6.4 Naming Compounds Containing Polyatomic Ions
Learning Objectives
By the end of this section, you will be able to:
- Generate a proper name for an ionic compound containing polyatomic ions.
Naming Compounds with Polyatomic ions
There also exists a group of ions that contain more than one atom. These are called polyatomic ions. Table 6.4a “Common Polyatomic Ions” lists the formulas, charges, and names of some common polyatomic ions. Only one of them, the ammonium ion, is a cation; the rest are anions. Most of them also contain oxygen atoms, so sometimes they are referred to as oxyanions. Some of them, such as nitrate and nitrite, and sulfate and sulfite, have very similar formulas and names, so care must be taken to get the formulas and names correct. Note that the -ite polyatomic ion has one less oxygen atom in its formula than the -ate ion but with the same ionic charge. For an accessible version of Table 6.4a, refer to Appendix E.
| Symbol | Name | Symbol | Name | Symbol | Name |
|---|---|---|---|---|---|
| CrO42- | Chromate | BO33- | Borate | SO42- | Sulfate |
| CrO72- | Dichromate | AsO43- | Arsenate | SO32- | Sulfite |
| CN– | Cyanide | BrO– | Hypobromite | HSO4– | Hydrogen sulfate (bisulfate) |
| SCN– | Thiocyanide | BrO3– | Bromate | HSO3– | Hydrogen sulfite (bisulfate) |
| NO3– | Nitrate | CIO– | Hypochlorite | PO43- | Phosphate |
| NO2– | Nitrite | CIO2– | Chlorite | PO33- | Phosphite |
| MnO4– | Permanganate | CIO3– | Chlorate | HPO42- | Hydrogen phosphate |
| OH– | Hydroxide | CIO4– | Perchlorate | H2PO42- | Dihydrogen phosphate |
| O22- | Peroxide | IO4– | Periodate | CO32- | Carbonate |
| NH2– | Amide | IO3– | Iodate | HCO3– | Hydrogen carbonate |
| C2H3O2– | Acetate | IO– | Hypoiodite | HC2O4– | Hydrogen oxalate |
| C2O42- | Oxalate | NH4+ | Ammonium |
The naming of ionic compounds that contain polyatomic ions follows the same rules as the naming for other ionic compounds: simply combine the name of the cation and the name of the anion. Do not use numerical prefixes in the name if there is more than one polyatomic ion; the only exception to this is if the name of the ion itself contains a numerical prefix, such as dichromate or triiodide.
Writing the formulas of ionic compounds has one important difference. If more than one polyatomic ion is needed to balance the overall charge in the formula, enclose the formula of the polyatomic ion in parentheses and write the proper numerical subscript to the right and outside the parentheses. Thus, the formula between calcium ions, Ca2+, and nitrate ions, NO3−, is properly written Ca(NO3)2, not CaNO32 or CaN2O6. Use parentheses where required. The name of this ionic compound is simply calcium nitrate.
Example 6.4a
Problems
Write the proper formula and give the proper name for each ionic compound formed between the two listed ions.
- NH4+ and S2−
- Al3+ and PO43−
- Fe2+ and PO43−
Solutions
- Because the ammonium ion has a 1+ charge and the sulfide ion has a 2− charge, we need two ammonium ions