Animated Solution for Chemistry - s and p-Block Elements: Aluminium chloride exists as dimer, ${\text{Al}}_2{\text{Cl}}_6$ in solid state as well as in solution of non-polar solvents such as benzene. When dissolved in water, it gives

Imagine you are an aluminium atom in a molecule of aluminium chloride ($AlC{l}_3$). You have three valence electrons, and you've shared them with three chlorine atoms. But wait, your outermost shell only has six electrons! You are electron-deficient. You are hungry for more electrons to complete your octet. What do you do?

In the solid state, or when dissolved in a non-polar solvent like benzene, aluminium chloride finds a clever workaround. Two $AlC{l}_3$ molecules come together. A chlorine atom from one molecule donates a lone pair of electrons to the empty orbital of the aluminium atom in the other molecule, and vice versa.

This mutual sharing creates coordinate covalent bonds, forming a dimer: $A{l}_{2}C{l}_6$. In this dimeric structure, both aluminium atoms happily achieve their octets. Because benzene is a non-polar solvent, it doesn't interact strongly with the $A{l}_{2}C{l}_6$ molecules. It simply surrounds them, allowing the dimer to remain intact. The solvent lacks the energetic 'muscle' to pull the atoms apart.

The Power of Hydration Energy

But what happens when you take this covalent dimer and drop it into water? The story changes dramatically. Water is a highly polar molecule. It has a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms.

When $A{l}_{2}C{l}_6$ enters water, the water molecules swarm around it like bees to honey. The oxygen atoms, with their lone pairs and partial negative charges, are intensely attracted to the aluminium atoms.

Here is where the magic happens: the $A{l}^{3+}$ ion is very small and carries a high $+3$ charge. This creates an enormous charge density. When water molecules bind to this highly charged ion, a massive amount of energy is released. This is called hydration energy.

Hexaaquaaluminium(III)

The hydration energy of the $A{l}^{3+}$ ion is so incredibly high that it completely overwhelms the energy of the covalent bonds holding the $A{l}_{2}C{l}_6$ dimer together. The dimer is ripped apart!

But the aluminium doesn't just float around as a bare $A{l}^{3+}$ ion. That would be energetically unfavorable due to its high charge density. Instead, six water molecules arrange themselves around the central aluminium ion. They donate their lone pairs of electrons into the empty $s{p}^3{d}^2$ hybridized orbitals of the aluminium ion.

This forms a beautiful, highly stable octahedral complex ion: $[Al(H_{2}O{)}_6{]}^{3+}$, known as the hexaaquaaluminium(III) ion.

The three chloride ions ($C{l}^{-}$) are also hydrated by water molecules (with the hydrogen atoms pointing towards them) and float freely in the solution.

So, when we look at our options, we can see why simply writing $A{l}^{3+}$ is an oversimplification. In reality, the intense hydration energy drives the formation of the complex ion.

The correct representation of aluminium chloride dissolved in water is the hexaaquaaluminium(III) complex along with three chloride ions. This perfectly matches option (b).

Understanding this process isn't just about memorizing a reaction; it's about appreciating the profound impact a solvent can have on the very nature of chemical bonding. It's a beautiful dance of energy, charge density, and molecular geometry!