Polyatomic ions
Names, formulas and charges, grouped the way you actually use them. 30 of them link through to a Lewis structure worked out from the formula.
A polyatomic ion is a group of atoms bonded to each other that carries a charge as a group. Inside it, the atoms share electrons like any covalent molecule. The charge sits on the whole thing, which is why the structure is drawn inside square brackets with the charge outside - it belongs to the ion, not to whichever atom looks most likely.
The charge is not something you derive from the formula. It is part of the ion's identity and you memorise it. What you can derive is everything downstream: sulfate carries 32 valence electrons because its atoms bring 30 and the 2− charge adds two more, and from that count the whole structure follows.
1− ions
| Name | Formula | Charge | Lewis structure |
|---|---|---|---|
| Acetate | C2H3O2− | 1− | Structure → |
| Amide | NH2− | 1− | Structure → |
| Bicarbonate | HCO3− | 1− | Structure → |
| Bromate | BrO3− | 1− | Structure → |
| Chlorate | ClO3− | 1− | Structure → |
| Chlorite | ClO2− | 1− | Structure → |
| Cyanide | CN− | 1− | Structure → |
| Dichloroiodate | ICl2− | 1− | Structure → |
| Dihydrogen phosphate | H2PO4− | 1− | Structure → |
| Hydrogen sulfate | HSO4− | 1− | Structure → |
| Hydrogen sulfite | HSO3− | 1− | Structure → |
| Hydrosulfide | HS− | 1− | Structure → |
| Hydroxide | OH− | 1− | Structure → |
| Hypochlorite | ClO− | 1− | Structure → |
| Iodate | IO3− | 1− | Structure → |
| Nitrate | NO3− | 1− | Structure → |
| Nitrite | NO2− | 1− | Structure → |
| Perchlorate | ClO4− | 1− | Structure → |
| Permanganate | MnO4− | 1− | Manganese is a transition metal. The octet rule does not describe its bonding, so no Lewis structure is drawn here. |
| Tetrachloroiodate | ICl4− | 1− | Structure → |
| Thiocyanate | SCN− | 1− | Structure → |
| Triiodide | I3− | 1− | Structure → |
2− ions
| Name | Formula | Charge | Lewis structure |
|---|---|---|---|
| Carbonate | CO32− | 2− | Structure → |
| Chromate | CrO42− | 2− | Chromium is a transition metal, so the same applies: the ion is real, a textbook Lewis structure for it is not. |
| Dichromate | Cr2O72− | 2− | Two chromium centres bridged by an oxygen. Same reason, twice over. |
| Hydrogen phosphate | HPO42− | 2− | Structure → |
| Oxalate | C2O42− | 2− | Structure → |
| Peroxide | O22− | 2− | Structure → |
| Sulfate | SO42− | 2− | Structure → |
| Sulfite | SO32− | 2− | Structure → |
3− ions
| Name | Formula | Charge | Lewis structure |
|---|---|---|---|
| Phosphate | PO43− | 3− | Structure → |
1+ ions
| Name | Formula | Charge | Lewis structure |
|---|---|---|---|
| Ammonium | NH4+ | 1+ | Structure → |
| Hydronium | H3O+ | 1+ | Structure → |
Why sulfate and sulfite are not the same ion
Most of this list is four short families rather than thirty unrelated facts. Take the chlorine oxyanions, which all carry a 1− charge and differ only in how many oxygens they hold:
| Oxygens | Name | Formula | |
|---|---|---|---|
| 1 | Hypochlorite | ClO− | Structure → |
| 2 | Chlorite | ClO2− | Structure → |
| 3 | Chlorate | ClO3− | Structure → |
| 4 | Perchlorate | ClO4− | Structure → |
The pattern reads outwards from the middle. -ate is the common one, -ite has one oxygen fewer, hypo-…-ite one fewer again, and per-…-ate one more. Nitrate and nitrite work the same way, as do sulfate and sulfite and the bromine and iodine series. The charge does not change along a series, only the oxygen count - so learning one ion in a family gets you the rest.
Checking a charge you are unsure of
There is no rule that turns a formula into a charge, but there is a check. Draw the Lewis structure, work out the formal charge on every atom, and add them up: the total has to equal the charge on the ion. If it does not, the structure is wrong, not the charge. Every structure page on this site shows that sum - for example phosphate, where four oxygens at 1− and a phosphorus at 1+ come to 3−.