The valence of aluminum is + 3 and the balance of chlorine is -1 the formula for aluminum chloride is correctly written as

The formula for aluminium chloride will be correctly written as AlCl3. 
Aluminium is a metal and in this reaction it is an electron donor. It has three electrons in its outermost shell and it is ready to give out all the three electrons to chlorine. Chlorine is a non metal and is an electron acceptor and it is more than ready to accept electrons from aluminium. But a chlorine atom can only receive only one electron from aluminium because, it already has seven electrons in its outermost shell and it only needs eight electrons in its outermost shell to be stable. To solve this problem, three chlorine atoms will react with one aluminium atom in order to accept all the three electrons from aluminium, hence the chemical formula AlCl3..

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