Memorizing Polyatomic Ions w/O4 & O3
If you're having trouble memorizing the common polyatomic ions table due to the seemingly inconsistant patterns of O3 -ate and O4 -ate, here is my mneumonic.
The abbreviation of the common polyatomic ions with O4 -ate and O3 -ite. W/exception of thiosulfate, which I included because thiosulfate [O3] is contradicted by sulfate [O4]. Oxalate doesn't have an O3 variant on the table but I included it because it's not a common word most people see used compared to others.
Chromate - CrO4
Oxalate - C2O4
Phosphate - PO4
Silicate - SiO4
Sulfate - SO4
Thiosulfate - S2O3
You could add H in there to represent hydrogen sulfate/phosphate, but that's up to you since sulfate and phosphate are already in there. [CrHOPSST].
All of these have variants with O3 -ite. That will help you remember when to call something -ite or -ate. If it's in CrOPSS, you'll remember that SO3 is sulfite, not sulfate.
For 2- charges, I used this mneumonic since I couldn't identify the correlation between compounds that had 2- and those that didn't.
Only 2 negative Perro Saw a Crazy Democrat Called HarryPotter.
Oxalate – O22-
Peroxide – O22-
Sulfur – 2- [all sulfur variants on the common polyatomic table have 2- charge]
Chromate – CrO42-
Dichromate – C2O72-
Carbonate – CO32-
Hydrogen Phosphate – HPO42-