A yellow coloured crystalline substance gave a colourless gas `X` on reaction with fluorine, which is thermally stable and has octahedral geometry. `X` can be

Options

A. `SF_4`
B. `S_2 F_2`
C. `SF_6`
D. `S_2 F_6`

Explanation

To solve the problem, we need to identify the colorless gas `X` produced when a yellow crystalline substance reacts with fluorine. We will follow these steps:

### Step 1: Identify the yellow crystalline substance
The yellow crystalline substance mentioned in the question is likely sulfur (S). Sulfur is known to be yellow in color and is a common element that can react with fluorine.

**Hint:** Consider the common characteristics of elements in the oxygen family (Group 16) and their colors.

### Step 2: Reaction with fluorine
When sulfur reacts with fluorine (F2), it forms sulfur hexafluoride (SF6). The balanced chemical equation for this reaction is:

\[ S + 3F_2 \rightarrow SF_6 \]

**Hint:** Recall the typical reactions of sulfur with halogens.

### Step 3: Identify the properties of the product
The product SF6 is a colorless gas. It is also known to be thermally stable and has an octahedral geometry.

**Hint:** Think about the physical state and stability of common sulfur compounds.

### Step 4: Determine the geometry of SF6
To confirm the geometry of SF6, we can analyze its hybridization. Sulfur has six valence electrons, and when it forms SF6, it uses these electrons to bond with six fluorine atoms.

The hybridization can be calculated using the formula:
\[ \text{Hybridization} = \frac{1}{2} \times (\text{Valence electrons of central atom} + \text{Number of monovalent atoms}) \]

For SF6:
- Valence electrons of sulfur = 6
- Number of monovalent fluorine atoms = 6

So,
\[ \text{Hybridization} = \frac{1}{2} \times (6 + 6) = 6 \]

This corresponds to sp³d² hybridization, which gives an octahedral geometry.

**Hint:** Remember the relationship between hybridization and molecular geometry.

### Conclusion
Based on the above steps, the colorless gas `X` produced from the reaction of sulfur with fluorine is sulfur hexafluoride (SF6).

**Final Answer:** The colorless gas `X` is SF6.