Assertion : The shape of `XeF_(4)` is square- planar.
Reason : In an octahedral geometry, a single lone pair can occupy any position but a second lone pair will occupy the opposite position to the first lone pair.

Options

A. If assertion is true but the reason is false
B. If assertion is false but reason is true
C. IF both assertion and reason are true and the reason is the correct explanation of assertion
D. If both assertion and reason are true but reason is not the correct explanation of assertion

Explanation

To analyze the assertion and reason provided in the question, let's break down the concepts step by step:

### Step 1: Understanding the Assertion
- **Assertion**: The shape of `XeF4` is square-planar.
- **Explanation**: Xenon tetrafluoride (XeF4) has a central xenon atom bonded to four fluorine atoms. In the case of XeF4, the molecular geometry can be determined by considering the number of bonding pairs and lone pairs around the xenon atom.

### Step 2: Determining the Valence Electrons
- **Valence Electrons of Xenon**: Xenon is in Group 18 of the periodic table and has 8 valence electrons.
- **Fluorine Atoms**: Each fluorine atom contributes 1 electron, and since there are 4 fluorine atoms, they contribute a total of 4 electrons.
- **Total Electrons**: Therefore, the total number of valence electrons in XeF4 is 8 (from Xe) + 4 (from 4 F) = 12 electrons.

### Step 3: Drawing the Lewis Structure
- **Bonding Pairs**: In the Lewis structure, Xe forms four single bonds with the four fluorine atoms, using 8 electrons (4 bonds).
- **Lone Pairs**: After forming the bonds, there are 4 electrons left, which will form 2 lone pairs on the xenon atom.

### Step 4: Determining the Steric Number
- **Steric Number Calculation**: The steric number is calculated as the sum of the number of bonding pairs and lone pairs. In XeF4:
- Number of sigma bonds = 4 (from the four Xe-F bonds)
- Number of lone pairs = 2
- Steric number = 4 (bonds) + 2 (lone pairs) = 6.

### Step 5: Hybridization and Geometry
- **Hybridization**: The hybridization corresponding to a steric number of 6 is `sp^3d^2`.
- **Molecular Geometry**: With 4 bonding pairs and 2 lone pairs, the arrangement of the atoms is octahedral, but the presence of lone pairs leads to a square-planar shape. The lone pairs occupy axial positions, while the fluorine atoms occupy the equatorial positions.

### Step 6: Analyzing the Reason
- **Reason**: In an octahedral geometry, a single lone pair can occupy any position, but a second lone pair will occupy the opposite position to the first lone pair.
- **Explanation**: This statement is partially correct. In an octahedral geometry, lone pairs do tend to occupy positions that minimize repulsion, which often means they will be opposite each other. However, the assertion that a lone pair can occupy any position is misleading; they will preferentially occupy positions that reduce electron-electron repulsion.

### Final Conclusion
- **Assertion**: True (the shape of XeF4 is indeed square-planar).
- **Reason**: True, but it does not correctly explain the assertion.
- **Final Answer**: Both the assertion and reason are true, but the reason is not the correct explanation of the assertion.