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NiNO3 -> Nickel (I) Nitrate .Each NO3 is -1...Therefore, nickel must be +1
Cr3(Po4)2 -> Chromium (II) phosphate...Po4 has -3.so total -6..cr should be +2.
All compounds must have a total charge of zero, so sometimes you must use subscripts to balance the charges of the cations and anions.
When forming compounds with transition metals, it is necessary to know which of the different charges for the metal is being used. This information is either provided or can be figured out from the charge of the anion.
The subscript for the polyatomic ion is located outside of the parentheses.
Please let me know if you need any clarification. I'm always happy to answer your questions.