Why does alcl3 form a dimer in vapour state

Explanation:

AlCl₃ forms a dimer in vapor state in order to compensate for the insufficient number of electrons that will make it attain stability in its structure.

Dimerization is a situation in which a molecule of a compound is derived from two identical monomers.

Dimerization yields very weak/strong intermolecular interaction producing some chemical bonds.  In the monomeric state, AlCl₃ does not have a complete octet. When in the vapor state, they attach to one another to compensate for this deficiency to attain stability.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.

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