Question

Which of the following pairs of ions are isoelectronic and isostructural?

(A) $C{O_3}^{2-}$, $N{O_3}^{-}$

(B) $Cl{O_3}^{-}$, $S{O_3}^{2-}$

(C) $Cl{O_3}^{-}$, $C{O_3}^{2-}$

(D) $S{O_3}^{2-}$, $N{O_3}^{-}$

Answer 1

Hint: Write the expanded structure of the above ions. Isoelectronic species have the same number of electrons which is the sum of electrons of all the atoms present in the ion. Now isostructural as the name suggests means that the ions have the same shape and arrangement. The shape is irrespective of the atoms attached to the central atom.

Complete answer:

In the options provided, we can clearly identify 4 distinct ions. We will draw the expanded structure of these 4 ions,  $ ClO_{3}^{-} $ ,  $ CO_{3}^{2-} $ ,  $ SO_{3}^{2-} $ ,  $ NO_{3}^{-} $ . With this we will determine the number of electrons in the valence shell and its structure based on the valence bond theory.

$NO_{3}^{-}$:

Total number of electrons in valence shell: 7 + 2 + 24 = 33

Hybridization:  $ s{{p}^{2}} $ 

Shape : trigonal planar

$CO_{3}^{2-}$:

Total number of electrons in valence shell: 6 + 2 + 24 = 32

Hybridization:  $ s{{p}^{2}} $ 

Shape : trigonal planar

$SO_{3}^{2-}$:

Total number of electrons in valence shell: 16 + 2 + 24 = 42

Hybridization:  $ s{{p}^{3}} $ 

Shape : tetrahedral

$ClO_{3}^{-}$:

Total number of electrons in valence shell: 17 + 24 + 1 = 42

Hybridization:  $ s{{p}^{3}} $ 

Shape : tetrahedral

From the above explanation we can conclude that the pair of isoelectronic and isostructural species is  $ ClO_{3}^{-} $ ,  $ SO_{3}^{2-} $ .

Therefore, the correct answer is option (B).

Note:

It is important to understand that while number of electrons we need to take the surrounding atoms into consideration as well and not just the central atom. This is because we consider all the atoms and ions as one single system while checking for isoelectronic species.