Problem 171 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following constitutes a group of the isoelectronic species? (a) \(\mathrm{C}_{2}^{2-}, \mathrm{O}_{2}^{-}, \mathrm{CO}, \mathrm{NO}\) (b) \(\mathrm{NO}^{+}, \mathrm{C}_{2}^{2}, \mathrm{C}^{\mathrm{N}-}, \mathrm{N}_{2}\) (c) \(\mathrm{CN}^{-}, \mathrm{N} 2, \mathrm{O}_{2}^{2-}, \mathrm{C}_{2}^{2-}\) (d) \(\mathrm{N}_{2}, \mathrm{O}_{2}^{-}, \mathrm{NO}^{+}, \mathrm{CO}\)

Step-by-Step Solution

Answer

Option (d) is the correct group of isoelectronic species.

Step 1: Identify the Meaning of Isoelectronic Species

Isoelectronic species are molecules, ions, or atoms that have the same number of electrons.

Step 2: Calculate the Number of Electrons for Each Species in Option (a)

- \(\mathrm{C}_{2}^{2-}\): Carbon atom (atomic number 6) has 6 electrons. For \(\mathrm{C}_{2}^{2-}\), each carbon atom contributes 6 electrons, totaling 12. Adding 2 for the charge, we have 14.- \(\mathrm{O}_{2}^{-}\): Oxygen has 8 electrons. Each oxygen in \(\mathrm{O}_{2}^{-}\) contributes 8, totaling 16, minus 1 for the charge, gives 15.- \(\mathrm{CO}\): Carbon and oxygen have 6 and 8 electrons, respectively, totaling 14.- \(\mathrm{NO}\): Nitrogen (7 electrons) and oxygen (8 electrons) total 15.

Step 3: Calculate the Number of Electrons for Each Species in Option (b)

- \(\mathrm{NO}^{+}\): \(\mathrm{N}\) has 7 electrons and \(\mathrm{O}\) has 8, totaling 15 - 1 for \(\mathrm{NO}^{+}\) = 14.- \(\mathrm{C}_{2}^{2}\): This is likely a misprint as \(\mathrm{C}_{2}^{2}\) doesn't exist with a neutral charge.- \(\mathrm{CN}^{-}\): \(\mathrm{C}\) is 6 and \(\mathrm{N}\) is 7 plus 1 for the charge = 14.- \(\mathrm{N}_{2}\): Two nitrogen atoms at 7 each total 14.

Step 4: Check Option (c)

- \(\mathrm{CN}^{-}\): \(\mathrm{C}\) has 6 and \(\mathrm{N}\) has 7; plus 1 for the charge totals 14.- \(\mathrm{N}_{2}\): Two \(\mathrm{N}\) each having 7 totals 14.- \(\mathrm{O}_{2}^{2-}\): Two \(\mathrm{O}\) each with 8, plus 2 for the charge totals 18.- \(\mathrm{C}_{2}^{2-}\): Each \(\mathrm{C}\) with 6 electrons, adds 2 for the charge totaling 14.

Step 5: Calculate the Number of Electrons for Each Species in Option (d)

- \(\mathrm{N}_{2}\): Two nitrogen atoms, each having 7 electrons, total 14.- \(\mathrm{O}_{2}^{-}\): Each oxygen has 8 electrons, totaling 16 minus 1 due to the charge gives 15.- \(\mathrm{NO}^{+}\): \(\mathrm{N}(7)+\mathrm{O}(8)-1 = 14\).- \(\mathrm{CO}\): \(\mathrm{C}(6)+\mathrm{O}(8) = 14\).

Step 6: Determine the Correct Group of Isoelectronic Molecules

From the calculations in the previous steps, option (d), \(\mathrm{N}_{2}, \mathrm{NO}^{+},\) and \(\mathrm{CO}\), both have 14 electrons, corresponding to a group of isoelectronic species.

Key Concepts

Electron Configuration

Electron configuration is an arrangement of electrons in an atom or molecule. Think of it as a blueprint that shows us where electrons are located relative to the nucleus. Organizing these electrons helps chemists predict chemical properties and behavior of an element. This setup is typically represented using a notation that indicates which shells and subshells the electrons occupy, following the order of the periodic table.

• The simplest example is hydrogen, with its one electron in the 1s orbital, given as 1s¹.
• Heavier elements like oxygen, which has eight electrons, have a more complex configuration: 1s² 2s² 2p⁴.

Each element's electron configuration is unique, very much like a fingerprint, and tells us which element it is. By studying electron configurations, we analyze chemical trends across the periodic table, such as atomic radius or ionization energy.

Molecular Ions

Molecular ions are molecules that have gained or lost electrons, thus acquiring a charge. This charge can either be positive, resulting in cations, or negative, which creates anions. Understanding molecular ions is crucial for predicting how substances react in chemical equations.

• For example, the nitrate ion \( \text{NO}_3^- \), is formed when nitric acid loses a hydrogen ion (H⁺).
• Similarly, an ammonium ion \( \text{NH}_4^+ \) is a positive ion formed by adding a proton to ammonia \( \text{NH}_3 \).

The formation of these ions alters the electron configuration of the molecule, which may affect its reactivity or strength of its chemical bonds. In chemical reactions, these ions tend to achieve a stable electron configuration, often similar to the nearest noble gas.

Atomic Number

The atomic number is fundamental to understanding the properties of an element. It represents the number of protons in an atom's nucleus, and it is unique to each element. This number also tells us how many electrons an atom has in a neutral state, thus directly influencing its electron configuration.

• For example, carbon has an atomic number of 6, indicating six protons and, when neutral, six electrons.
• Oxygen, with an atomic number of 8, has eight protons and typically eight electrons.

The atomic number is the defining property of an element and determines its place on the periodic table. Understanding atomic numbers helps us explore how and why elements interact with each other, forming compounds through chemical reactions.