Problem 169 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following sets of ions represents a collection of isoelectronic species? (a) \(\mathrm{K}^{+} \cdot \mathrm{Cl}^{-}, \mathrm{Ca}^{2+}, \mathrm{Sc}^{3+}\) (b) \(\mathrm{Ba}^{2+}, \mathrm{Sr}^{2+} \mathrm{K}^{+}, \mathrm{Ca}^{2+}\) (c) \(\mathrm{N}^{3-}, \mathrm{O}^{2-}, \mathrm{F}^{-}, \mathrm{S}^{2-}\) (d) \(\mathrm{Li}^{+}, \mathrm{Na}^{+}, \mathrm{Mg}^{2+} \mathrm{Ca}^{2+}\)

Step-by-Step Solution

Answer

Option (a) is the correct set of isoelectronic species.

Step 1: Understand the Concept of Isoelectronic Species

Isoelectronic species are atoms, molecules, or ions that have the same number of electrons. To determine if a collection of species is isoelectronic, compare the total number of electrons in each species.

Step 2: Determine the Number of Electrons in Option (a)

- \(\mathrm{K}^{+}\): Potassium has 19 electrons, losing one electron (forming \(\mathrm{K}^{+}\)) leaves 18 electrons.- \(\mathrm{Cl}^{-}\): Chlorine has 17 electrons, gaining one electron (forming \(\mathrm{Cl}^{-}\)) results in 18 electrons.- \(\mathrm{Ca}^{2+}\): Calcium has 20 electrons, losing two electrons results in 18 electrons.- \(\mathrm{Sc}^{3+}\): Scandium has 21 electrons, losing three electrons results in 18 electrons.All species in this option have 18 electrons.

Step 3: Determine the Number of Electrons in Option (b)

- \(\mathrm{Ba}^{2+}\): Barium has 56 electrons, losing 2 results in 54 electrons.- \(\mathrm{Sr}^{2+}\): Strontium has 38 electrons, losing 2 results in 36 electrons.- \(\mathrm{K}^{+}\): As previously calculated, it has 18 electrons.- \(\mathrm{Ca}^{2+}\): As previously calculated, it has 18 electrons.Thus, the species in this option are not isoelectronic.

Step 4: Determine the Number of Electrons in Option (c)

- \(\mathrm{N}^{3-}\): Nitrogen has 7 electrons, gaining 3 results in 10 electrons.- \(\mathrm{O}^{2-}\): Oxygen has 8 electrons, gaining 2 results in 10 electrons.- \(\mathrm{F}^{-}\): Fluorine has 9 electrons, gaining 1 results in 10 electrons.- \(\mathrm{S}^{2-}\): Sulfur has 16 electrons, gaining 2 results in 18 electrons.The first three species are isoelectronic with 10 electrons each, but \(\mathrm{S}^{2-}\) is not.

Step 5: Determine the Number of Electrons in Option (d)

- \(\mathrm{Li}^{+}\): Lithium has 3 electrons, losing one results in 2 electrons.- \(\mathrm{Na}^{+}\): Sodium has 11 electrons, losing one results in 10 electrons.- \(\mathrm{Mg}^{2+}\): Magnesium has 12 electrons, losing two results in 10 electrons.- \(\mathrm{Ca}^{2+}\): As previously calculated, it has 18 electrons.These species are not isoelectronic.

Step 6: Identify the Correct Isoelectronic Set

After evaluating all options, option (a) \(\mathrm{K}^{+}, \mathrm{Cl}^{-}, \mathrm{Ca}^{2+}, \mathrm{Sc}^{3+}\) has all species with the same number of electrons (18) and is the correct set of isoelectronic species.

Key Concepts

Electron Configuration

Electron configuration is a systematic way to describe the distribution of electrons in atoms, ions, or molecules. It helps us understand the arrangement and number of electrons in various energy levels or shells around a nucleus. This representation employs the principles of quantum mechanics and is expressed using a notation.

• For instance, for the neutral atom of potassium (K), which has 19 electrons, its electron configuration can be written as 1s²2s²2p⁶3s²3p⁶4s¹.
• When forming ions, such as the potassium ion \(\text{K}^{+}\), you must account for the loss or gain of electrons. Potassium loses one electron, leading to an electron configuration of \([\text{K}^+]\) 1s²2s²2p⁶3s²3p⁶, equating to 18 electrons.

Understanding electron configuration is crucial as it explains the chemical behavior of elements and ions. It serves as the foundational element used in identifying isoelectronic species.

Ions

Ions are charged particles that form when atoms gain or lose electrons.
A neutral atom becomes an ion by either acquiring extra electrons (negative charge) or losing some (positive charge).

• Cations are positively charged ions, formed by the loss of electrons. For example, \(\text{Ca}^{2+}\) signifies a calcium atom that has lost two electrons.
• Anions are negatively charged ions, created when atoms gain electrons. For example, the chloride ion, denoted as \(\text{Cl}^{-}\), results from chlorine gaining one electron.

The formation of ions is central to the creation of isoelectronic species, as it alters the electron count while the nucleus remains unchanged. The neutrality of the whole system changes, leaving an imbalance between the number of protons and electrons, thus identifying the type of ion.

Chemical Species

A chemical species refers to an atom, molecule, ion, or radical that can be found in a specific chemical environment and is involved in chemical reactions. It can exist in different forms, such as metal atoms, molecular ions, or complexes.

• An isoelectronic species is a group of species that has the same number of electrons. These can be atoms, ions, or even molecules that appear different but share the same electronic structure.
• The term encompasses a diverse range of particles, like \(\text{K}^{+}, \ \text{Cl}^{-}, \ \text{Ca}^{2+},\text{Sc}^{3+}\), which despite their different identities, carry an identical electron count, i.e., 18 as shown through their electron configurations.

Understanding different chemical species is vital to grasp how chemical reactions occur and how particles interact during these processes. Particularly, isoelectronic species allow scientists to explore structural and chemical properties shared by otherwise different elements or compounds.

Atomic Structure

Atomic structure refers to the arrangement of subatomic particles: electrons, protons, and neutrons within an atom. This concept is fundamental to chemistry as it influences an element's chemical properties and behavior.

• The nucleus contains positively charged protons and neutral neutrons, with the number of protons defining the element's identity.
• Electrons, which are negatively charged, orbital around the nucleus creating electron clouds or shells at varying energy levels depending on their distance from the nucleus.

The atomic structure guides chemical interactions because the most reactive elements have electrons in outer shells that can be gained, lost, or shared. For ions, this structure shifts, altering the electron cloud while the nucleus stays the same, crucially affecting chemical identity and behavior. Isoelectronic species particularly showcase how variance in their atomic structure (i.e., electron count) affects their chemistry without changing their group members.