Now the other polyatomic ions don't fit into predictable patterns and so must be memorized. So we're going to start out first with the other tetraoxides. Now these are called the others because they don't quite fit in with the other 2 major tetraoxides we covered in sulfate and phosphate. Now, permanganate. Permanganate's formula is MnO4−1. Then we have chromate, which is CrO42−, and then finally, oxalate, which is C2O42−. Now remember, they're all tetraoxides because they all possess 4 oxygens. But we call them the others because we have manganese, chromium, and carbon. These elements are in different places on the periodic table, so it's hard to form a real pattern with them.
Then we have the other polyatomic ions. We don't really classify them as trioxides or tetraoxides because some of them don't possess that many oxygens or any oxygens at all. So here we have cyanide. Cyanide is CN−. We have hydroxide, which is OH−, peroxide which is O22−. This kind of reminds us a little bit of the mercury 1 ion where we have, in this case, 2 oxygens. Each one is minus 1, so collectively there are 2 minus. Then we have dichromate, which is a little bit similar to chromate. So di meaning that we kind of double things a little bit, but here it's Cr2O72− instead of CrO42−. Then we have cyanate, which is related to cyanide. Cyanate is CNO−, and actually OCN− is the correct way to write it. So, cyanide doesn't possess an oxygen, cyanate does possess an oxygen.
Then finally, we have the acetate ion. Acetate is written as C2H3O2−. Now, this will be the predominant form that you will see, but later on in chemistry, you may see them showing in another form. You may also see it as CH3COO−. So just keep your eyes open. When you see either form, both of them represent the acetate ion.