Question

The pair of ions having the same electronic configuration is?
(A) $C{{r}^{3+}}$,$F{{e}^{3+}}$
(B) $F{{e}^{3+}}$,$M{{n}^{2+}}$
(C) $F{{e}^{3+}}$,$C{{o}^{3+}}$
(D) $S{{c}^{3+}}$, $C{{r}^{3+}}$

Answer 1

Isoelectronic species are defined as those ions which have the same electronic configuration, that is they have the same number of electrons and protons. For isoelectronic species the size is determined by the number of protons present in it. The example of isoelectronic species is carbon monoxide and dinitrogen.

Complete step-by-step answer: the electronic configuration of the valence shell for the atoms and their ions are the following
So for option a, firstly the atomic number of Cr is 24 and electronic configuration of Cr is $Z=21:3{{d}^{0}}4{{s}^{2}}$ and the electronic configuration for $C{{r}^{3+}}$ is $3{{d}^{3}}$. The atomic number for Fe is 26 and the electronic configuration of Fe is $Z=26:3{{d}^{6}}4{{s}^{2}}$ and the electronic configuration for $F{{e}^{3+}}$ is $3{{d}^{5}}$. As we can see that both the ions have different electronic configurations so this option is incorrect.
Now for option b, firstly $3{{d}^{3}}$ the atomic number for Fe is 26 and the electronic configuration of Fe is $Z=26:3{{d}^{6}}4{{s}^{2}}$ and the electronic configuration for $F{{e}^{3+}}$is $3{{d}^{5}}$. The atomic number for Mn is 25 and the electronic configuration of Mn is $Z=25:3{{d}^{5}}4{{s}^{2}}$ and the electronic configuration for $M{{n}^{2+}}$ is $3{{d}^{5}}$. As the both ions have the same electronic configuration so they are isoelectronic species and the option ‘b’ is correct.
Now for option c, firstly the atomic number for Fe is 26 and the electronic configuration of Fe is $Z=26:3{{d}^{6}}4{{s}^{2}}$ and the electronic configuration for $F{{e}^{3+}}$is $3{{d}^{5}}$.The atomic number for Co is 27 and the electronic configuration for Co is $Z=27:3{{d}^{7}}4{{s}^{2}}$ and the electronic configuration for the $C{{o}^{3+}}$ is $3{{d}^{6}}$. As the electronic configurations of the ions are not same so this option is incorrect.
Now for option d, firstly the atomic number for Sc is 21 and the electronic configuration of Sc is $Z=21:3{{d}^{1}}4{{s}^{2}}$ and the electronic configuration for $S{{c}^{3+}}$is $3{{d}^{0}}$. the atomic number of Cr is 24 and electronic configuration of Cr is $Z=21:3{{d}^{0}}4{{s}^{2}}$ and the electronic configuration for $C{{r}^{3+}}$ is $3{{d}^{3}}$.As the electronic configuration of both the ions is not same so this option is incorrect.

The correct answer for this question is option (B).

Note: Electronic configuration determines the way the electrons are distributed in the orbitals. It helps in determining the valency and predicting the properties of the elements. It is written with the help of the subshells notation. The orbitals are filled by the Aufbau principle which states that the electron will occupy firstly the lower energies orbitals and then the higher energy orbitals.