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.

The separation of lanthanoids in ion exchange method is based on

size of lanthanoids

The correct answer is option 2. size of lanthanoids.

The ion exchange method for separating lanthanoids exploits the subtle differences in their ionic sizes, which are a result of the lanthanoid contraction. Here’s a detailed explanation:

As we move across the lanthanoid series from lanthanum (La) to lutetium (Lu), the atomic and ionic radii of the elements gradually decrease. This phenomenon is known as the lanthanoid contraction. The contraction occurs because the addition of electrons to the 4f orbitals does not effectively shield the increasing nuclear charge. As a result, the electrons are drawn closer to the nucleus, reducing the size of the ion.

Ion exchange chromatography is a technique used to separate ions based on their affinity to an ion exchange resin. The resin is typically composed of a polymeric material with charged functional groups that can attract and hold ions of the opposite charge.

Separation Process

Preparation: The lanthanoid ions, typically in the form of their trivalent cations (Ln³⁺), are introduced into the ion exchange column containing the resin.

Interaction with Resin: The lanthanoid ions interact with the resin. Due to the lanthanoid contraction, the smaller ions (those further along the series) bind more strongly to the resin compared to the larger ions (those at the beginning of the series). This difference in binding strength is primarily due to the size and charge density of the ions.

Elution: A suitable eluent, often a solution with varying pH or ionic strength, is passed through the column. The ions with weaker binding to the resin elute first, while the ions with stronger binding elute later. Because the binding strength correlates with the ionic size, the lanthanoids can be separated sequentially.

The size of the lanthanoid ions affects how tightly they bind to the resin:

Larger ions (e.g., La³⁺, Ce³⁺) have a lower charge density and form weaker interactions with the resin.

Smaller ions (e.g., Lu³⁺, Yb³⁺) have a higher charge density and form stronger interactions with the resin.

These differences in binding strengths due to the varying ionic sizes allow for the sequential separation of the lanthanoids.

The ion exchange method separates lanthanoids based on the subtle differences in their ionic radii, which directly affect their interaction with the ion exchange resin. This method efficiently exploits the size variation resulting from the lanthanoid contraction, allowing for the effective separation of these chemically similar elements.