Question

Which of the following options are correct for isoelectronic pairs?
A. ${{\text{H}}_{2}}\text{,HD}$
B. $\text{CO, }{{\text{N}}_{2}}$
C. $\text{HCl, }{{\text{H}}_{2}}\text{S}$
D. ${{\text{D}}_{2}}\text{O, Ne}$

Answer 1

Hint: The term isoelectronic species needs to be defined first, as the name suggests ‘iso’ means the same and ‘electronic’ means electrons. So, atoms or ions that have the same electron configuration are isoelectronic in nature.

Complete step by step solution:
Two or more molecular entities are said to be isoelectronic if they have the same number of valence electrons and the same structure that is the number and connectivity of atoms but differ in elements involved. Let us now find the isoelectronic pairs, to find that we need to check options one by one-

A.${{\text{H}}_{2}}\text{,HD}$: We will count the total number of electrons in the elements described. One element is hydrogen and another is deuterium. Hydrogen and deuterium are isotopic species. The two differ in mass number but the number of protons and electrons present in their 1s shell is the same that is 1 electron. As both elements have the same number of electrons. So, the number of electrons present in ${{\text{H}}_{2}}$ is (1+1) or 2 electrons. Similarly, $\text{HD}$ also has 2 electrons. Thus, ${{\text{H}}_{2}}\text{ and HD}$ are isoelectronic.

B. $\text{CO, }{{\text{N}}_{2}}$: To find the total number of electrons in $\text{CO and }{{\text{N}}_{2}}$; we need to find the electrons present in their elements $\left( \text{C,O,N} \right)$ and then add. The electrons present in $\text{C}$are 6$\left( \text{1}{{\text{s}}^{2}}\text{2}{{\text{s}}^{2}}\text{2}{{\text{p}}^{2}} \right)$, the electrons in$\text{N}$ are 7 $\left( \text{1}{{\text{s}}^{2}}\text{2}{{\text{s}}^{2}}\text{2}{{\text{p}}^{3}} \right)$ and in$\text{O}$ are 8 $\left( \text{1}{{\text{s}}^{2}}\text{2}{{\text{s}}^{2}}\text{2}{{\text{p}}^{4}} \right)$. The total electrons present in ${{\text{N}}_{2}}$ are (7+7) or 14 electrons. Total electrons in $\text{CO}$ are (8+6) or 14. Hence, this is an isoelectronic pair.

C. $\text{HCl, }{{\text{H}}_{2}}\text{S}$: To find the total number of electrons in $\text{HCl and }{{\text{H}}_{2}}\text{S}$; we need to find the electrons present in their elements $\left( \text{H,Cl,S} \right)$ and then add. The electrons present in $\text{H}$is 1, the electrons present in $\text{Cl}$ are $17\left( \text{1}{{\text{s}}^{2}}\text{2}{{\text{s}}^{2}}\text{2}{{\text{p}}^{6}}\text{3}{{\text{s}}^{2}}\text{3}{{\text{p}}^{5}} \right)$ and electrons in$\text{S}$ are 16$\left( \text{1}{{\text{s}}^{2}}\text{2}{{\text{s}}^{2}}\text{2}{{\text{p}}^{6}}\text{3}{{\text{s}}^{2}}\text{3}{{\text{p}}^{4}} \right)$. The total electrons in $\text{HCl}$ are (1+17) or 18. Electrons present in ${{\text{H}}_{2}}\text{S}$ are (1+1+16) or 18. This is also an isoelectronic pair.

D. ${{\text{D}}_{2}}\text{O, Ne}$: To find the total number of electrons in ${{\text{D}}_{2}}\text{O and Ne}$; we need to find the electrons present in their elements $\left( \text{D,O,Ne} \right)$ and then add. The electrons present in$\text{D}$is 1, the electrons present in$\text{O}$ are $8\left( \text{1}{{\text{s}}^{2}}\text{2}{{\text{s}}^{2}}\text{2}{{\text{p}}^{4}} \right)$and electrons in Ne are$10\left( \text{1}{{\text{s}}^{2}}\text{2}{{\text{s}}^{2}}\text{2}{{\text{p}}^{6}} \right)$. Total electrons in ${{\text{D}}_{2}}\text{O}$are (1+1+8) or 10. Electrons present in $\text{Ne}$ are 10. This is an isoelectronic pair.

The correct to this question is option ‘a’, ‘b’, ‘c’ and ‘d’. Here the pairs are ${{\text{H}}_{2}}\text{,HD}$;$\text{CO, }{{\text{N}}_{2}}$;$\text{HCl, }{{\text{H}}_{2}}\text{S}$and ${{\text{D}}_{2}}\text{O, Ne}$.

Note: The terms isotopes and isoelectronic are different. Isotopes are species that have the same number of protons but a different number of neutrons. Species can be said to be isoelectronic when they have the same number of electrons like ${{\text{O}}^{2-}}\text{ and }{{\text{F}}^{-}}$are isoelectronic ions as they both have 10 electrons.