Problem 154 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following sets of ions represents a collection of isoelectronic species? [2006] (a) \(\mathrm{K}^{+}, \mathrm{Cl}^{-}, \mathrm{Ca}^{2+}, \mathrm{Sc}^{3+}\) (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

Set (a) is isoelectronic.

Step 1: Understanding Isoelectronic Species

Isoelectronic species are atoms or ions that have the same number of electrons. To determine if a set of ions is isoelectronic, we need to find the total number of electrons for each species in the set.

Step 2: Calculate Electrons in Group (a)

Review the ions \(\text{K}^+, \text{Cl}^-, \text{Ca}^{2+}, \text{Sc}^{3+}\). \(\text{K}^+\) has 18 electrons (K: 19 - 1 = 18), \(\text{Cl}^-\) has 18 electrons (Cl: 17 + 1 = 18), \(\text{Ca}^{2+}\) has 18 electrons (Ca: 20 - 2 = 18), and \(\text{Sc}^{3+}\) has 18 electrons (Sc: 21 - 3 = 18). All ions have the same number of electrons, 18.

Step 3: Calculate Electrons in Group (b)

Consider the ions \(\text{Ba}^{2+}, \text{Sr}^{2+}, \text{K}^+, \text{Ca}^{2+}\). Calculate their electrons: \(\text{Ba}^{2+}\) has 54 (Ba: 56 - 2 = 54), \(\text{Sr}^{2+}\) has 36 (Sr: 38 - 2 = 36), \(\text{K}^+\) has 18 (K: 19 - 1 = 18), \(\text{Ca}^{2+}\) has 18 (Ca: 20 - 2 = 18). The numbers of electrons do not match.

Step 4: Calculate Electrons in Group (c)

Examine ions \(\text{N}^{3-}, \text{O}^{2-}, \text{F}^-, \text{S}^{2-}\). \(\text{N}^{3-}\) has 10 electrons (N: 7 + 3 = 10), \(\text{O}^{2-}\) has 10 (O: 8 + 2 = 10), \(\text{F}^-\) has 10 (F: 9 + 1 = 10), but \(\text{S}^{2-}\) has 18 electrons (S: 16 + 2 = 18). One ion is not matching.

Step 5: Calculate Electrons in Group (d)

Review ions \(\text{Li}^+, \text{Na}^+, \text{Mg}^{2+}, \text{Ca}^{2+}\). Calculate: \(\text{Li}^+\) has 2 (Li: 3 - 1 = 2), \(\text{Na}^+\) has 10 (Na: 11 - 1 = 10), \(\text{Mg}^{2+}\) has 10 (Mg: 12 - 2 = 10), \(\text{Ca}^{2+}\) has 18 (Ca: 20 - 2 = 18). Their numbers of electrons vary.

Key Concepts

Electron Configuration

Understanding electron configuration is key to grasping how atoms and ions form isoelectronic species. Electron configuration refers to the arrangement of electrons around the nucleus of an atom or ion. By knowing the electron configuration, we can determine how many electrons are present in a particular ion.
This is crucial when identifying isoelectronic species, as these species will have identical electron configurations. For example, the electron configuration of \(\text{K}^+\) is \(1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6\), which matches that of \(\text{Cl}^-\), \(\text{Ca}^{2+}\), and \(\text{Sc}^{3+}\). All these species share the configuration ending in \(3p^6\), indicating they have the same number of electrons, despite having different atomic numbers and positions in the periodic table.

Ions

Ions are charged particles that form when atoms gain or lose electrons. This gain or loss results in a net positive or negative charge, and ions can be classified as either cations, which are positively charged, or anions, which are negatively charged.
The concept of ions is crucial in identifying isoelectronic species, as each ion's charge affects its electron count. For instance, when potassium donates an electron, it forms \(\text{K}^+\), a cation with 18 electrons, aligning it with the electron configuration of \(\text{Cl}^-\), \(\text{Ca}^{2+}\), and others in its isoelectronic group.

• Cations: Formed by loss of electrons, e.g., \(\text{Ca}^{2+}\).
• Anions: Formed by gain of electrons, e.g., \(\text{Cl}^-\).

Understanding how ions form and their resulting charges helps clarify the electron redistribution that leads to isoelectronicity.

Chemical Species

In chemistry, a chemical species refers to atoms, ions, molecules, or radicals that take part in chemical reactions. These entities are distinct forms of matter.
Isoelectronic species are a unique category of chemical species whose electron configurations are identical, despite differing in atomic structure and electron count. The exercise asks us to find sets of ions that represent collections of isoelectronic species, highlighting the importance of understanding that these species retain identical electron counts regardless of differences in atomic makeup.
Isoelectronic chemical species, such as \(\text{K}^+\) and \(\text{Cl}^-\), are crucial in studying chemical reactivity and stability as they can predict similarities in behavior.

Ion Charge

Ion charge is the electric charge that an ion carries, determined by the number of electrons lost or gained compared to the neutral state of the atom. This charge is pivotal for identifying and calculating electron counts in isoelectronic studies.

• Neutral atoms become positively charged cations when they lose electrons, e.g., \(\text{Ca}^{2+}\), which loses two electrons, resulting in a \(+2\) charge.
• On the other hand, gaining electrons leads to negatively charged anions, as seen in \(\text{Cl}^-\).

Isoelectronic sets, like the one including \(\text{K}^+\), must account for charge to confirm matching electron numbers. Changes in electron count due to charge fluctuations are the main focus during these calculations. Understanding ion charge aids in visualizing how atomic adjustment results in consistent electron shares across different ions.