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.

What is the prominent oxidation state of lanthanoid?

+3

The correct answer is option 1. +3.

The lanthanoids (or lanthanides) are a series of elements in the periodic table that range from lanthanum (La) to lutetium (Lu). The most prominent oxidation state for these elements is +3. Here’s a detailed explanation of why this is the case:

The general electron configuration of lanthanoids is [Xe] 4f^n 5d^0-1 6s^2, where \( n \) ranges from 1 to 14 as you move from lanthanum to lutetium.

Stability of the +3 Oxidation State

1. Energy Considerations:

Removing the first two 6s electrons and one 4f or 5d electron to achieve a +3 oxidation state requires less energy compared to removing additional electrons. This is because the 4f orbitals are relatively deep within the electron cloud and well-shielded by the 5s and 5p electrons.

2. Ionic Radius and Effective Nuclear Charge:

In the +3 state, lanthanoid ions have a high charge density, meaning they have a relatively small ionic radius for a given charge. This high charge density stabilizes the +3 oxidation state.  Additionally, as electrons are removed, the effective nuclear charge (the net positive charge experienced by electrons) increases, which further stabilizes the +3 state.

3. Crystal Field Stabilization:

The +3 ions of lanthanoids exhibit significant crystal field stabilization when they form compounds, such as oxides and halides, which further stabilizes the +3 oxidation state.

Examples of +3 Oxidation State

Lanthanum (La): The electron configuration is \([Xe] 5d^1 6s^2\), and in the \(+3\) state, it becomes \([Xe]\), a stable configuration.

Neodymium (Nd): The electron configuration is \([Xe] 4f^4 6s^2\), and in the \(+3\) state, it becomes \([Xe] 4f^3\).

Gadolinium (Gd): The electron configuration is \([Xe] 4f^7 5d^1 6s^2\), and in the \(+3\) state, it becomes \([Xe] 4f^7\), which is half-filled and particularly stable.

Other Oxidation States

While +3 is the most stable and common oxidation state, some lanthanoids can exhibit other oxidation states such as +2 and +4, but these are less common:

+2 Oxidation State: This occurs in a few lanthanoids where the 4f^n configuration can provide additional stability (e.g., Eu^2+ with a half-filled 4f^7 configuration).

+4 Oxidation State: This is less common and occurs in a few cases where additional stabilization is possible (e.g., Ce^4+).

The prominent oxidation state of lanthanoids is +3 due to the relatively low energy required to remove three electrons, the stability provided by the resulting electron configuration, and the favorable ionic size and charge density. The +3 oxidation state leads to stable compounds and is energetically favorable compared to other possible oxidation states.