Problem 124 - A Complete Resource Book in Chemistry for JEE Main

Question

\begin{aligned} &\text { Match the following }\\\ &\begin{array}{ll} \hline \text { Column-I } & \text { Column-II } \\ \hline \text { (a) } \mathrm{Na}^{+}<\mathrm{F}^{-}<\mathrm{O}^{2-}<\mathrm{N}^{3-} & \text { (p) Electronegativity } \\ \text { (b) } \mathrm{Li}^{+}<\mathrm{Na}^{+}<\mathrm{K}^{+}<\mathrm{Rb}^{+}<\mathrm{Cs}^{+} & \text {(q) } \begin{array}{l} \text { Mobility of } \\ \text { hydrated ions } \end{array} \\ \text { (c) } \mathrm{O}<\mathrm{S}<\mathrm{F}<\mathrm{Cl} & \text { (r) } \text { Ionic size } \\ \text { (d) } \mathrm{Cl}^{-}<\mathrm{K}^{+}<\mathrm{Ca}^{2+}<\mathrm{Sc}^{3+} & \text { (s) Electron affinity } \\ \hline \end{array} \end{aligned}Reason: Fluorine has small size and high electronegativity.

Step-by-Step Solution

Answer

(a)-(r), (b)-(q), (c)-(s), (d)-(p)

Step 1: Understand Column-I

In Column-I, we have four sets of elements and ions listed in order of certain properties. Each set represents a sequence from smallest to largest: \( \mathrm{Na}^{+}<\mathrm{F}^{-}<\mathrm{O}^{2-}<\mathrm{N}^{3-} \), \( \mathrm{Li}^{+}<\mathrm{Na}^{+}<\mathrm{K}^{+}<\mathrm{Rb}^{+}<\mathrm{Cs}^{+} \), \( \mathrm{O}<\mathrm{S}<\mathrm{F}<\mathrm{Cl} \), and \( \mathrm{Cl}^{-}<\mathrm{K}^{+}<\mathrm{Ca}^{2+}<\mathrm{Sc}^{3+} \).

Step 2: Understand Column-II

In Column-II, the properties to match with the sequences from Column-I are listed: **(p) Electronegativity**, **(q) Mobility of hydrated ions**, **(r) Ionic size**, and **(s) Electron affinity**.

Step 3: Match Sequence (a)

Sequence \( \mathrm{Na}^{+}<\mathrm{F}^{-}<\mathrm{O}^{2-}<\mathrm{N}^{3-} \) represents an increase in ionic size as negative charge increases for isoelectronic species, matching with **(r) Ionic size**.

Step 4: Match Sequence (b)

Sequence \( \mathrm{Li}^{+}<\mathrm{Na}^{+}<\mathrm{K}^{+}<\mathrm{Rb}^{+}<\mathrm{Cs}^{+} \) shows alkali metals arranged by increasing ionic size and correlated mobility of hydrated ions, matching with **(q) Mobility of hydrated ions**.

Step 5: Match Sequence (c)

Sequence \( \mathrm{O}<\mathrm{S}<\mathrm{F}<\mathrm{Cl} \) represents increasing atomic size from top to bottom of groups, matching with **(s) Electron affinity** where effective nuclear charge decreases down the group affecting affinity.

Step 6: Match Sequence (d)

Sequence \( \mathrm{Cl}^{-}<\mathrm{K}^{+}<\mathrm{Ca}^{2+}<\mathrm{Sc}^{3+} \) shows ions listed by decreasing ionic size corresponding to increased nuclear charge for the same electron count, matching with **(p) Electronegativity**.

Key Concepts

Ionic Size

Ionic size, or ionic radius, refers to the size of an ion. This concept is essential to understand because ions do not have fixed radii like atoms; their size changes depending on their environment.
When atoms lose electrons to form cations, they generally lose an entire electron shell, leading to a smaller size. Conversely, when atoms gain electrons and form anions, they become larger as the electron-electron repulsion in the outer shell increases.
Here's an interesting list to consider:

• Cations (⁺ ions) are smaller than their parent atoms due to the loss of electrons and a decrease in electron-electron repulsion.
• Anions (^- ions) are larger than their parent atoms because of the gain of electrons which increases repulsion between electrons.
• Within a series like isoelectronic species (ions having the same number of electrons), the ion with the greatest positive charge is the smallest, while the one with the greatest negative charge is the largest. For example, in the sequence \( \mathrm{Na}^{+}<\mathrm{F}^{-}<\mathrm{O}^{2-}<\mathrm{N}^{3-} \), the ionic size increases as the negative charge increases.

Understanding ionic size helps explain many physical and chemical properties of elements and their ions, such as solubility and reactivity.

Electronegativity

Electronegativity is a measure of how strongly an atom attracts shared electrons in a chemical bond. This property varies across the periodic table and impacts how atoms bond with one another.
There are a few key points to remember about electronegativity:

• Fluorine is the most electronegative element, which means it has a very strong pull on shared electrons.
• Electronegativity generally increases across a period (from left to right) in the periodic table and decreases down a group (from top to bottom).
• As you move from left to right across a period, atoms have more protons and a greater effective nuclear charge, attracting electrons more strongly.
• Down a group, atoms have more electron shells, which distances the valence electrons from the nucleus, reducing the attraction.

Electronegativity helps predict the type of bond that will form between atoms. For instance, a large electronegativity difference between two atoms often results in an ionic bond, while a smaller difference may result in a covalent bond.

Electron Affinity

Electron affinity is the energy change that occurs when an electron is added to a neutral atom in the gas phase, forming a negative ion. It is an indicator of the attraction of an atom for additional electrons.
Key aspects of electron affinity include:

• Generally, elements with high electron affinity release more energy when they gain an electron, indicating a strong attraction to the added electron.
• Electron affinity tends to increase across a period from left to right as atoms have higher effective nuclear charges and smaller radii.
  For instance, in the sequence \( \mathrm{O}<\mathrm{S}<\mathrm{F}<\mathrm{Cl} \), we see an increase in electron affinity.
• Down a group, electron affinity typically decreases as the added electron would be further away from the nucleus (due to an increased number of electron shells).

Understanding electron affinity is crucial in predicting reactions involving negative ions and assessing an element’s behavior in gaining electrons.

Mobility of Hydrated Ions

When ions are dissolved in water, they often surround themselves with water molecules, becoming 'hydrated.' The mobility of these hydrated ions – or the ability of the ions to move through the solvent – is affected by their size and charge, impacting solutions' conductive properties.
Some points to understand these effects:

• Smaller ions typically have lower mobility because they are often tightly surrounded by water molecules, forming a robust hydration shell.
• Larger ions may have a weaker hydration shell, making them more mobile in a solution.
• The mobility can also depend on the charge of the ion; higher charged ions often have stronger interactions with water molecules.

In the sequence \( \mathrm{Li}^{+}<\mathrm{Na}^{+}<\mathrm{K}^{+}<\mathrm{Rb}^{+}<\mathrm{Cs}^{+} \), the increasing size of the ions correlates with increasing mobility when hydrated. This information is crucial for understanding how ions conduct electricity in solutions and for designing substances for specific uses in electrochemical applications.