Question

Which of the following are isoelectronic specials, i.e., those having the same number of electrons?
$Na^+$, $K^+$, $Mg^{2+}$, $Ca^{2+}$, $S^{2-}$, $Ar$.

Answer 1

Isoelectronicity is an effect observed when distinct molecular entities have identical electron configuration but they are different in their physical and chemical properties.

Complete step by step answer:

As we already know, two or more molecular entities described as isoelectronic if they have the same number of valence electrons and the same structures i.e. number and connectivity of atoms, but differ in some of the elements involved. The importance of the concept lies in identifying significantly related species, as pairs or series. Isoelectronic species can be expected to show useful consistency and predictability in their properties.
The N atom and the ${O^ + }$ ion are isoelectronic because each has five electrons in the outer electron shell.
Let us find out the number of isoelectronic species from the given ions/atoms:
Now, a positive charge denotes the loss of an electron & a negative charge denotes the gain of an electron by a species.

1. Number of electrons in sodium $\left( {Na} \right)$= 11
   Therefore, number of electrons in $(N{a^ + })$= 10
2. Number of electrons in $K +$= 18
3. Number of electrons in $M{g^{2 + }}$= 10
4. Number of electrons in $C{a^{2 + }}$= 18
5. Number of electrons in sulphur $\left( S \right)$= 16
   $\therefore$ Number of electrons in ${S^{2 - }}$= 18
6. Number of electrons in argon $\left( {Ar} \right)$= 18

Hence, the following are isoelectronic species:
a) $N{a^ + }$ and $M{g^{2 + }}$ (10 electrons each)
b) ${K^ + }, {\text{ }}C{a^{2 + }}, {\text{ }}{S^{2 - }}$ and $Ar$ (18 electrons each)

Note: We observe that isoelectronic species having the same number of valence electrons only the difference is that they carry different charges according to their ability.