Question

Lanthanides are characterized by the filling of the _ _ _ _ _ _ _ _ _ _.
(A) penultimate 4f energy level
(B) antepenultimate 4f energy level
(C) penultimate 5f energy level
(D) antepenultimate 5f energy level

Answer 1

Hint: The Lanthanides are the element of the first inner transition series. The starting element of this series is Lanthanum. So, the series is named as the Lanthanide series. The 4f is the first f-orbital to be filled. It has energy higher than 6s orbital which is valence in the Lanthanides.

Complete step by step solution:
The Lanthanides are the elements of the inner transition series. These elements start with Lanthanum as the first element. So, named lanthanides. The inner transition elements are called as f-block elements and they involve the filling of electrons in f- orbitals. There are two series of inner transition elements that involve filling of 4f and 5f orbitals. The first inner transition series starts with Lanthanum and is called the lanthanide series. It involves filling of 4f orbitals and the second inner transition series starts with actinium and involves filling of 5f orbitals.
So, the lanthanides series involve filling of 4f orbitals.
Now, the concept comes from whether this 4f is the penultimate.
First, let us see what are penultimate and antepenultimate shells.
Penultimate shell is the shell which is filled just before the valence shell i.e. the shell just inner to valence shell. It will have a number one less than valence shell. If the valence shell has value ‘n’ then the penultimate shell will have value (n-1).
Anti-penultimate shell is the shell which is filled two shells before the valence shell i.e. the shell inner to the just inner shell. It will have a number two less than valence shell. If the valence shell has value ‘n’ then the penultimate shell will have value (n-2).
We know that the Lanthanides have 6s as the valence shell. So, the 4f will be antepenultimate to it.
Thus, the Lanthanides are characterized by the filling of the anti-penultimate 4f energy level.

So, the option (B) is the correct answer.

Note:It must be noted that the electrons are filled in the shells based on the Aufbau principle which says that the orbitals must be filled in order of increasing energy level. The increasing energy level order is as -
1s,2s,2p,3s,3p,4s,3d,4p5s,4d,5p,6s,4f and so on.
It is not there that first all 2nd shell orbitals will be filled and then 3rd and so on. This is the wrong way.