Problem 12 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The correct statement for the molecule, \(\mathrm{CsI}_{3}\) is (a) It is a covalent molecule. (b) It contains \(\mathrm{Cs}^{+}\)and \(\mathrm{I}_{3}\) ions. (c) It contains \(\mathrm{Cs}^{3+}\) and \(\mathrm{I}^{-}\)ions. (d) It contains \(\mathrm{Cs}^{+}, \mathrm{I}^{-}\)and lattice \(\mathrm{I}_{2}\) molecule.

Step-by-Step Solution

Answer

(b) It contains \(\mathrm{Cs}^{+}\) and \(\mathrm{I}_{3}^{-}\) ions.

Step 1: Analyze Electronegativity

Cesium (Cs) is an alkali metal with very low electronegativity, while iodine (I) has a higher electronegativity. Typically, when there is a significant difference in electronegativity, ionic bonds are likely. This means Cs often forms cations (Cs\(^+\)) with iodine forming anions (I\(^-\)).

Step 2: Consider Possible Iodine Molecules

Given the compound is CsI\(_3\), we suspect that it involves more than one iodine atom. Common forms of polyiodides include I\(_2\) and I\(_3\) molecules, which suggests possible ion types are I\(_3\)^-.

Step 3: Evaluate Each Option

(a) Covalent bonding isn't likely due to the ionic nature of the components. (b) CsI\(_3\) can dissociate into Cs\(^+\) and I\(_3\)^- due to its structure. (c) Cs often forms Cs\(^+\) rather than Cs\(^{3+}\). (d) I\(_3\) ions are more plausible than separate I\(_2\) molecules.

Step 4: Select the Correct Statement

Option (b) correctly matches our analysis: CsI\(_3\) contains Cs\(^+\) and I\(_3\)^-.

Key Concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry that helps us understand how atoms interact in bonds. It is a measure of an atom's ability to attract and hold onto electrons. In general, the greater the electronegativity of an atom, the more it pulls electrons towards itself in a chemical bond.

- **Cesium (Cs)**: As an alkali metal, cesium has a very low electronegativity. This means it does not hold onto its electrons tightly and readily loses an electron to form positive ions.- **Iodine (I)**: This non-metal has a higher electronegativity compared to cesium. Iodine tends to gain electrons, forming negative ions.

When cesium and iodine come together, the difference in their electronegativities leads to the formation of an ionic bond. In such bonds, electrons are transferred from the less electronegative cesium to the more electronegative iodine, resulting in cesium cations (Cs\(^+\)) and iodine anions (I\(^-\)). This fundamental property of ionic bonding is critical to understanding compounds like cesium iodide in its various forms.

Polyiodide Ions

Polyiodide ions are an interesting and complex part of inorganic chemistry. These ions are formed in compounds that contain more than one iodine atom. The most common forms are diiodide (I\(_2\)^-) and triiodide (I\(_3\)^-) ions.

- **Triiodide Ion (I\(_3\)^-)**: This is a linear anion made up of three iodine atoms. It occurs when extra iodine atoms are involved in the reaction, forming a bridge with existing ions.

In the case of cesium iodide (CsI\(_3\)), the presence of I\(_3\)^- ions indicates that more than one iodine atom is involved in the compound. This helps explain why cesium iodide can form different structures. The triiodide ion plays a key role in determining the overall ionic structure and contributes to the unique properties of polyiodide compounds. Compounds with these iodine molecules exhibit interesting electrical and magnetic properties, largely due to their complex bonding arrangements.

Cesium Iodide Dissociation

Dissociation is important for understanding how ionic compounds like cesium iodide (CsI\(_3\)) dissolve in solution. When cesium iodide comes into contact with water or another solvent, it dissociates into ions.

- **Ionic Dissociation**: In water, cesium iodide separates into its ions; cesium cations (Cs\(^+\)) and triiodide anions (I\(_3\)^-). These ions are hydrated by water molecules and become free to move, conducting electricity.- **Stability of Ions**: The ease of dissociation is due to the respective stability of the ions formed. Cesium prefers to exist as a positive ion while iodine, particularly in triiodide form, stabilizes as a negative ion.

Cesium iodide’s ability to dissociate is a crucial aspect of its chemical behavior, impacting everything from its solubility to its role in different chemical reactions. Understanding this dissociation helps explain why options like (b) in the original exercise correctly describe the structure present in cesium iodide solutions.