Arrange the following in the decreasing order of their thermal stability of `Cl_(2), F_(2), I_(2), Br_(2)`.

Options

A. `F_(2)gt Cl_(2)gt Br_(2)gt I_(2)`
B. `Cl_(2)gt Br_(2)gt F_(2)gt I_(2)`
C. `Cl_(2)gt F_(2)gt Br_(2)gt I_(2)`
D. `F_(2)gt Br_(2)gt Cl_(2)gt I_(2)`

Explanation

To determine the decreasing order of thermal stability for the molecules \( Cl_2, F_2, I_2, \) and \( Br_2 \), we need to analyze the bond strengths and lengths of these diatomic molecules. Here’s a step-by-step solution:

### Step 1: Understand Thermal Stability
Thermal stability refers to the ability of a compound to remain intact at elevated temperatures. In the context of diatomic molecules, it is often related to the strength of the bond between the two atoms. Stronger bonds are generally more stable and thus have higher thermal stability.

### Step 2: Analyze Bond Strengths
The bond strength of diatomic molecules can be influenced by factors such as atomic size, bond length, and the presence of lone pairs. The bond strength can be ranked based on the type of overlap between atomic orbitals:

- **Fluorine (\( F_2 \))**: The bond is formed by the overlap of 2p orbitals. However, \( F_2 \) has three lone pairs on each atom, leading to significant repulsion, which weakens the bond.
- **Chlorine (\( Cl_2 \))**: The bond is formed by the overlap of 3p orbitals. Chlorine has fewer lone pairs compared to fluorine, leading to less repulsion and a stronger bond than \( F_2 \).
- **Bromine (\( Br_2 \))**: The bond is formed by the overlap of 4p orbitals. The bond length is longer than that of \( Cl_2 \), which generally leads to a weaker bond.
- **Iodine (\( I_2 \))**: The bond is formed by the overlap of 5p orbitals. Iodine has the longest bond length, making it the weakest bond among these molecules.

### Step 3: Rank the Molecules
Based on the analysis of bond strengths:
1. **\( Cl_2 \)** has the strongest bond due to less lone pair repulsion and optimal orbital overlap.
2. **\( Br_2 \)** has a weaker bond than \( Cl_2 \) but stronger than \( F_2 \).
3. **\( F_2 \)** has a weak bond due to lone pair repulsion, making it less stable than \( Br_2 \).
4. **\( I_2 \)** has the weakest bond due to the longest bond length and the largest atomic size.

### Final Order of Thermal Stability
Thus, the decreasing order of thermal stability is:
\[ Cl_2 > Br_2 > F_2 > I_2 \]