Question

An element with the electronic configuration $\left[ {{\text{Xe}}} \right]{\text{4}}{{\text{f}}^{\text{7}}}{\text{5}}{{\text{d}}^{\text{1}}}{\text{6}}{{\text{s}}^{\text{2}}}$ lies in the:
(A) S- block II-A group
(B) D- block III-B group
(C) F- block III-B group
(D) D- block VIII group

Answer 1

To answer this question, you must recall the positions of elements in the periodic table on the basis of their electronic configurations. The valence shell electrons of an element determine its properties to a large extent and this is the reason why elements of a group have similar physical and chemical properties.

Complete step by step solution:
We know that the periodic table can be majorly distinguished into four blocks, namely s- block, p- block d- block and f- block which correspond to the filling of the s- orbitals, p- orbitals, d- orbitals and f- orbitals respectively.
In a given shell, the first orbital to be filled is the s- orbital. After two electrons fill up the s- orbital, the p- orbitals are filled next. But period 4 onwards, the d- orbital of the penultimate shell starts filling p after the s- orbital of the valence shell followed by the p- orbitals. Period 6 onwards, after filling of only one d- orbital the f- orbital of the inner shell starts to fill up. And no more electrons are filled in the d- orbitals until the f- orbital is filled with 14 electrons. This order of filling of orbitals is given by the Auf- Bau principle.
From the above information we can see in the given electronic configuration, i.e. $\left[ {{\text{Xe}}} \right]{\text{4}}{{\text{f}}^{\text{7}}}{\text{5}}{{\text{d}}^{\text{1}}}{\text{6}}{{\text{s}}^{\text{2}}}$ , that there are 7 electrons in the $4f$ orbital. Thus, the element corresponds to the filling of the f- orbital. Thus, it belongs to the f- block
Thus, the correct option is C.

Note:
The Auf- bau principle: It states that atomic orbitals in an atom are filled in the increasing order of the energy level of the orbitals. The electronic configuration of an element is written according to the above principle and two other principles, namely the Hund’s rule of maximum multiplicity and the Pauli’s exclusion principle.