The elements in which the last electron enter the ante-penultimate 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?

The elements of both the series are radioactive

The correct answer is option 3. The elements of both the series are radioactive.

Let us delve into the details of each statement regarding the lanthanoids and actinoids:

1. Oxidation state of +3 is predominant in both the series

The elements in the lanthanoid series (elements 57 to 71, from lanthanum to lutetium) predominantly exhibit a +3 oxidation state. This is because the removal of three electrons (two from the 6s orbital and one from the 5d or 4f orbital) results in a relatively stable electron configuration. For example, for cerium (Ce), the configuration changes from [Xe] 4f¹ 6s² to [Xe] after losing three electrons.

Similarly, the actinoids (elements 89 to 103, from actinium to lawrencium) predominantly exhibit a +3 oxidation state. This is due to the removal of electrons from the 7s and 5f orbitals. For example, uranium (U) changes from [Rn] 5f³ 6d¹ 7s² to [Rn] 5f³ after losing three electrons.

2. In both the series, f-orbitals are being progressively filled

In the lanthanoid series, electrons are added to the 4f orbitals as we move across the period. For example, lanthanum (La) starts with [Xe] 5d¹ 6s², and by the time we reach lutetium (Lu), the configuration is [Xe] 4f¹⁴ 5d¹ 6s².

In the actinoid series, electrons are added to the 5f orbitals. For instance, actinium (Ac) has the configuration [Rn] 6d¹ 7s², and by the time we reach lawrencium (Lr), the configuration is [Rn] 5f¹⁴ 6d¹ 7s².

3. The elements of both the series are radioactive

Most of the lanthanoids are not radioactive. They have stable, non-radioactive isotopes, except for promethium (Pm). Promethium has no stable isotopes and is naturally radioactive. The other lanthanoids like neodymium (Nd), samarium (Sm), and europium (Eu) have stable isotopes and are not radioactive under normal conditions.

All actinoids are radioactive. This is due to their large atomic numbers, which lead to a greater number of protons in the nucleus. The strong electrostatic repulsion between these protons makes their nuclei unstable, leading to radioactive decay. Elements like uranium (U), thorium (Th), and plutonium (Pu) are well-known for their radioactive properties.

4. Both the series show contraction known as lanthanoid and actinoid contraction

As we move from lanthanum (La) to lutetium (Lu) in the lanthanoid series, the atomic and ionic radii decrease. This is known as lanthanoid contraction. The poor shielding effect of the 4f electrons means the increasing nuclear charge pulls the outer electrons closer to the nucleus, reducing the size of the atoms and ions.

Similarly, in the actinoid series, there is a gradual decrease in the size of the atoms and ions from actinium (Ac) to lawrencium (Lr). This is called actinoid contraction. The 5f electrons do not shield the nuclear charge effectively, leading to a stronger pull on the outer electrons and thus a decrease in atomic and ionic radii.

While the first, second, and fourth statements are correct, the third statement is not correct. Only the actinoids are universally radioactive due to their large atomic numbers and instability. In contrast, most of the lanthanoids are stable, with the notable exception of promethium, which is radioactive. This differentiation is key to understanding the unique properties of these two series of elements.