Binary-type nomenclature can be extended beyond simple stoichiometric names. Let’s have a look at the compound with empirical formula HKO. If we were trying to come up with purely stoichiometric name, it would be either potassiumhydride oxide or hydrogen potassiumoxide, but nobody calls it that. Moreover, it is customary to write its formula not like I did (with element symbols ordered alphabetically), but KOH. Why? Because it is known that KOH is an ionic compound which will dissociate in water into cations K+ and anions OH−. Attention please: we have just zoomed from (macroscopic) compound to (microscopic) molecular entities.
So we’ve got some extra structural information, viz. that the anion is composed of oxygen and hydrogen. The anion OH− is known as hydroxide and thus our compound is named potassiumhydroxide.
Likewise, it is known that NH4NO3 dissociates into cations NH4+ and anions NO3−. The NH4+ cation is known as ammonium and the NO3− anion as nitrate so our compound is is named ammoniumnitrate.
As you can see, to use this extended binary-type nomenclature we need to know more than just the empirical formula. First, we need to know the correct way to divide the formula into positive and negative parts. Second, we have to be able to name the heteropolyatomic ions.
The way the formulae (both empirical and molecular) are written often provide clues about the composition of the cationic and anionic parts: KOH rather than HKO, NH4NO3 rather than H4N2O3, H2CO3 rather than CH2O3 and so on. That can be helpful.
It is possible to name the heteropolyatomic ions systematically, but there is an extensive list of trivial names (or “acceptable non-systematic names”, in IUPAC parlance) that are shorter and more widely used. The names of anions are derived from corresponding inorganic acids, the names of cations from corresponding bases.
| Base | Cation | |||
|---|---|---|---|---|
| Formula | Name | Formula | Name | |
| AsH3 | arsane | → | AsH4+ | arsonium |
| BH3 | borane | → | BH4+ | boronium |
| H2O | water | → | H3O+ | oxonium |
| H2S | hydrogen sulfide | → | H3S+ | sulfonium |
| NH3 | ammonia | → | NH4+ | ammonium |
| PH3 | phosphane | → | PH4+ | phosphonium |
| SbH3 | stibane | → | SbH4+ | stibonium |
| Acid | Anion | |||
|---|---|---|---|---|
| Formula | Name | Formula | Name | |
| H3BO3 | boric acid | → | BO33− | borate |
| HBrO | hypobromous acid | → | BrO− | hypobromite |
| HBrO2 | bromous acid | → | BrO2− | bromite |
| HBrO3 | bromic acid | → | BrO3− | bromate |
| HBrO4 | perbromic acid | → | BrO4− | perbromate |
| HCN | hydrogen cyanide | → | CN− | cyanide |
| H2CO3 | carbonic acid | → | CO32− | carbonate |
| HClO | hypochlorous acid | → | ClO− | hypochlorite |
| HClO2 | chlorous acid | → | ClO2− | chlorite |
| HClO3 | chloric acid | → | ClO3− | chlorate |
| HClO4 | perchloric acid | → | ClO4− | perchlorate |
| H2CrO4 | chromic acid | → | CrO42− | chromate |
| H2Cr2O7 | dichromic acid | → | Cr2O72− | dichromate |
| HIO | hypoiodous acid | → | IO− | hypoiodite |
| HIO2 | iodous acid | → | IO2− | iodite |
| HIO3 | iodic acid | → | IO3− | iodate |
| HIO4 | periodic acid | → | IO4− | periodate |
| H5IO6 | orthoperiodic acid | → | IO65− | orthoperiodate |
| HMnO4 | permanganic acid | → | MnO4− | permanganate |
| H2MnO4 | manganic acid | → | MnO42− | manganate |
| H3MnO4 | hypomanganic acid | → | MnO43− | hypomanganate |
| HNO2 | nitrous acid | → | NO2− | nitrite |
| HNO3 | nitric acid | → | NO3− | nitrate |
| H2O | water | → | OH− | hydroxide |
| H3PO3 | phosphorous acid | → | PO33− | phosphite |
| H3PO4 | phosphoric acid | → | PO43− | phosphate |
| H4P2O7 | diphosphoric acid | → | P2O74− | diphosphate |
| H5P3O10 | triphosphoric acid | → | P3O105− | triphosphate |
| H2SO3 | sulfurous acid | → | SO32− | sulfite |
| H2SO4 | sulfuric acid | → | SO42− | sulfate |
| H2S2O7 | disulfuric acid | → | S2O72− | disulfate |
| H2SeO3 | selenous acid | → | SeO32− | selenite |
| H2SeO4 | selenic acid | → | SeO42− | selenate |
| H4SiO4 | silicic acid | → | SiO44− | silicate |
| H6Si2O7 | disilicic acid | → | Si2O76− | disilicate |
Some of these acceptable non-systematic names themselves start with a “Greek prefix”, for example dichromate, triphosphate, disulfate, disilicate. To avoid ambiguity, the alternative multiplicative prefixes bis-, tris-, tetrakis-, pentakis- etc. are used, with the name of the “multiplied” ion placed in parentheses. Within the complete molecular formulae, the formulae of polyatomic ions being multiplied are also enclosed in parentheses and followed by the corresponding multiplicative subscript.
| Formula | “Greek prefix” name | “Oxidation state” name | “Charge number” name | ChEBI |
|---|---|---|---|---|
| Ca3(PO4)2 | tricalcium bis(phosphate) | calcium(II) phosphate | calcium(2+) phosphate | CHEBI:9679 |
| Cu(NO3)2 | copper dinitrate | copper(II) nitrate | copper(2+) nitrate | CHEBI:78036 |
| FeSO4 | iron sulfate | iron(II) sulfate | iron(2+) sulfate | CHEBI:75832 |
| Fe2(SO4)3 | diiron trisulfate | iron(III) sulfate | iron(3+) sulfate | CHEBI:53438 |
| Fe4(P2O7)3 | tetrairon tris(diphosphate) | iron(III) diphosphate | iron(3+) diphosphate | CHEBI:132767 |
| (NH4)2Fe(SO4)2 | diammonium iron bis(sulfate) | ammonium iron(II) sulfate | ammonium iron(2+) sulfate | CHEBI:76243 |
Among other things, this table shows that the naming of chemicals is an art. The multiplicative name for Ca3(PO4)2 is totally unambiguous but cumbersome, while, knowing that calcium is almost always divalent, both oxidation state and charge number are redundant in the names. I’d just call it “calcium phosphate”. On the other hand, “iron sulfate” for FeSO4, although formally correct, sounds ambiguous, because we know that iron can have different oxidation states. In case of Fe4(P2O7)3, the “Greek prefix” name is my favourite since it makes perfectly clear that we are talking about diphosphate anion, not two phosphate anions.