Consider the following statements about the f-block elements.
(A) Lanthanum is usually included in any discussion of the lanthanoids because it closely resembles them.
(B) The lanthanoids resemble one another less closely than do the members of an ordinary transition series.
(C) The earlier members of the actinoid series have relatively long half-lives.
(D) The later members of the actinoid series could be prepared only in nanogram quantities.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (C) and (D) only
The f-block has two series: the lanthanoids, which are very much alike, and the actinoids, which are radioactive and harder to study.
(A) Lanthanum is usually included in any discussion of the lanthanoids because it closely resembles them. — True. For this reason the general symbol Ln is often used for lanthanum and the lanthanoids together.
(B) The lanthanoids resemble one another less closely than do the members of an ordinary transition series. — False. They resemble one another more closely. They have only one stable oxidation state, so their chemistry shows the effect of small changes in size and nuclear charge along the series.
(C) The earlier members of the actinoid series have relatively long half-lives. — True. All actinoids are radioactive. The earlier ones have relatively long half-lives, while the later ones have half-lives from a day down to 3 minutes for lawrencium (Z = 103).
(D) The later members of the actinoid series could be prepared only in nanogram quantities. — True. These tiny amounts and the short half-lives make the study of the later actinoids more difficult.
NCERT (2026-27 edition):
The Inner Transition Elements (f-Block) – “The f-block consists of the two series, lanthanoids (the fourteen elements following lanthanum) and actinoids (the fourteen elements following actinium). Because lanthanum closely resembles the lanthanoids, it is usually included in any discussion of the lanthanoids for which the general symbol Ln is often used. … The lanthanoids resemble one another more closely than do the members of ordinary transition elements in any series. They have only one stable oxidation state and their chemistry provides an excellent opportunity to examine the effect of small changes in size and nuclear charge along a series of otherwise similar elements.”
4.6 The Actinoids – “The actinoids are radioactive elements and the earlier members have relatively long half-lives, the latter ones have half-life values ranging from a day to 3 minutes for lawrencium (Z =103). The latter members could be prepared only in nanogram quantities. These facts render their study more difficult.”