Q.(a) Why AlCl3 exist as dimers?

AlCl3 dimerises to satisfy Al's incomplete octet via coordinate Cl-bridge bonds; all silicate structures are built from a repeating SiO4 tetrahedral unit.

(a) In monomeric AlCl3, aluminium forms three covalent bonds to three chlorine atoms using all three valence electrons, leaving Al with only 6 electrons in its valence shell (incomplete octet) -- electron-deficient and a strong Lewis acid. To achieve greater stability, two AlCl3 units combine: two chlorine atoms (one from each unit) each donate a lone pair to the electron-deficient Al of the other unit, forming two coordinate (dative) Al-Cl-Al bridge bonds. This gives the dimer Al2Cl6, in which each Al is surrounded by 8 electrons (tetrahedrally coordinated to 4 Cl atoms: 2 terminal + 2 bridging).

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