[填空题]
Gaseous aluminium chloride has the molecular formula $AlCl_3$.
1.By counting the valence electrons of the Al atom, explain why $AlCl_3$ is described as electron deficient.
2.Aluminium chloride changes from a gas to a solid when it is cooled. The molecules of $AlCl_3$ dimerise to form $Al_2Cl_6$,which has the following structural formula.
1.Calculate the formal charge of each aluminium atom in $Al_2Cl_6$,Formal charge =- or -.
2.State how the molecular geometry around each aluminium atom changes when $AlCl_3$ dimerises to form $Al_2Cl_6$.In $AlCl_3$ is geometry ;In $Al_2Cl_6$ is geometry.
3.Iodine trichloride also forms a dimeric structure. The structural formula of the $I_2Cl_6$ dimer is shown.
1.Explain why the electron domain geometry of the iodine atoms in $I_2Cl_6$ differs from that of the aluminium atoms in $Al_2Cl_6$.
2.Explain how repulsion between the valence electron pairs determines the molecular geometry around each iodine atom.
