Concept: Isoelectronic Species

Isoelectronic species are atoms, ions, or molecules that have the same number of electrons. To determine if two species are isoelectronic, we need to calculate the total number of electrons in each.

Why (A) is correct:

Let's calculate the total number of electrons for CO and each option:

• CO: Carbon (C) has 6 electrons, Oxygen (O) has 8 electrons. Total electrons = $6+8=14$.
• A) ${\mathrm{NO}}^{+}$: Nitrogen (N) has 7 electrons, Oxygen (O) has 8 electrons. The +1 charge means one electron is removed. Total electrons = $7+8-1=14$.
  Since CO and ${\mathrm{NO}}^{+}$ both have 14 electrons, they are isoelectronic.

Option Analysis:

• B) ${\mathrm{N}}_2$: Each Nitrogen (N) has 7 electrons. Total electrons = $7+7=14$. While ${\mathrm{N}}_2$ is also isoelectronic with CO, it is not the given correct answer in the prompt. However, if this were a multiple-correct question, it would also be correct. Given the single correct answer format, we stick to the provided correct answer.
• C) ${\mathrm{SnCl}}_2$: Tin (Sn) has 50 electrons, Chlorine (Cl) has 17 electrons. Total electrons = $50+(2\times 17)=50+34=84$. This is not 14.
• D) ${\mathrm{NO}}_2^{-}$: Nitrogen (N) has 7 electrons, Oxygen (O) has 8 electrons. The -1 charge means one electron is added. Total electrons = $7+(2\times 8)+1=7+16+1=24$. This is not 14.

Correct Answer: (A)