Final answer:
The central atom in AlCl3, aluminum, exhibits sp² hybridization, due to it being surrounded by three regions of electron density in the dimeric (Al2Cl6) structure of the solid form.
Step-by-step explanation:
The hybridization of the central atom in AlCl3 can be determined by considering the number of regions of electron density around the aluminum atom. In the solid state, AlCl3 exists as a molecular framework, where each aluminum atom is surrounded by three chlorine atoms in a planar arrangement, and it also has a partial bond with chloride ions, hence it's often described as an Al2Cl6 dimer. This arrangement suggests that the aluminum atom is surrounded by three regions of electron density (the three Al-Cl bonds) and therefore exhibits sp² hybridization, as seen in other molecules with three regions of electron density such as formaldehyde (CH2O) and ethene (H2CCH2).
The Al2Cl6 dimer has a structure where each aluminum atom is bonded to four chlorines, with two chlorines being shared between two aluminum atoms. The shared chlorines are involved in three-center two-electron bonds, which can be described as coordinate covalent bonds. Even though aluminum in AlCl3 has an additional bond in the dimeric form, it is still considered to exhibit sp² hybridization because these three-center two-electron bonds do not change the hybridization from the simple trivalent state of aluminum.