Question

The element “X” has electronic configuration $\left[ Rn \right]6{{d}^{2}}7{{s}^{2}}$. It belongs to
A) f-block
B) s-block
C) p-block
D) d-block

Answer 1

Hint: Check the atomic number of the atom given.
Then consider the general electronic configuration of the block elements. The valence electron will decide the block that it falls into.

Complete Solution :
Here an element is give with the electronic configuration $\left[ Rn \right]6{{d}^{2}}7{{s}^{2}}$, and we have to find in which block does this element belongs to. For that let’s see the general electronic configuration of all the above blocks given,
A) For f - block the general electronic configuration is: ${{(n-2)}^{(0-14)}}f(n-1){{d}^{(0-1)}}n{{s}^{2}}$
B) For s - block elements the general electronic configuration: $n{{s}^{1-2}}$
C) For p - block elements, the general configuration is: $n{{s}^{2}}n{{p}^{1-6}}$
D) For d - block element, the general electronic configuration is: $(n-1){{d}^{(1-10)}}n{{s}^{2}}$$(n-1){{d}^{(1-10)}}n{{s}^{2}}$ ${{(n-2)}^{(0-14)}}f(n-1){{d}^{(0-1)}}n{{s}^{2}}$

- Now consider the atomic number of the given element, Radon (Rn) has an atomic number 86 and in this there are 4 electrons, two electrons in the s-orbital and d orbitals so,
X = $\left[ Rn \right]6{{d}^{2}}7{{s}^{2}}$ = 86 + 4 = 90
So, the element has an atomic number 90.
- The element with atomic number 90 is thorium. From elements 89-102, the elements are considered as f-block elements, but for the given configuration there are no f-subshells.
We usually define f-blocks as the elements, in which the electrons are filled in the valence f shell. But in f-block elements there are some exceptional cases too, i.e empty, half-filled and completely filled f-orbitals have special stability effect and to obtain special stability ,the electrons that should be filled in 5f orbitals are filled in 6d.
Thorium is an exceptional case in the actinide series.
So, the correct answer is “Option A”.

Additional information-
Actinides and lanthanides are the sub-groups of f-block elements.
Actinides are the elements from 89-102 and lanthanides are placed from 58-71.
Some of the actinides and lanthanide series elements show deviation from the general electronic configuration of the group.
In actinides the energy difference between 5f and 6d is very small for the first four elements I e thorium, Protactinium, Uranium and Neptunium, so the electrons may enter in 5f or 6d or may enter in both to yield extra stability.
So, the correct answer is “Option A”.

Note: While solving these types of questions we should calculate the atomic number first and then the special stability factors i.e. the empty, half-filled and completely filled subshell configuration should be a prior factor.