Question

Which of the following are iso-electronic species?
A.$C{O_2},{\text{ }}N{O_2}$
B. $N{O_2}^ - ,{\text{ }}C{O_2}$
C. $C{N^ - },{\text{ }}CO$
D. $S{O_2},{\text{ }}C{O_2}$

Answer 1

We must know that the iso-electronic species are the ones which have similar electron configurations. Check the valence electron configuration.

Complete step by step answer:
Let’s start with discussing iso-electronic species. Iso-electronic species are the ones which have similar electron configurations. Like, ${O^{2 - }}$ is having similar electronic configurations as ${N^{ - 3}}$ which is $1{s^2}2{s^2}2{p^6}$.
But how to check electronic configuration is the case of molecules? To check we need to check the valence electrons of the molecules. So, let’s check the valence electron of molecules $C{O_2},{\text{ }}N{O_2},{\text{ }}N{O_2}^ - ,{\text{ }}C{N^ - },{\text{ }}CO{\text{ }} and {\text{ }}S{O_2}$. For $C{O_2}$ the number of valence electrons is $4+6+6$ which is $16$ valence electrons.
For $N{O_2}$ the number of valence electrons is $5+6+6$ which is $17$ valence electrons. For $N{O_2}^ -$ the number of valence electrons is $5+6+6+1$ which is 18. For $C{N^ - }$ the number of valence electrons is $4+5+1$ which is $10$. For $CO$ the number of valence electrons is $4+6$ which is $10$ valence electrons. For $S{O_2}$ the number of valence electrons is $6+6+6$ which is $18$ valence electrons.
So, the only two with similar valence electron configurations are $CO$ and $C{N^ - }$ which are $10$ valence electrons each.

Hence, the answer to this question is C..

$C{N^ - }and{\text{ }}CO$.

Note: Most of the times we get confused between iso-electronic species and iso-electric point. Both the terms are totally different in meaning, iso-electronic species are the one having the same electronic configuration whereas the iso-electric point is the point at which the net charge of any solution becomes 0.