Coordination Number of Al in AlCl3
In the crystalline state of AlCl3, aluminum (Al) has a coordination number of 6. This means that each aluminum atom is surrounded by 6 chlorine atoms in a regular octahedral arrangement.

Explanation:
- Coordination number: The coordination number of an atom in a compound is the number of atoms bonded to it. In the case of AlCl3, each aluminum atom is bonded to 6 chlorine atoms.
- Crystal structure: In the crystalline state of AlCl3, the aluminum atoms are arranged in a close-packed structure with each aluminum atom at the center of an octahedron formed by 6 chlorine atoms.
- Octahedral arrangement: The octahedral arrangement around the aluminum atom is a result of the aluminum atom's ability to form 6 bonds with the surrounding chlorine atoms.
- Stability: The coordination number of 6 provides stability to the AlCl3 crystal structure by allowing each aluminum atom to achieve a full coordination sphere of 6.
- Geometry: The octahedral geometry around the aluminum atom in AlCl3 is a common coordination geometry for metal atoms with a coordination number of 6.
In conclusion, the coordination number of 6 for aluminum in the crystalline state of AlCl3 is a result of the octahedral arrangement of 6 chlorine atoms around each aluminum atom, providing stability to the compound's crystal structure.