Question

The coordination number of Al in crystalline AlCl$_3$ and liquid AlCl$_3$, respectively, is

• A. 4 and 4
• B. 6 and 6
• C. 6 and 4
• D. 3 and 6

Hint:

AlCl$_3$ increases its coordination number upon melting because it forms the dimer Al$_2$Cl$_6$.

Answer 1

The Correct Option is C

Step 1: Structure of crystalline AlCl$_3$.
Solid AlCl$_3$ has a layer-type structure similar to AlBr$_3$. Each Al atom is bonded to 3 Cl atoms in a trigonal planar arrangement. Thus, its coordination number is 3.
Step 2: Structure of liquid AlCl$_3$.
On melting, AlCl$_3$ dimerizes to form Al$_2$Cl$_6$, where each Al atom achieves octahedral coordination through bridging chlorides. This raises the coordination number to 6.
Step 3: Conclusion.
Hence the coordination numbers are 3 (solid state) and 6 (liquid state).