Here's why:

* Aluminum (Al) has three valence electrons.

* Chlorine (Cl) has seven valence electrons.

* In AlCl6, aluminum is bonded to six chlorine atoms. This means that the aluminum atom has six bonding pairs of electrons and no lone pairs.

According to the VSEPR (Valence Shell Electron Pair Repulsion) theory, six bonding pairs around a central atom will arrange themselves in an octahedral shape to minimize electron repulsion.

Therefore, AlCl6 has an octahedral molecular shape.