Problem 159 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following grouping represents a collection of isoelectronic species? (At. numbers \(\mathrm{Cs}-55, \mathrm{Br}-35)\) (a) \(\mathrm{Na}^{+}, \mathrm{Ca}^{2+}, \mathrm{Mg}^{2+}\) (b) \(\mathrm{N}^{3-}, \mathrm{F}^{-} \mathrm{Na}^{+}\) (c) \(\mathrm{Be}, \mathrm{Al}^{3+}, \mathrm{Cl}^{-}\) (d) \(\mathrm{Ca}^{2+}, \mathrm{Cs}^{+}, \mathrm{Br}\)

Step-by-Step Solution

Answer

The isoelectronic species are in option (b): N³⁻, F⁻, Na⁺, each with 10 electrons.

Step 1: Understand Isoelectronic Species

Isoelectronic species are atoms, ions, or molecules that have the same number of electrons. When determining if species are isoelectronic, count the electrons in each ion or atom.

Step 2: Determine the Number of Electrons

Let's determine the number of electrons for each option: - (a) Na⁺, Ca²⁺, Mg²⁺ - Na (11 protons) loses 1 electron to become Na⁺: 10 electrons. - Ca (20 protons) loses 2 electrons to become Ca²⁺: 18 electrons. - Mg (12 protons) loses 2 electrons to become Mg²⁺: 10 electrons. - (b) N³⁻, F⁻, Na⁺ - N (7 protons) gains 3 electrons to become N³⁻: 10 electrons. - F (9 protons) gains 1 electron to become F⁻: 10 electrons. - Na (11 protons) loses 1 electron to become Na⁺: 10 electrons. - (c) Be, Al³⁺, Cl⁻ - Be (4 protons) does not gain or lose electrons: 4 electrons. - Al (13 protons) loses 3 electrons to become Al³⁺: 10 electrons. - Cl (17 protons) gains 1 electron to become Cl⁻: 18 electrons. - (d) Ca²⁺, Cs⁺, Br - Ca (20 protons) loses 2 electrons to become Ca²⁺: 18 electrons. - Cs (55 protons) loses 1 electron to become Cs⁺: 54 electrons. - Br (35 protons) does not gain or lose electrons in neutral form: 35 electrons.

Step 3: Identify Isoelectronic Collection

From the above calculations: - (a) contains different electron numbers: 10, 18. - (b) all three have the same number of electrons: 10 electrons. - (c) has different electron numbers: 4, 10, 18. - (d) has different electron numbers: 18, 54, 35. Thus, only (b) is a collection where all species are isoelectronic.

Key Concepts

Electron Configuration

Electron configuration is a way to describe how electrons are distributed in an atom or ion. Electrons occupy energy levels, or shells, around the nucleus, and these shells have specific arrangements and capacities.
The configuration begins by filling the lowest energy level with electrons and proceeds to fill higher levels. Understanding electron configurations helps predict chemical behavior, such as bonding and ion formation.

• The Aufbau principle guides the order in which atomic orbitals are filled with electrons.
• Pauli's exclusion principle states that no two electrons can have the same set of quantum numbers in an atom.
• Hund's rule explains that electrons will fill an unoccupied orbital before they pair up in an occupied one.

For instance, sodium (Na) has 11 electrons in its neutral state. Its electron configuration is 1s² 2s² 2p⁶ 3s¹. When it loses one electron to become Na⁺, it has the same electron configuration as the noble gas neon, 1s² 2s² 2p⁶.

Atomic Structure

Atomic structure is all about understanding what an atom looks like and how it behaves. An atom consists of a nucleus, composed of protons and neutrons, with electrons buzzing around the nucleus in defined spaces called orbitals.

• Protons are positively charged particles in the nucleus that determine the atomic number of an element.
• Neutrons have no charge but add to the mass of the atom and affect stability.
• Electrons are negatively charged and occupy various shells around the nucleus, creating a sort of 'cloud'.

The most critical feature in atomic structure for understanding chemical reactions is the electron cloud. The arrangements of the electrons, particularly the valence electrons in the outermost shell, define how an atom will interact with others.
The atomic number is essentially the identity of an element, as it represents the number of protons in the atom's nucleus. For example, sodium (4Na5) will always have 11 protons and, in a neutral state, 11 electrons.

Ion Formation

Ion formation occurs when an atom gains or loses electrons, resulting in a charge imbalance. This process is essential for forming compounds, as ions readily interact with each other due to their charges.

• Cations are positively charged ions formed when an atom loses electrons. For example, Na⁺ is formed from sodium losing an electron.
• Anions are negatively charged ions created when an atom gains electrons. For example, Cl⁻ forms when chlorine gains an electron.

In isoelectronic configurations, ions or atoms share the same number of electrons, resulting in similar electron arrangements. This makes isoelectronic species share some similar chemical properties, even if their identities are quite different.
For instance, consider the set 4N³⁻, F⁻, Na⁺.5 Each has 10 electrons, mirroring the electron configuration of the noble gas neon. This makes them isoelectronic, though they are fundamentally different atoms or ions. This similarity in configuration can affect intermolecular interactions and compound formation.