Binary-type, extended

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 potassium hydride oxide or hydrogen potassium oxide, 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 potassium hydroxide.

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 ammonium nitrate.

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.