Polyatomic Ions

Polyatomic ions are tightly bound groups composed of multiple elements that possess an overall charge. The positively charged ions are called cations, while the negatively charged ions are called anions. 

The charts below show some of the most common polyatomic ions grouped by their respective charges. Unfortunately when it comes to learn them all you will have to seat down and memorize. 


The one major +1 charge polyatomic ion is the ammonium ion. 



NH4+



Ammonium 



The +2 charge cation is the mercury (I) ion, which is two mercury ions that are bonded together to form a dimer.  



Hg22+



Mercury (I) 


There are many - 1 charge anions and these are the most commonly recognized. 



CH3CO2  or  C2H3O2




Acetate




CN





Cyanide


OH





Hydroxide 



MnO4



Permanganate





NO3





Nitrate



NO2





Nitrite



HCO3





Hydrogen Carbonate or Bicarbonate



H2PO4





Dihydrogen phosphate



H2PO3




Dihydrogen phosphite



HSO4





Hydrogen sulfate or Bisulfate



HSO3





Hydrogen sulfite or Bisulfite



SCN





Thiocyanate



OCN 





Cyanate



Polyatomic ions containing halogens are referred to as oxyhalogens or halogen oxyanions. 



Halogen


Hypohalite 


Halite 


Halate 


Perhalate 


Fluorine


FO

hypofluorite


–––


–––


–––


Chlorine


ClO

Hypochlorite


   ClO2

Chlorite


  ClO3

Chlorate


      ClO4

Perchlorate


Bromine



BrO

Hypobromite


   BrO2

Bromite


  BrO3

Bromate


      BrO4

Perbromate


Iodine



IO

Hypoiodite


   

–––



  lO3

Iodate


        lO4

Periodate




















The most common – 2 charge polyatomic ions are: 



C2O42–  




Oxalate



CO32–


Carbonate



CrO42–


Chromate


Cr2O72–


Dichromate



HPO42–


Hydrogen phosphate



O22–


Peroxide



SO32–


Sulfite



SO42–


Sulfate



SiO32–


Silicate



S2O32–


thiosulfate



The most common – 3 charge polyatomic ions are: 



PO43–  




Phosphate



PO33–


Phosphite



AsO43–


Arsenate

Memorizing these polyatomic ions will be essential later on when dealing with calculating oxidation states and oxidation numbersnaming binary molecular compounds, ionic compounds, and oxyacids.