Question

Which of the following pairs of molecules or ions are isoelectronic with each other? (Sections \(4.9-4.10\) ) (a) \(\mathrm{CO}\) and \(\mathrm{NO}^{-}\) (b) \(\mathrm{CN}^{-}\) and \(\mathrm{NO}^{+}\) (c) \(\mathrm{N}_{2}^{-}\) and \(\mathrm{O}_{2}^{-}\)

Short answer

(b) CN⁻ and NO⁺ are isoelectronic.

Step-by-step solution

Understanding Isoelectronic Species

Isoelectronic species are atoms, molecules, or ions that have the same number of total electrons. To determine this for a pair, we need to compare their electron count.

Calculate Electrons in CO and NO⁻

For carbon monoxide (\(\mathrm{CO}\)), carbon has 6 electrons and oxygen has 8, totaling 14 electrons. For \(\mathrm{NO}^{-}\), nitrogen has 7 electrons, oxygen has 8, and the extra negative charge adds 1 more, totaling 16 electrons. CO and NO⁻ are not isoelectronic.

Calculate Electrons in CN⁻ and NO⁺

For \(\mathrm{CN}^{-}\), carbon has 6 electrons, nitrogen has 7, plus 1 extra for the negative charge, totaling 14 electrons. For \(\mathrm{NO}^{+}\), nitrogen has 7 electrons, and oxygen has 8, minus 1 for the positive charge, totaling 14 electrons. CN⁻ and NO⁺ are isoelectronic.

Calculate Electrons in N₂⁻ and O₂⁻

For \(\mathrm{N}_{2}^{-}\), each nitrogen has 7 electrons, so 14 in total, plus 1 for the negative charge, totaling 15 electrons. For \(\mathrm{O}_{2}^{-}\), each oxygen has 8 electrons, totaling 16, plus 1 for the negative charge, totaling 17 electrons. N₂⁻ and O₂⁻ are not isoelectronic.

Key concepts

Electron Count

Understanding electron count is essential to determine whether two chemical species are isoelectronic. An isoelectronic status is achieved when two or more species have the same total number of electrons. This involves calculating the sum of electrons from each atom in a molecule or ion. For example:

• Carbon Monoxide (\(\mathrm{CO}\)) consists of a carbon atom with 6 electrons and an oxygen atom with 8 electrons, leading to a total of 14 electrons.
• The ion \( \mathrm{NO}^{-} \) includes nitrogen with 7 electrons, oxygen with 8 electrons, and an additional electron from the negative charge, totaling 16 electrons.

These calculations help us see that the electron count is different, and thus, \(\mathrm{CO}\) and \(\mathrm{NO}^{-}\) are not isoelectronic. In contrast, to assess \(\mathrm{CN}^{-}\) and \(\mathrm{NO}^{+}\):

• \( \mathrm{CN}^{-} \) has 6 electrons from carbon, 7 from nitrogen, plus 1 from its negative charge, summing to 14 electrons.
• \( \mathrm{NO}^{+} \) possesses 7 electrons from nitrogen and 8 from oxygen, but one electron is taken away due to its positive charge, also resulting in 14 electrons.

Thus, this pair is isoelectronic since both have an equal electron count.

Molecular Ions

Molecular ions are charged species that form when neutral molecules gain or lose electrons. The presence of a charge influences the electron count of the species, potentially impacting their isoelectronic status with other species. The charge of a molecular ion affects its total electron count in the following way:

• A positive charge (cation) indicates a loss of electrons, which reduces the total electron count.
• A negative charge (anion) represents a gain of electrons, increasing the total electron count.

For instance, in the example of \( \mathrm{NO}^{+} \) and \( \mathrm{CN}^{-} \):

• One positive charge on \( \mathrm{NO}^{+} \) means we subtract an electron, affecting its isoelectronic comparison.
• One negative charge on \( \mathrm{CN}^{-} \) means we add an electron, bringing its electron count to match that of \( \mathrm{NO}^{+} \).

This manipulation highlights how molecular ions' charges are fundamental in determining if they are isoelectronic with other species.

Atomic Structure

Atomic structure refers to the arrangement of protons, neutrons, and electrons in an atom. In the context of isoelectronic species, we focus on the outer electron arrangements that influence chemical properties and bonding patterns.While protons and neutrons contribute to the atomic mass, the number of electrons directly affects the electron configuration and thus, the chemical behavior of the species. When molecules or ions are isoelectronic, they share an identical number of electrons leading to similar electron configurations. However, they might still differ in properties due to the presence of different nuclei.Considering \( \mathrm{N}_{2}^{-} \) and \( \mathrm{O}_{2}^{-} \):

• Both involve diatomic molecules with combinations of nitrogen or oxygen, but due to differing charges \(-1\), their total electron count varies, \(\mathrm{N}_{2}^{-}\) having 15 and \(\mathrm{O}_{2}^{-}\) having 17.
• This distinction shows that even slight changes in atomic or molecular structure, like an extra electron, can alter electron configurations and decide if species are isoelectronic.

Therefore, understanding the atomic structure is key to grasping differences and similarities in chemical species regarding their electron counts.