Core Concepts
Learning the common polyatomic ions is a rite of passage for all chemistry students. The good news is that many of these ions are really cool, reactive, and colorful. So let’s jump right in and look at the list of common polyatomic ions. Afterwards, we’ll share some cool facts about these ions, and talk about some not-so-common ones. If you haven’t yet, review our article on cations and anions.
Topics covered in other articles:
Monoatomic vs Polyatomic Ions
Chemists use the term “polyatomic” to refer to ions that have more than one atom. This is in contrast to so-called “monoatomic” ions that, predictably, only have one ion. Monoatomic ions are more straightforward to understand and there’s no memorization necessary, since they’re simply ionic forms of the periodic table elements.
However, you should know one important piece of nomenclature when dealing with monoatomic ions: anions have the suffix “-ide”. For instance, O2- is called “oxide”, S2- is “sulfide”, F– is fluoride, and so on. Cations are even easier: chemists just call them by their element names with a roman numeral indicating charge. So Na+ is called “sodium (I)”, Ca2+ is “calcium (II)”, and Fe3+ is “iron (III)”. The one exception is H+, which a lot of chemists just call a proton, because cationic hydrogen is literally just a proton!
Most Common Polyatomic Ions
List of the polyatomic ions you will see most often. Be sure to know these by heart!
Acetate C2H3O2–
Ammonium NH4+
Bicarbonate HCO3– (Hydrogen Carbonate)
Carbonate CO32-
Cyanide CN–
Bisulfate HSO4– (Hydrogen Sulfate)
Hydroxide OH–
Nitrate NO3–
Nitrite NO2–
Peroxide O22-
Phosphate PO43-
Sulfate SO42-
Sulfite SO32-
Less Common Polyatomic Ions
The ions in this list, although less common than the first list, still occur quite frequently in chemistry – so we suggest you learn these as well.
Arsenate AsO43-
Borate BO33- (Orthoborate)
Bromate BrO3–
Chlorate ClO3–
Chlorite ClO2–
Chromate CrO42-
Citrate C6H5O73-
Dihydrogen Phosphate H2PO4–
Ferricyanide Fe(CN)63-
Ferrocyanide Fe(CN)64-
Formate HCO2–
Monohydrogen Phosphate HPO42-
Hydrogen Sulfite HSO3– (Bisulfite)
Hypochlorite ClO–
Dichromate Cr2O72-
Iodate IO3–
Mercury (I) Hg22+ (Mercurous)
Oxalate C2O42-
Perchlorate ClO4–
Permanganate MnO4–
Phosphite PO33-
Metasilicate SiO32-
Thiocyanate SCN–
Thiosulfate S2O32-
Tungstate WO42-
Polyatomic Ions list of fun facts
- Thiocyanate ion is used as a sensitive test for Fe3+ – Iron (III) thiocyanate is very red, even in small concentrations
- There are many different silicate ions, with different quantities of Si and O; the same is true of molybdates
- Explosives commonly inlcude chlorates and perchlorates
- Dichromates are often colored a brilliant yellow, but are also considered carcinogenic
- The mercury (I) ion, while still toxic, is less so than mercury (II)
- Ants use the formate ion as a weapon, in the form of formic acid
- Nitrate salts are universally soluble (inorganic ones; oxynitrates not included)
- Thiosulfate ion has a great use – cleaning iodine stains
- Some metal oxalates decompose when heated to form a pyrophoric elemental powder. Cool!
- The silicate ion can be used to form a chemical garden when colorful metal salts are added to the solution
- Sodium dihydrogen phosphate is also called monosodium phosphate. Yeah, confusing.
- An emergency cure for thallium poisoning is ferric ferrocyanide.
- Toxic ions like arsenate can be detected using arsenic speciation
- Sodium acetate produces hot ice
- Polyatomic ion lists can lead to a lifetime love of descriptive chemistry. Beware!