The f-block elements are those in which the differentiating electron enters the (n-2)f orbitals. There are two series of f-Block elements corresponding to filling of 4f and 5f-orbitals. The series of 4f-orbitals is called lanthanides. Lanthanides show different oxidation states depending upon stability of f0, f7 and f14 configurations, though the most common oxidation state is +3. There is a regular decrease in the size of lanthanide ions with increase in atomic number which is known as lanthanide contraction.
Which of the following statements is incorrect?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The different radii of Zr and Hf are a consequence of the lanthanide contraction.
Lanthanide contraction (also called lanthanoid contraction) is the steady fall in the size of atoms and ions from La to Lu.
Along the series each new electron enters a 4f orbital, and a 4f electron shields the others poorly, so the outer electrons feel a larger nuclear pull at every step.
Hf (5d series) comes right after the lanthanides, so this contraction cancels the size increase expected on going down from Zr (4d series) to Hf: Zr (160 pm) and Hf (159 pm) have almost identical radii, not different ones.
Option (1) Lanthanide contraction is the accumulation of successive shrinkages. — A correct statement. The small decrease from one element to the next adds up along the series: the lanthanide contraction is the cumulative effect of these shrinkages.
Option (2) The different radii of Zr and Hf are a consequence of the lanthanide contraction. — An incorrect statement. The lanthanide contraction makes the radii of Zr and Hf almost the same, which is why they occur together in nature and are hard to separate.
Option (3) Shielding power of 4f electrons is quite weak. — A correct statement. The shielding of one 4f electron by another is even less than that of one d electron by another; this weak shielding is the cause of the contraction.
Option (4) The radii of the atoms and ions decrease from La to Lu. — A correct statement. This decrease from lanthanum to lutetium is the lanthanide contraction itself.
NCERT (2026-27 edition):
4.5.2 Atomic and Ionic Sizes – “… The almost identical radii of Zr (160 pm) and Hf (159 pm), a consequence of the lanthanoid contraction, account for their occurrence together in nature and for the difficulty faced in their separation.”