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Its about writing formulas.

For example, aluminum chloride. I know that Al has a valence number of +3, and I know that Cl has a valence number of -1.

So applying the "crossover rule" the answer is AlCl3.

Now the only reason I know the valence numbers of Aluminum and Chlorine is because I got them from a Valence number table with about 10 different elements.

My question is, how do I figure out the valence number for all the other elements?

thanks