The elements in which the last electron enters the antepenultimate energy level, i.e., (n - 2)f-orbital are called f-block elements. These elements have been termed as f-block elements as the last electron enters in one of the f-orbitals. These elements are also known as the inner transition elements. This is because the last electron in them enters into (n - 2)-orbital, i.e., inner to the penultimate energy level and they form a transition series within the transition series. The general electronic configuration is:
\[(n - 2)f^{1-14}(n - 1)d^{0-1}ns^2\]
Classification of f-block elements:
Depending upon whether the last electron enters a 4f-orbital or a 5f-orbital, the f-block elements have been divided into two series as follows:
(i) Lanthanides: The elements in which the last electron enters one of the 4f-orbitals are called 4f-block elements or first inner transition series. These are also called lanthanides or lanthanons or lanthanoids because they come immediately after lanthanum.
(ii) Actinides: The elements in which the last electron enters one of the 5f-orbitals are called 5f-block elements or second inner transition series. These are also called actinides or actinons or actinoids because they come immediately after actinium.
Which of the following statements is not correct regarding lanthanoids and actinoids?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The elements of both the series are radioactive
The f-block has two series: the lanthanoids, in which the 4f orbitals are filled, and the actinoids, in which the 5f orbitals are filled. Radioactivity is a mark of the actinoid series and one reason why its chemistry is harder to study.
Option (1) The actinoids show a greater range of oxidation states than the lanthanoids — Correct about lanthanoids and actinoids. The lanthanoids keep mainly to the +3 state, while in the actinoids the 5f, 6d and 7s levels are of comparable energies, so they show a wider range of oxidation states.
Option (2) In both the series, f-orbitals are progressively filled — Correct about lanthanoids and actinoids. Across the lanthanoids the 4f orbitals fill, and across the actinoids the 5f orbitals fill.
Option (3) The elements of both the series are radioactive — Not correct about lanthanoids and actinoids. The actinoids are radioactive elements, but radioactivity is not a property of the lanthanoid series as a whole, so the statement fails for “both the series”.
Option (4) Both the series show a contraction in size, known as the lanthanoid contraction and the actinoid contraction respectively — Correct about lanthanoids and actinoids. The size of the atoms and M3+ ions falls gradually across each series; in the actinoids this is the actinoid contraction, like the lanthanoid contraction.
NCERT (2026-27 edition):
The Inner Transition Elements (f-Block) – “… The chemistry of the actinoids is, on the other hand, much more complicated. The complication arises partly owing to the occurrence of a wide range of oxidation states in these elements and partly because their radioactivity creates special problems in their study; the two series will be considered separately here.”
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).”