There are different ways to write different formulas.
Calcium & Nitrogen
1. Identify the two elements
Ca & N
2. Identify the charges of the two elements.
Ca + 2 N -3
3. Make sure the charges balance by multiplying them together if they do not add up to zero.
Ca 2(3) N -3(2)
Ionic Compounds with Polyatomic Ions
Iron III & Sulfate
1. Identify the 2 elements.
Fe & SO4
2. Identify the charges. Make sure you don't mix up the charge with the number of elements!
Fe +3 (SO4)-2
3. Balane the charges. The charge goes on the OUTSIDE of the parenthesese.
1. Identify the elements.
Ca & Cl
2. Identify the NUMBER in each molecule.
Ca has 2 (DIcalcium) and Cl has 7 (HEPTAchloride)
3. Write that number behind the element's symbol.