Valency of aluminium:

The atomic number of Aluminium is 13 ,so its electronic configuration is 2, 8, 3 .It has 3 electrons in outermost SHELL . The aluminium ATOM can lose these 3 electrons to form the aluminium ION ,Al³+, having an inert gas electron configuration of 2,8.

Since one atom of aluminium loses 3 (or 3+) electrons to achieve the inert gas electron configuration.

Valency of chlorine:

The atomic number of chlorine is 17 ,so its electronic configuration is 2, 8, 7 . The chlorine atom has 7 electrons in its outermost shell and it needs one electron to achieve the 8 electron configuration. So the chlorine atom gains 1 electron to form chloride Ion Cl- having an inert gas electron ARRANGEMENT of 2,8,8.

Since one chlorine atom gains 1 electron to achieve the inert gas configuration so the valency of chlorine is 1(or 1-)..

In AlCl3

Al³+ Cl¹-

3+ 1-

On crossing over the valency of aluminium and chloride ions the FORMULA of aluminium chloride becomes AlCl3

Aluminium chloride contains aluminium ions Al³+ and chloride ions Cl-. Now the valency of aluminium ion Al³+ and is 3 and that of chloride ion is 1. So one aluminium ion Al³+ will unite with 3 chloride ions to form AlCl3.