Problem 20 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is isoelectronic? (a) \(\mathrm{CO}_{2}, \mathrm{NO}_{2}\) (b) \(\mathrm{NO}_{2}^{-}, \mathrm{CO}_{2}\) (c) \(\mathrm{CN}^{-}, \mathrm{CO}\) (d) \(\mathrm{SO}_{2}, \mathrm{CO}_{2}\)

Step-by-Step Solution

Answer

Option (c): \(\mathrm{CN}^{-}, \mathrm{CO}\) are isoelectronic.

Step 1: Understanding Isoelectronic Species

Isoelectronic species are atoms or molecules that have the same number of electrons. To find these, we'll determine the total electron count of each given molecule and ion.

Step 2: Calculate Electrons in Each Option

(a) For \(\mathrm{CO}_{2}\), carbon has 6 electrons, and each oxygen has 8. Thus, total = \(6 + (2 \times 8) = 22\). For \(\mathrm{NO}_{2}\), nitrogen has 7 electrons, each oxygen has 8. Total = \(7 + (2 \times 8) = 23\).(b) For \(\mathrm{NO}_{2}^{-}\), nitrogen has 7, each oxygen 8, and -1 charge adds 1 electron. Total = \(7 + (2 \times 8) + 1 = 24\). For \(\mathrm{CO}_{2}\), total is 22.(c) For \(\mathrm{CN}^{-}\), carbon has 6, nitrogen 7, and -1 adds 1 electron. Total = \(6 + 7 + 1 = 14\). For \(\mathrm{CO}\), carbon has 6, oxygen 8. Total = \(6 + 8 = 14\).(d) For \(\mathrm{SO}_{2}\), sulfur has 16, and each oxygen has 8. Total = \(16 + (2 \times 8) = 32\). For \(\mathrm{CO}_{2}\), total is 22.

Step 3: Determine Isoelectronic Pair

Option (c) is the only one where both species have the same total number of electrons: \(\mathrm{CN}^{-}\) and \(\mathrm{CO}\) both have 14 electrons.

Key Concepts

Electron Count

Understanding the electron count is crucial in identifying isoelectronic species. Isoelectronic species are atoms, molecules, or ions that have the same number of electrons, but not necessarily the same number of protons. This identical electron count allows them to have similar electronic structures and properties, even if their compositions differ.

To calculate the electron count of a chemical entity, you need to add up the electrons from individual atoms and adjust for any electric charge. Here's how you can do it:

• *Look at the atomic number* - The atomic number tells you the number of protons and, in a neutral atom, the number of electrons.
• *Account for charges* - If the species is an ion, adjust the electron count by adding or subtracting electrons based on the charge. Add one electron for each negative charge and remove one for each positive charge.

By applying these steps, you can determine the electron count for species like \(\mathrm{CO}_2\) (22 electrons), \(\mathrm{NO}_2^-\) (24 electrons), and so on, making it easier to identify isoelectronic pairs.

Chemical Species

The term "chemical species" encompasses a broad range of entities including atoms, molecules, ions, and radicals. Each species can have unique chemical properties due to differences in their number of electrons, protons, and neutrons.

Chemical species can be defined as:

• *Atoms* - The basic unit of a chemical element, containing a nucleus surrounded by electrons.
• *Molecules* - Groups of two or more atoms bonded together, representing the smallest fundamental unit of a chemical compound.
• *Ions* - Atoms or molecules that have gained or lost one or more electrons, resulting in a charged species.

In the context of identifying isoelectronic species, it's important to understand how these players interact. Even if two chemical species are different in terms of element composition or charge, they can still be isoelectronic, meaning they share an electron count, potentially giving rise to similar shapes and behaviors.

Molecular Ions

Molecular ions are charged chemical species consisting of two or more atoms. They can result from either gaining or losing electrons, which alters their total charge. These ions often play crucial roles in chemical reactions, serving as intermediates.

• *Formation of molecular ions* - When a molecule loses or gains electrons, it becomes a molecular ion. Common examples include polyatomic ions such as \(\mathrm{NO}_2^-\) and \(\mathrm{CN}^-\).
• *Significance* - Because molecular ions have a net charge, they often participate in ionic bonds and influence the chemical behavior of compounds.

Isoelectronic considerations often involve molecular ions because their ability to gain or lose electrons brings them to the same electron count as other neutral molecules or ions.

Thus, determining the electron count is vital for identifying when a molecular ion like \(\mathrm{CN}^-\) is isoelectronic with a neutral molecule like \(\mathrm{CO}\), as both share the same electron count, affecting interactions at the molecular level.