Problem 164 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

Option (a) is the set of isoelectronic species.

Step 1: Understanding Isoelectronic Species

Isoelectronic species are ions or atoms that have the same number of electrons. We will examine the electron count of each ion in the given options to determine isoelectronic species.

Step 2: Counting Electrons in Option (a)

- \(K^+\) has 18 electrons (19-1=18).- \( ext{Ca}^{2+}\) has 18 electrons (20-2=18).- \( ext{Sc}^{3+}\) has 18 electrons (21-3=18).- \( ext{Cl}^-\) has 18 electrons (17+1=18). \Option (a) represents a set of isoelectronic species with each ion having 18 electrons.

Step 3: Counting Electrons in Option (b)

- \( ext{Na}^+\) has 10 electrons (11-1=10).- \( ext{Ca}^{2+}\) has 18 electrons (20-2=18).- \( ext{Sc}^{3+}\) has 18 electrons (21-3=18).- \( ext{F}^-\) has 10 electrons (9+1=10). \In this set, not all ions have the same number of electrons. There are two different electron counts.

Step 4: Counting Electrons in Option (c)

- \( ext{K}^+\) has 18 electrons (19-1=18).- \( ext{Cl}^-\) has 18 electrons (17+1=18).- \( ext{Mg}^{2+}\) has 10 electrons (12-2=10).- \( ext{Sc}^{3+}\) has 18 electrons (21-3=18). \In this set, most ions have 18 electrons, but \( ext{Mg}^{2+}\) does not.

Step 5: Counting Electrons in Option (d)

- \( ext{Na}^+\) has 10 electrons (11-1=10).- \( ext{Mg}^{2+}\) has 10 electrons (12-2=10).- \( ext{Al}^{3+}\) has 10 electrons (13-3=10).- \( ext{Cl}^-\) has 18 electrons (17+1=18). \Most ions in this set have 10 electrons except for \( ext{Cl}^-\).

Step 6: Comparing Options for Isoelectronic Species

Option (a) has all ions with 18 electrons. Hence, it is the only option where all ions are truly isoelectronic.

Key Concepts

Electron Configuration

Understanding "electron configuration" is fundamental in determining why certain species are isoelectronic. Electron configuration refers to how electrons are distributed around an atom's nucleus. It essentially defines the arrangement of electrons in "shells" and "orbitals," providing a map of where electrons reside around a nucleus.

When atoms or ions are isoelectronic, they share the same electron configuration, despite being different species. This is because their electron count is identical. For instance,

• Neutral potassium (K) has an electron configuration of \[ 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 \].
• When it becomes \( K^+ \), the 4s electron is lost and the configuration becomes \[ 1s^2 2s^2 2p^6 3s^2 3p^6 \], similar to argon \( Ar \).

This identical electron configuration across different species determines their isoelectronic nature.

Ionic Species

"Ionic species" refer to atoms or molecules that have gained or lost electrons, resulting in a net electrical charge. This charge indicates the number of electrons that have been either shed (positive ions) or gained (negative ions), compared to the number of protons in their nuclei.

Let’s break it down:

• Metals, such as sodium \(Na\) and potassium \(K\), tend to lose electrons to become positively charged cations, like \(Na^+\) and \(K^+\)
• Non-metals, like fluorine \(F\) and chlorine \(Cl\), tend to gain electrons, forming negatively charged anions, seen as \(F^-\) and \(Cl^-\)

In the context of isoelectronic species, recognizing the electron gain or loss is crucial as it directly impacts the electron count, paving the way to compare different ions.

Electron Count

Understanding "electron count" is essential when identifying isoelectronic species. This count tells us the number of electrons present in an atom or ion.

Usually, we start with the number of electrons in a neutral atom, captured by its atomic number. When an atom forms an ion, however:

• Electrons are removed from or added to the atom's outer shell.
• Each added electron increases the electron count by 1, while each lost electron decreases it by 1.

For example, consider the ion \( K^+ \):the neutral potassium atom has 19 electrons. Upon losing one electron to form \( K^+ \), the electron count decreases to 18. Therefore, when comparing ion sets for isoelectronic properties, ensuring each species results in the same electron count is the key factor.