Question

In most of the lanthanides, the penultimate shell contains ____ electrons in s,p orbitals.
A.6
B.7
C.8
D.10

Answer 1

We know that the lanthanides are the element of the primary inner transition series. The starting element of this series is lanthanum. So, the series is known as lanthanides series. The 4f is the first $f$ orbital to be filled. It’s energy is higher than $6s$ orbital which is valence within the lanthanides.

Complete step by step answer:
As we know the lanthanides are the elements of the inner transition series. These elements start with lanthanum because the first element. So, named lanthanides the inner transition elements as f-block elements and that they involve the filling of $f -$ orbitals. There are two series of inner transition elements that involve filling of $4f$ and $5d$ orbitals.
First allow us to see the penultimate shell: Penultimate shell is inner to valence shell in which penultimate shell filled first and valence shell filled next. It will have a variety one less than valence shell. If the valence shell has a ‘n’ value, the value of the penultimate shell will become (n-1).
Now from the given question: It is considered that the $5d$ orbitals remain vacant and therefore the electrons enter into the $4f$ orbital. Hence, as electrons enter the n-1 shell it is called a penultimate shell.
Hence, before filling $d$ orbital surely it might have filed s and p orbitals during which we will accommodate 8 electrons. Therefore, in the lanthanides series, the penultimate shell contains 8 electrons in s and p orbital.

So, the correct answer is Option C.

Note:
It must be noted that the electrons are filled within the subshells based on the aufbau principle when it says that the orbitals must be filled in order of increasing energy level. It isn't that first all 2nd shell orbitals will be filed and then 3rd and then on.