To DrBob222
posted by Sara .
Can you please explain thoroughly to me about all this.
Why do we criss cross the formulas of compounds?
How do we find out which combining capacity to use if there are more than one? Is there a special formula we use to find out or something? I really do not understand.

Criss cross to obtain the formula of a compound is a simple technique working out the formula. In a formula, the + charges must equal to the negative charges because compounds are neutral. IN Fe2O3, for example, the 2*+3 for Fe = 3*2 for oxgen and +6  6 = 0.That kind of calculation can be done every time a formula is needed; however, criss crossing is a simpler method of obtaining the same thing. If we write +3 above the iron and 2 above the O atom, we obtain Fe2O3 if we criss cross those numbers (we ignore the signs) AND if we obtain numbers that are multiples we usually reduce to the smallest set of numbers. For example MgO we would write +2 above Mg and 2 above O which would give us Mg2O2 which we reduce to MgO.
I don't know of any way to know for sure about the combining capacity when more than one is available. Iron forms so many +2 and +3 compounds; experience helps but I've had years of experience and sometimes I'm not sure either. 
Thank you, I'm kind of getting it. I am developing some understandings on this topic. Thank you very much for the information:)