Monday, June 3, 2019

inorganic chemistry - The Crisscross method for finding the chemical formula



I am reading this wikipedia article that I don't understand. What I don't understand is:



suppose we have two elements X and Y having oxidation numbers x and y respectively. Can we prove that the compound formed will be XyYx.



I tried to understand this method as: I think this method comes from the fact of charge neutrality. Suppose the compound to be formed has formula XyYx. The charge on y number of X elements is xy. The charge on x number of Y elements is xy. Now by charge neutrality we should have : |x|y=x|y|


this implies |x||y|=xy.



But I could not further understand why the we divide x and y with their greatest common factor to find the required solution.



I have also found some contradiction examples to this Crisscross rule. Example is : Solid-state-semiconductor GaAs, in this Gallium has valancy 3 and As has valancy 5.

The main points on which I want answers are:




  • What is the logic behind this method, how to prove this method?

  • Who, how and when developed this method?

  • Is this method only applicable to ionic compounds?

  • Why compounds like GaAs do not follow this rule, how charge neutrality is preserved in these compounds. I think these don't follow this rule because these are not ionic compounds, if yes then how the chemical formula of compounds like solid state GaAs is known(or decided) and how it is justified, how one atom of Ga balances one atom of As ,their valencies are different, how they are bonded(covalently I guess) together?





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