How to find the valency of ionic compounds
For finding valencies of ions in compounds like A3B2 we can refer to Crisscross method.
The Crisscross method is a method used to find the chemical formula of ionic compounds, with the help of oxidation number of the ions in the compound. The oxidation number of the ions gives the valency of that ion.
Oxidation number of the first ion is used as the subscript of the second, and vice versa. The subscripts are then reduced by the greatest common factor.
Let us find the formula for Barium chloride
The oxidation state of Barium (Ba) is +2 (valency 2) and chlorine (Cl) is -1 (valency 1) i.e. Ba2+ Cl-.
Now cross the oxidation number of both ions, 2 becomes the subscript of chloride ion and 1 becomes the subscript of Ba. It becomes BaCl2. This is the chemical formula of barium chloride.
Let us find the formula for aluminium chloride
The oxidation state of Aluminium (Al) is +3 (valency 3) and chlorine (Cl) is -1 (valency 1) i.e. Al3+Cl-.
Now cross the oxidation number of both ions, 3 becomes the subscript of chloride ion and 1 becomes the subscript of Al. It becomes AlCl3. This is the chemical formula of aluminium chloride.
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