In f-block elements, the electrons enter in which of the following shell?

Antipenultimate shell

The correct answer is option 2. Antipenultimate shell.

Let us break this down clearly with an explanation of the electron configurations and the terminology used for the shells: Shells and Subshells

Principal Quantum Number (n):

The principal quantum number \( n \) indicates the main energy level (shell) of an atom.

For example, \( n = 1, 2, 3, 4, \ldots \).

Subshells:

Within each shell, there are subshells designated by the angular momentum quantum number \( l \). For f-block elements, \( l = 3 \) (f subshell).

f-block Elements

Lanthanides: These elements fill the 4f subshell.

Actinides: These elements fill the 5f subshell.

Penultimate Shell:

The second-to-last shell (n-1).  Example: For \( n = 6 \), the penultimate shell is \( n-1 = 5 \).

Antipenultimate Shell:

The third-to-last shell (n-2). Example: For \( n = 6 \), the antipenultimate shell is \( n-2 = 4 \).

Electron Configuration in f-block Elements

For lanthanides (period 6):

The principal quantum number \( n = 6 \). Electrons are added to the 4f subshell. This subshell belongs to the \( (6-2) = 4 \) shell, which is the antipenultimate shell.

For actinides (period 7):

The principal quantum number \( n = 7 \). Electrons are added to the 5f subshell. This subshell belongs to the \( (7-2) = 5 \) shell, which is the antipenultimate shell.

Summary

The f-block elements have electrons filling the f subshells of the shell that is two levels below the outermost shell. This subshell is in the antipenultimate shell (n-2).

Therefore, the electrons in f-block elements enter the antipenultimate shell.