When you write the formula for an ionic compound, remember that the positive ion is always listed first. When there are two or more polyatomic ions in a formula, enclose the polyatomic ion in parentheses. Write down the information you have for the charges of the component ions and balance them to answer the problem.

Polyatomic Ion Charge = -1
acetate - C2H3O2-
bicarbonate (or hydrogen carbonate) - HCO3-
bisulfate (or hydrogen sulfate) - HSO4-
chlorate - ClO3-
chlorite - ClO2-
cyanate - OCN-
cyanide - CN-
dihydrogen phosphate - H2PO4-
hydroxide - OH-
nitrate - NO3-
nitrite - NO2-
perchlorate - ClO4-
permanganate - MnO4-
thiocyanate - SCN-

Polyatomic Ion Charge = -2
carbonate - CO32-
chromate - CrO42-
dichromate - Cr2O72-
hydrogen phosphate - HPO42-
peroxide - O22-
sulfate - SO42-
sulfite - SO32-
thiosulfate - S2O32-

Polyatomic Ion Charge = -3
borate - BO33-
phosphate - PO43-