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.
Consider the following statements about the actinoids.
(A) The actinoid contraction is greater from element to element than the lanthanoid contraction.
(B) Hydrochloric acid attacks all the actinoid metals, but most of them are only slightly affected by nitric acid.
(C) The magnetic properties of the actinoids are simpler than those of the lanthanoids.
(D) The ionisation enthalpies of the early actinoids are higher than those of the early lanthanoids.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) and (B) only
In the actinoids (Th to Lr) the 5f orbitals are filled. The 5f electrons are less firmly held than the 4f electrons of the lanthanoids, and this explains most of the differences between the two series.
(A) The actinoid contraction is greater from element to element than the lanthanoid contraction. — True. The size of the atoms and M3+ ions falls across both series, but the fall is larger in the actinoids because of poor shielding by 5f electrons.
(B) Hydrochloric acid attacks all the actinoid metals, but most of them are only slightly affected by nitric acid. — True. The actinoids are highly reactive metals, but nitric acid forms a protective oxide layer on most of them.
(C) The magnetic properties of the actinoids are simpler than those of the lanthanoids. — False. The magnetic properties of the actinoids are more complex. Their magnetic susceptibility varies with the number of unpaired 5f electrons roughly as in the lanthanoids, but the lanthanoids have higher values.
(D) The ionisation enthalpies of the early actinoids are higher than those of the early lanthanoids. — False. They are lower. The 5f orbitals penetrate less into the inner core, so the 5f electrons are more effectively shielded from the nuclear charge and are removed more easily.
NCERT (2026-27 edition):
4.6.2 Ionic Sizes – “The general trend in lanthanoids is observable in the actinoids as well. There is a gradual decrease in the size of atoms or M3+ ions across the series. This may be referred to as the actinoid contraction (like lanthanoid contraction). The contraction is, however, greater from element to element in this series resulting from poor shielding by 5f electrons.”
4.6.4 General Characteristics and Comparison with Lanthanoids – “The actinoids are highly reactive metals, especially when finely divided. … Hydrochloric acid attacks all metals but most are slightly affected by nitric acid owing to the formation of protective oxide layers; alkalies have no action.”
“The magnetic properties of the actinoids are more complex than those of the lanthanoids.”
“It is evident from the behaviour of the actinoids that the ionisation enthalpies of the early actinoids, though not accurately known, but are lower than for the early lanthanoids.”