour in lanthanoid ions arises from f-f transitions due to absorption in the visible region. La^3+ has a 4f^0 configuration (no f electrons) and therefore cannot undergo f-f transitions, making it colourless. The other ions have unpaired f electrons and exhibit colour.
7. In the periodic table, the lanthanoids (or lanthanides) are placed in:
- A) Group 3, Period 6
- B) Group 3, Period 7
- C) The same row as actinoids
- D) The f-block, which is placed separately below the main table
Answer: D) The f-block, which is placed separately below the main table
Lanthanoids (elements 58-71) and actinoids (90-103) are f-block elements placed in two rows below the main periodic table. They are not placed in specific periods of the main table but are often shown as separate rows. Option D accurately describes their placement.
8. The most predominant and stable oxidation state exhibited by lanthanoids is:
- A) +2
- B) +3
- C) +4
- D) +5
Answer: B) +3
Lanthanoids predominantly show a +3 oxidation state because after losing two 6s electrons and one 4f electron (or one 5d electron), they achieve a stable configuration. Though +2 and +4 are also known for some, +3 is the most common and stable.
9. The general electronic configuration of the f-block elements is:
- A) (n-2)f^(1-14) (n-1)d^(0-1) ns^2
- B) (n-1)d^(1-10) ns^2
- C) ns^2 np^1-6
- D) (n-2)f^(1-14) ns^2 np^6
Answer: A) (n-2)f^(1-14) (n-1)d^(0-1) ns^2
f-block elements have the general configuration (n-2)f^(1-14) (n-1)d^(0-1) ns^2, where n=6 for lanthanoids and n=7 for actinoids. Options B, C, D correspond to d-block, p-block, and incorrect f-block configurations respectively.
10. Which of the following statements about the oxidation states of actinoids is correct?
- A) Actinoids show only +3 oxidation state
- B) Actinoids show lower oxidation states compared to lanthanoids
- C) Actinoids exhibit a wide range of oxidation states from +3 to +7
- D) Actinoids never exceed +4 oxidation state
Answer: C) Actinoids exhibit a wide range of oxidation states from +3 to +7
Unlike lanthanoids, actinoids show variable oxidation states because of the small energy gap between 5f, 6d, and 7s orbitals. Common states are +3, +4, +5, +6, and +7 (e.g., Np, Pu, Am). Options A, B, D are incorrect.