Problem 203 - A Complete Resource Book in Chemistry for JEE Main

Question

Of the following sets, which one does not contain isoelectronic species? (a) \(\mathrm{CN}^{-}, \mathrm{N}_{2}, \mathrm{C}_{2}^{2-}\) (b) \(\mathrm{PO}_{4}^{3-}, \mathrm{SO}_{4}^{2-}, \mathrm{ClO}_{4}\) (c) \(\mathrm{SO}_{3}^{2-}, \mathrm{CO}_{3}^{2-}, \mathrm{NO}_{3}\) (d) \(\mathrm{BO}_{3}^{3-}, \mathrm{CO}_{3}^{2-}, \mathrm{NO}_{3}^{3}\)

Step-by-Step Solution

Answer

Set (b) does not contain isoelectronic species.

Step 1: Define Isoelectronic Species

Isoelectronic species are atoms, molecules, or ions with the same number of electrons and the same electron configuration.

Step 2: Determine Electron Count for Each Species in Set (a)

- For \( \text{CN}^- \), the electron count is 7 (C) + 7 (N) + 1 (charge) = 15 electrons. - For \( \text{N}_2 \), the electron count is 7 (N) + 7 (N) = 14 electrons. - For \( \text{C}_2^{2-} \), the electron count is 6 (C) + 6 (C) + 2 (charge) = 14 electrons.

Step 3: Determine Electron Count for Each Species in Set (b)

- For \( \text{PO}_4^{3-} \), calculate: 15 (P) + 8 \times 4 = 32, then add 3 for charge = 35 electrons. - For \( \text{SO}_4^{2-} \), calculate: 16 (S) + 8 \times 4 = 32, then add 2 for charge = 34 electrons. - Calculate for \( \text{ClO}_4^- \): 17 (Cl) + 8 \times 4 = 32, then add 1 for charge = 33 electrons.

Step 4: Determine Electron Count for Each Species in Set (c)

- For \( \text{SO}_3^{2-} \), calculate: 16 (S) + 8 \times 3 = 24, then add 2 for charge = 26 electrons. - For \( \text{CO}_3^{2-} \), calculate: 6 (C) + 8 \times 3 = 24, then add 2 for charge = 26 electrons. - Calculate for \( \text{NO}_3 \): 7 (N) + 8 \times 3 = 31 electrons.

Step 5: Determine Electron Count for Each Species in Set (d)

- For \( \text{BO}_3^{3-} \), calculate: 5 (B) + 8 \times 3 = 29, then add 3 for charge = 32 electrons. - For \( \text{CO}_3^{2-} \), calculate: 6 (C) + 8 \times 3 = 24, then add 2 for charge = 26 electrons. - For \( \text{NO}_3^{3} \), calculate: 7 (N) + 8 \times 3 = 31 electrons no additional charge to consider at \( \text{NO}_3^0 \) assumption.

Step 6: Identify the Set That Does Not Contain Isoelectronic Species

Compare the electron counts for each set. Set (b) does not have a consistent electron count: \( \text{PO}_4^{3-} \) has 35 electrons, \( \text{SO}_4^{2-} \) has 34 electrons, and \( \text{ClO}_4^- \) has 33 electrons. They are not isoelectronic.

Key Concepts

Electron Configuration

The concept of electron configuration plays a significant role in understanding both isoelectronic species and chemical behavior. Electron configuration refers to the arrangement of electrons in an atom, a molecule, or an ion. For every element, electrons fill orbitals in a structured manner based on increasing energy levels. This sequence follows the Aufbau principle, where lower energy orbitals fill before higher ones.

In general, the electron configuration is denoted by characters such as 1s² 2s² 2p⁶, indicating the number of electrons in atomic orbitals. This arrangement decides an element's chemical properties and its placement in the periodic table.

Isoelectronic species share identical electron configurations, despite possessing different atomic structures or charges. For instance, the ion \( ext{N}^3-\) and the neon atom \( ext{Ne}\) both have an electron configuration of 1s² 2s² 2p⁶. This shared configuration among different entities allows further exploration of how particular atomic arrangements influence bonding and behavior in chemical reactions.

Anions and Cations

Anions and cations are ions with negative and positive charges, respectively, and understanding these charged particles is crucial in the study of chemistry and electron configurations.

• Anions are formed when atoms gain electrons. This addition increases the overall negative charge because electrons have a negative charge. The electron count for anions is determined by adding the charge of the ion to the total electrons of the neutral atom.
• Cations, on the other hand, are created when atoms lose electrons, resulting in a positive charge. The number of electrons is counted by subtracting the charge of the ion from the total electrons of the neutral atom.

Understanding the formation of anions and cations can help one predict and comprehend chemical reactions and phenomena, such as ionic bonds, electrical conductivity, and the solubility of compounds. These ionic transformations are foundational in determining whether species are isoelectronic by adjusting their electron counts to match those of other entities.

Molecular Ions

Molecular ions are a type of ion where the charge is distributed across multiple atoms, creating structures that hold unique characteristics different from their neutral counterparts. These entities occur when molecules gain or lose electrons, forming polyatomic ions, which can be either anions or cations.

For example, the sulfate ion \( ext{SO}_4^{2-}\) is a molecular ion, as sulfur and oxygen atoms together hold a collective negative charge. Similarly, the ammonium ion \( ext{NH}_4^+\) represents a molecular cation with a positive charge.

In terms of electron configuration examination, molecular ions offer a remarkable way to understand charge distribution across a group of atoms. Evaluating these ions involves calculating the total electron count by considering the atom-specific electrons and the charge of the ion itself. Such knowledge is critical to assess and compare different sets of species to determine their isoelectronic status, making sense of how electron adjustments influence molecular stability and reactions.