Elements like iron have 2 valencies. So, how would I know which one to use?

Answer 1

It depends on the particular situation. If given the name, such as iron(III) chloride, you would use the Fe³⁺ (FeCl₃) . If given iron(II) chloride, you'd use Fe²⁺ (FeCl₂). If given the formula, e.g. Cu(NO₃)₂, you know it is Cu²⁺ but if given CuNO₃, it would be Cu⁺. Does that answer your question?