Sample 1Easy
The principal oxidation state shown by the lanthanoids is
- A
+2
- B
+4
- C
+3
- D
+1
Show the answer
The answer is C.Lanthanoids principally exhibit the +3 oxidation state. Some show +2 or +4 occasionally, for example Eu2+ and Ce4+, when this leads to an empty, half-filled or filled f subshell.
- A.
+2 occurs only occasionally, as in Eu2+ and Yb2+.
- B.
+4 occurs only occasionally, as in Ce4+.
- D.
+1 is not a characteristic oxidation state of the lanthanoids.
NCERT: Class 12 Chemistry, Unit 4 (The d- and f-Block Elements), The Lanthanoids: Oxidation States
Sample 2Medium
A graph of the ionic radius of M³⁺ (y-axis) against atomic number (x-axis) for the lanthanoids from La³⁺ (Z = 57) to Lu³⁺ (Z = 71) shows a steady, smooth fall. What causes this fall, and what is one consequence of it?
- A
Imperfect shielding by 4f electrons; zirconium and hafnium have very different radii
- B
Imperfect shielding of one 4f electron by another as nuclear charge rises; zirconium and hafnium have almost the same radii
- C
Addition of a new electron shell at each step; zirconium and hafnium differ greatly in radius
- D
Very effective shielding by 4f electrons; the lanthanoids are easy to separate from one another
Show the answer
The answer is B.Moving from La to Lu, each added 4f electron shields the others poorly from the rising nuclear charge, so the radius shrinks steadily. As a result the third transition series elements have radii almost equal to those of the second, as with Zr and Hf.
- A.
The cause is right, but the contraction makes the radii of Zr (160 pm) and Hf (159 pm) almost equal.
- C.
Across the lanthanoids electrons enter the same 4f subshell; no new shell is added, and the radius falls rather than rises.
- D.
The 4f electrons shield poorly, and the similar radii make the lanthanoids hard to separate.
NCERT: Class 12 Chemistry, Chapter 4