Predictable Monatomic Charges

Polyatomic Cations
 
Hg22+ mercury(I) NH4+ ammonium

Polyatomic Anions
 
1- 2- 3-
-ate, -ite
NO3- nitrate SO42- sulfate PO43- phosphate
NO2- nitrite SO32- sulfite PO33- phosphite
ClO- hypochlorite
ClO2- chlorite
ClO3- chlorate
ClO4- perchlorate
(also F, Br, I)
no -ite
C2H3O2- acetate CO32- carbonate
  C2O42-
Metal containing
MnO4- permanganate CrO42- chromate
Cr2O72- dichromate
 
-ide
N3- azide
OH- hydroxide
CN- cyanide O22- peroxide