Problem 199 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

Set (a) does not contain isoelectronic species.

Step 1: Define Isoelectronic Species

Isoelectronic species are atoms, molecules, or ions that have the same number of electrons. To determine if a set contains isoelectronic species, calculate the total number of electrons for each ion or molecule in the set.

Step 2: Examine Set (a)

For each species in (a), calculate the total electrons:- For \( \text{SO}_3^{2-} \): S(16) + 3×O(8) + 2 e = 42 electrons- For \( \text{CO}_3^{2-} \): C(6) + 3×O(8) + 2 e = 32 electrons- For \( \text{NO}_3^{-} \): N(7) + 3×O(8) + 1 e = 32 electronsBecause \( \text{SO}_3^{2-} \) has a different number of electrons (42), set (a) contains non-isoelectronic species.

Step 3: Check Set (b)

Now check the electron count for set (b):- For \( \text{CN}^- \): C(6) + N(7) + 1 e = 14 electrons- For \( \text{N}_2 \): 2×N(7) = 14 electrons- For \( \text{C}_2^{2-} \): 2×C(6) + 2 e = 14 electronsAll species in (b) have 14 electrons, making them isoelectronic.

Step 4: Assess Set (c)

Calculate electrons for set (c):- For \( \text{BO}_3^{3-} \): B(5) + 3×O(8) + 3 e = 32 electrons- For \( \text{CO}_3^{2-} \): C(6) + 3×O(8) + 2 e = 32 electrons- For \( \text{NO}_3^{-} \): N(7) + 3×O(8) + 1 e = 32 electronsAll species have 32 electrons, so they are isoelectronic.

Step 5: Verify Set (d)

Now for set (d), find electron counts:- For \( \text{PO}_4^{3-} \): P(15) + 4×O(8) + 3 e = 50 electrons- For \( \text{SO}_4^{2-} \): S(16) + 4×O(8) + 2 e = 50 electrons- For \( \text{ClO}_4^{-} \): Cl(17) + 4×O(8) + 1 e = 50 electronsAll components in (d) have 50 electrons, showing they are isoelectronic.

Key Concepts

Electron Configuration

Understanding electron configuration is crucial in chemistry, especially when exploring isoelectronic species. Electron configuration of an atom or ion describes how electrons are distributed in its atomic orbitals.
Each atom can be thought of as a small solar system, with the nucleus as the sun and the electrons orbiting like planets in specific paths or 'shells'. These shells are labeled as s, p, d, and f, indicating different energy levels and capacities.
When considering isoelectronic species, keep in mind that they share the same electron configuration despite possibly being different elements or ions. This means two species can have a similar number but different types of electrons, like a Swedish fish and a gummy bear, both being candy but having different flavors.

• Fill each **shell** in order from lower to higher energy levels.
• Anion and cation charges further adjust electron numbers compared to neutral atoms.
• Achieving isoelectronic states often involves gaining or losing electrons.

Anions and Cations

Anions and cations play a vital role in determining whether certain species are isoelectronic. These are ions, but the key difference lies in their charges. Anions are negatively charged ions, meaning they gain electrons, whereas cations lose electrons and are positively charged.
This electron exchange affects the total electron count and helps define the overall charge of a species. For example, when a neutral atom becomes a cation, it loses some of its electrons.
Conversely, if it becomes an anion, it gains extra electrons. Thus, two different species can share the same electron count through these ion transformations:

• Cations: **Positive Charge** 0 - 0 Losing Electrons
• Anions: **Negative Charge** 0 - 0 Gaining Electrons
• Changes in charge directly impact electron configuration, which is integral in identifying isoelectronic species.

Molecular Geometry

The concept of molecular geometry comes into play when considering the shape and arrangement of atoms that make up a molecule. This is important in understanding how isoelectronic species can behave similarly and why they might differ in properties.
While isoelectronic species have the same number of electrons, their molecular geometry might influence their functionalities and interactions. The geometry determines how atoms are spatially oriented and connected through bonds, explaining why their chemical behavior might differ despite having identical electron counts.
Geometric arrangements rely on the central atom's electron pairs' repulsion, as per VSEPR (Valence Shell Electron Pair Repulsion) theory, leading to differing shapes like linear, bent, trigonal planar, etc.

• Molecules are shaped based on the number of **bond pairs** and **lone pairs** around the central atom.
• Each shape results from **electron pair repulsions** trying to stay as far apart as possible.
• The actual geometry will affect physical properties and chemical reactivity, even among isoelectronic species.