Question

Hydrogen ion H^- is isoelectronic with
A. Li
B. He
C. H⁺
D. Li^-

Answer 1

Hint: When two or more different atoms, molecules, or ions contain the same number of valence electrons and have the arrangement of electrons in the same way these molecules, atoms, or ions are called isoelectronic molecules, atoms, or ions.

Complete Step by Step Answer:
For molecules to be isoelectric they need to have the same number of electrons in the valence shell. And mostly their geometry is also the same and also they have similar electronic configuration of the valence shell electron, now similar electronic configuration doesn’t that the electron is not supposed to be in the same period that means in one atom it can be 1s of one atom and can be 2s of another atom. In hydrogen ion H- the electronic configuration of H^- ion is:


Now the electronic configuration of given species Li, He, H+, Li- are:
Electronic configuration of Li


Electronic configuration of He


Electronic configuration of H⁺


Electronic configuration of Li^-


Now as we can see that the valence shell electrons are the same in He with the hydrogen ion H- so the species which is isoelectronic with hydrogen ion H^- is Helium He.

And it is not necessary that they are supposed to be only a pair of ions, a pair of molecules, or a pair of atoms. They can be a pair of atom-ion, ion-molecule, molecule-atom any pair can be possible.
Thus, Option (A) is correct

Note: There are several more species that are isoelectronic like O=C=O, N=N=O , H₂C=C=CH₂, N=N=N. One other example is NH₃ & PH₃. If you want to know if any diatomic species are isoelectronic or not we can use molecular orbital diagrams.