Which of the following represents the correct order of covalent character among the iodides of alkali metals ?

Options

A. Lil `gt` Nal `gt` Kl `gt` Rbl `gt` Csl
B. Nal `gt` Rbl `gt` Cal `gt` Lil
C. Lil `gt` csl `gt` Rbl `gt` Nal
D. Csl `gt` Rbl `gt` Kl `gt` Nal `gt` Lil

Answer

To determine the correct order of covalent character among the iodides of alkali metals, we can use Fajans' rules, which help us understand the polarizing power of cations and the polarizability of anions.

### Step-by-Step Solution:

1. **Understanding Fajans' Rules**:
- Fajans' rules state that the covalent character of an ionic compound increases with the polarizing power of the cation and the polarizability of the anion.
- Polarizing power is influenced by the size and charge of the cation. Smaller cations with higher positive charges have greater polarizing power.
- Polarizability of an anion increases with its size; larger anions are more easily distorted.

2. **Identify the Alkali Metal Iodides**:
- The alkali metals are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs).
- The corresponding iodides are LiI, NaI, KI, RbI, and CsI.

3. **Determine the Size of Cations**:
- The size of the cations decreases in the following order:
- Cs⁺ > Rb⁺ > K⁺ > Na⁺ > Li⁺
- Therefore, lithium is the smallest cation, and cesium is the largest.

4. **Determine the Polarizing Power**:
- Since smaller cations have higher polarizing power, the order of polarizing power among the alkali metal cations is:
- Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺

5. **Determine the Polarizability of Iodide Ion (I⁻)**:
- The iodide ion (I⁻) is the same for all alkali metal iodides. It is a relatively large anion, which means it has high polarizability.

6. **Combine the Information**:
- According to Fajans' rules, the covalent character will be greatest for the iodide with the smallest cation (highest polarizing power).
- Therefore, the order of covalent character among the iodides will be:
- LiI > NaI > KI > RbI > CsI

7. **Final Order**:
- The correct order of covalent character among the iodides of alkali metals is:
- **LiI > NaI > KI > RbI > CsI**