Stoichiometric names

The Red Book [1, p. 5] uses the term compositional nomenclature

to denote name constructions which are based solely on the composition of the substances or species being named, as opposed to systems involving structural information.

It is the simplest systematic way of naming chemical substances. Compositional nomenclature can be used for both compounds and elementary substances. In case of compounds, is is also known as binary-type nomenclature [2]. Why “binary”? Because the names of compounds named that way always consist of two parts, positive and negative.

The names of elements in the positive part are not changed. If the negative part consists of only one element, this element name is modified as to end with -ide, as in carbon dioxide, copper sulfide, or sodium chloride:

hydrogen	→	hydride	boron	→	boride	carbon	→	carbide
nitrogen	→	nitride	oxygen	→	oxide	fluorine	→	fluoride
phosphorus	→	phosphide	sulfur	→	sulfide	chlorine	→	chloride
selenium	→	selenide	bromine	→	bromide	iodine	→	iodide

In spite of lack of the structural information in compositional names or formulae, they are still widely used in inorganic chemistry.

According to the Red Book [1, p. 68],

The simplest type of compositional name is a stoichiometric name, which is just a reflection of the empirical formula or the molecular formula of the compound.

There is a scope for confusion as to which kind of formula the stoichiometric name may correspond. This is because the empirical formula (aka compositional formula) is not the same as the molecular formula.

Empirical formula is the simplest possible formula expressing the composition of a given (macroscopic) compound or elementary substance. It does not tell us anything about the composition of molecules, nor about the very existence of the discrete molecules. For example, SiO₂ is the empirical formula of silica (corresponding stoichiometric name silicon dioxide), NaCl is an empirical formula of table salt (stoichiometric name sodium chloride) and Au is the empirical formula of metallic gold.

On the other hand, the molecular formulae should be used only — and preferably — if we do know the composition of discrete molecular entities. Thus O₃ is a molecular formula of ozone (stoichiometric name trioxygen), CO₂ is a molecular formula of carbon dioxide and C₆H₁₂O₆ is a molecular formula of glucose.

There are three ways of indicating the proportions of constituents in compositional names: multiplicative prefixes, oxidation states and charge numbers.

Multiplicative prefixes

Multiplicative prefixes used in chemistry (di-, tri-, tetra-, penta-, hexa-, etc.) are of Greek origin. (Just like we don’t use the subscript 1 in empirical or molecular formulae, we don’t normally use the prefix mono-, but see below.) For me, compositional names constructed with these prefixes reflect empirical formulae the best. There are no assumptions about the nature of the bonding in the compound. Besides, this is the only sensible way to construct compositional names for elementary substances.

Comparing (compositional) formulae with the corresponding (compositional) names, one cannot help noticing that the latter may sometimes appear more ambiguous than the former. The reason is that these names often make implicit use of the concept of valency. For example, the compositional name “hydrogen oxide” for water (H₂O) implies that everybody knows that hydrogen is always monovalent and oxygen is always divalent, so a binary compound of hydrogen and oxygen has no choice but to have two atoms of hydrogen per one atom of oxygen. Of course, this logic breaks down with hydrogen peroxide which has the empirical formula HO (but molecular formula H₂O₂). The names like “dihydrogen oxide” or “dihydridooxygen” are unambiguous, while “dihydrogen monoxide” contains the prefix mono- that is usually superfluous. Not always though, as the case of carbon monoxide (CO) vs carbon dioxide (CO₂) demonstrates: saying just “carbon oxide” is not enough.

Binary-type names are not restricted to binary compounds. The names like magnesium potassium trichloride (KMgCl₃) or phosphorus trichloride oxide (PCl₃O) still have positive and negative parts.

