Question

Identify the isoelectronic pairs.
(A) $C{O_3}^{2-}\text{ and }N{O_3}^{-}$
(B) $P{O_4}^{3-}\text{ and S}{O_4}^{2-}$
(C) CO and $N_2$
(D) All of the above

Answer 1

Hint: Isoelectronic pairs are those atoms, ions and molecules that have the same number of electrons in them. To find the isoelectronic pairs, we can just add the number of electrons of each atom and also the charge of the ion (if present) to find which of the molecules have the same number of electrons in them.

Complete answer:
- For answering this question, we first need to understand the meaning of isoelectronic pairs.
- Isoelectronic pairs are those pairs of atoms, ions or molecules that have the same number of electrons in them.

- For finding the isoelectronic pairs, we can just add the number of electrons of each atom in the species and also the charge of the species (if present).
- Then, if the number of electrons are equal in both species, they are said to be isoelectronic pairs.

- First let us take a look at $C{O_3}^{2-}\text{ and }N{O_3}^{-}$.
- We know that the number of electrons in C, N, and O are - 6, 7 and 8 respectively and the charge in $C{O_3}^{2-}\text{ and }N{O_3}^{-}$ are 2 and 1 respectively.
- Therefore, the number of electrons in each species will be-

For $C{O_3}^{2-}$: 6 + (3 x 8) + 2 = 32 electrons
For $N{O_3}^{-}$: 7 + (3 x 8) + 1 = 32 electrons.
- As the number of electrons in both species is equal (32 electrons), they are an isoelectronic pair.
- Next, let us take a look at $P{O_4}^{3-}\text{ and S}{O_4}^{2-}$
- We know that the number of electrons in O, P, and S are - 8, 15 and 16 respectively and the charge in $P{O_4}^{3-}\text{ and S}{O_4}^{2-}$ are 3 and 2 respectively.

- Therefore, the number of electrons in each species will be-
For$P{O_4}^{3-}$: 15 + (4 x 8) + 3 = 50 electrons
For $\text{S}{O_4}^{2-}$: 16 + (4 x 8) + 2 = 50 electrons.
- As the number of electrons in both species is equal (50 electrons), they are an isoelectronic pair.
- Now, let us take a look at CO and $N_2$.
- We know that the number of electrons in C, N, and O are - 6, 7 and 8 respectively and the charge of both species is 0.
- Therefore, the number of electrons in each species will be-

For CO: 6 + 8 + 0 = 14 electrons
For $N_2$: (2 x 7) = 14 electrons.
- As the number of electrons in both species is equal (14 electrons), they are an isoelectronic pair.
- Hence, we can see that all the pairs given in the question are isoelectronic pairs.
- Therefore, the correct answer is option (D) All of the above

Note: It is of utmost importance that you take the net charge of the species into account while calculating the number of electrons. Failure to do so will give you the incorrect answer.