The coordination number of `Al` in the crystalline state of `AlCl_(3)` is _____.

Explanation

To determine the coordination number of aluminum (Al) in the crystalline state of aluminum chloride (AlCl₃), we can follow these steps:

### Step 1: Understand the Structure of AlCl₃
AlCl₃ in its crystalline form adopts a face-centered cubic (FCC) lattice structure. In this structure, the aluminum ions (Al³⁺) occupy the octahedral voids, while the chloride ions (Cl⁻) are located at the lattice points, which include the corners and face centers of the cubic unit cell.

**Hint:** Recall that the coordination number is defined as the number of nearest neighboring ions surrounding a central ion in a crystal lattice.

### Step 2: Identify the Positions of Ions
In the FCC lattice:
- The chloride ions (Cl⁻) are located at the corners and face centers of the cube.
- The aluminum ions (Al³⁺) occupy the octahedral voids, which are located at the body center and the centers of the edges of the cube.

**Hint:** Visualize or draw the FCC structure to better understand the arrangement of ions.

### Step 3: Determine the Coordination Number
The coordination number of an ion is determined by counting the number of closest neighboring ions surrounding it. For Al³⁺ ions in this structure:
- The closest chloride ions are located at the face centers of the cube.
- Each aluminum ion is surrounded by six chloride ions, which are the nearest neighbors.

**Hint:** Count the number of nearest Cl⁻ ions around the Al³⁺ ion to find the coordination number.

### Step 4: Conclusion
Thus, the coordination number of aluminum (Al) in the crystalline state of AlCl₃ is **6**.

**Final Answer:** The coordination number of Al in the crystalline state of AlCl₃ is **6**.