The Polyatomic Ion Problem!

Knowing dot structures (Using only the Octet Rule) of polyatomic ions (Corwin Chapter 12 section 12.5), and some keen observations you can boil it down to six questions to determine the formula and the charge on a polyatomic ion :

1.   What is the formula for the –ate polyatomic ion?


See Taylor’s 3/4 Rule:


What is the charge on –ate polyatomic ion?

See Taylor’s Charge Rule:




2.   What happens to the charge on the polyatomic ion when you attach hydrogen atom(s) to the polyatomic 2- and 3- anions?

The hydrogen fills one of the electron holes on the polyatomic ion and it results in an increase of +1 to the 2- charge ( making the charge1-) or 3- charge (making the charge 2-)

Carbonate is CO32-  so hydrogen carbonate or bicarbonate is HCO31-

Phosphate is PO43- so hydrogen phosphate is HPO42-




3.   What does ite mean in a polyatomic ion?

One less oxygen than the –ate ion, but with the same charge as the –ate ion.

 Nitrate is NO31- , but Nitrite is NO21-




5.   How do the hypo- and per- prefixes apply to polyatomic ions?

Hypo- means one oxygen less than ite but the same charge as ite

Chlorite is ClO21-  so hypochlorite is ClO1-

Per- means one oxygen more than –ate but the same charge as –ate


Chlorate is ClO31-  so Perchlorate is ClO41-



6.   What are the two ide polyatomic negative charged Anions and the two -ium positive charged polyatomic Cations?


Hydroxide  is OH1-                         Ammonium  is NH41+

Cyanide is CN1-                                  Hydronium   is H3O1+