Oxidation states

If, by some reason, you dislike Greek prefixes but don’t mind Roman numerals, you may consider using compositional names based on oxidation states (oxidation numbers). A Roman numeral, placed in parentheses after the name of the element, indicate its formal oxidation state, from which one can figure out the proportions of the elements in the name. Let’s see how it works (or doesn’t work) with nitrogen oxides:

Molecular formula	“Greek prefix” name	“Oxidation state” name	ChEBI
NO	nitrogen oxide; nitrogen monooxide; nitrogen monoxide	nitrogen(II) oxide	CHEBI:16480
NO2	nitrogen dioxide	nitrogen(IV) oxide	CHEBI:33101
NO3	nitrogen trioxide	nitrogen(VI) oxide	CHEBI:29329
N2O	dinitrogen oxide	nitrogen(I) oxide	CHEBI:17045
N2O2	dinitrogen dioxide	nitrogen(II) oxide	CHEBI:29797
N2O3	dinitrogen trioxide	nitrogen(III) oxide	CHEBI:29799
N2O4	dinitrogen tetraoxide	nitrogen(IV) oxide	CHEBI:29803
N2O5	dinitrogen pentaoxide	nitrogen(V) oxide	CHEBI:29802

As you can see, it does not work that well with nitrogen(II) oxide and nitrogen(IV) oxide, since each of these names refer to two different compounds (albeit with the same empirical formula).

The formal oxidation states may be positive, zero, or negative, although the ones most commonly used in chemical names are positive. To indicate the negative oxidation state, one has to put the minus sign in front of a Roman numeral. For instance, hydrogen is considered negative (oxidation state –I) in metal hydrides. For the zero oxidation state (found in elementary substances), the numeral 0 is used. As Romans did not have the concept of zero or negative numbers, this use of Roman numerals looks to me a bit weird.

It gets worse. For magnetite Fe₃O₄, the formal oxidation state of iron is fractional (⁸/₃). We can use the “oxidation state” name, iron(II) diiron(III) oxide, but that still means employing the Greek prefix di-. I’d scrap the oxidation states and go for “triiron tetraoxide”. (Incidentally, Romans did use fractions, to be precise duodecimal fractions, so one could express ⁸/₃ = ³²/₁₂ = 2⁸/₁₂ as IIS··, but let’s not do it here.)

Charge numbers

If neither Greek nor Roman ways appeal to you, don’t despair just yet. You can indicate proportions of constituent elements indirectly using charge numbers. In the names constructed in this fashion the charge numbers are expressed by Arabic numerals followed by the plus (for positive charges) or minus (for negative ones) signs. This method, however, only makes sense for ionic compounds (or for coordination compounds that could be approximated as ionic), that’s why I didn’t attempt to give any “charge number” names for nitrogen oxides.

Formula	“Greek prefix” name	“Oxidation state” name	“Charge number” name	ChEBI
CuS	copper sulfide	copper(II) sulfide	copper(2+) sulfide	CHEBI:51110
Cu2S	dicopper sulfide	copper(I) sulfide	copper(1+) sulfide	CHEBI:51114
FeO	iron oxide	iron(II) oxide	iron(2+) oxide	CHEBI:50820
Fe2O3	diiron trioxide	iron(III) oxide	iron(3+) oxide	CHEBI:50819
Fe3O4	triiron tetraoxide	iron(II) diiron(III) oxide	—	CHEBI:50821
AuCl3	gold trichloride	gold(III) chloride	gold(3+) chloride	CHEBI:30076
Au2Cl6	digold hexachloride	gold(III) chloride	gold(3+) chloride	CHEBI:30337
AuF5	gold pentafluoride	gold(V) fluoride	gold(5+) fluoride	CHEBI:30080
UF6	uranium hexafluoride	uranium(VI) fluoride	uranium(6+) fluoride	CHEBI:30235

Compounds such as AuCl₃ and Au₂Cl₆ present a problem as neither oxidation state nor charge number names can distinguish between them. As for Fe₃O₄, we cannot name it with charge numbers at all.

To sum up:

• Empirical formulae, molecular formulae, and stoichiometric names are based on proportions of elements. They don’t tell us anything about structure.
• Empirical formulae describe the composition of (macroscopic) compounds, while molecular formulae deal with the composition of (microscopic) molecular entities.
• A stoichiometric name reflect either empirical or molecular formula, but you can’t always guess which one.
• There could be more than one correct name for every empirical or molecular formula.
• Learn Greek prefixes, we’ll need them anyway.

References

1. Connelly, N.G., Hartshorn R.M., Damhus, T. and Hutton, A.T. Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005. Royal Society of Chemistry, Cambridge, 2005.
2. Leigh, G.J., Favre, H.A. and Metanomski, W.V. Principles of Chemical Nomenclature: A Guide to IUPAC Recommendations. Blackwell Science, 1998, p. 27.