IUPAC nomenclature of inorganic chemistry

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For the current Red Book version, see IUPAC nomenclature of inorganic chemistry 2005.

In chemical nomenclature, the IUPAC nomenclature of inorganic chemistry is a systematic method of naming inorganic chemical compounds, as recommended by the International Union of Pure and Applied Chemistry (IUPAC). It is published in Nomenclature of Inorganic Chemistry (which is informally called the Red Book).[1] Ideally, every inorganic compound should have a name from which an unambiguous formula can be determined. There is also an IUPAC nomenclature of organic chemistry.


The names "caffeine" and "3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione" both signify the same chemical. The systematic name encodes the structure and composition of the caffeine molecule in some detail, and provides an unambiguous reference to this compound, whereas the name "caffeine" just names it. These advantages make the systematic name far superior to the common name when absolute clarity and precision are required. However, for the sake of brevity, even professional chemists will use the non-systematic name almost all of the time, because caffeine is a well-known common chemical with a unique structure. Similarly, H2O is most often simply called water in English, though other chemical names do exist.

  1. Single atom anions are named with an -ide suffix: for example, H is hydride.
  2. Compounds with a positive ion (cation): The name of the compound is simply the cation's name (usually the same as the element's), followed by the anion. For example, NaCl is sodium chloride, and CaF2 is calcium fluoride.
  3. Cations which have taken on more than one positive charge are labeled with Roman numerals in parentheses. For example, Cu+ is copper(I), Cu2+ is copper(II). An older, deprecated notation is to append -ous or -ic to the root of the Latin name to name ions with a lesser or greater charge. Under this naming convention, Cu+ is cuprous and Cu2+ is cupric. For naming metal complexes see the page on complex (chemistry).
  4. Oxyanions (polyatomic anions containing oxygen) are named with -ite or -ate, for a lesser or greater quantity of oxygen, respectively. For example, NO2 is nitrite, while NO3 is nitrate. If four oxyanions are possible, the prefixes hypo- and per- are used: hypochlorite is ClO, perchlorate is ClO4.
  5. The prefix bi- is a deprecated way of indicating the presence of a single hydrogen ion, as in "sodium bicarbonate" (NaHCO3). The modern method specifically names the hydrogen atom. Thus, NaHCO3 would be pronounced sodium hydrogen carbonate.

Positively charged ions are called cations and negatively charged ions are called anions. The cation is always named first. Ions can be metals or polyatomic ions. Therefore the name of the metal or positive polyatomic ion is followed by the name of the non-metal or negative polyatomic ion. The positive ion retains its element name whereas for a single non-metal anion the ending is changed to -ide.

Example: sodium chloride, potassium oxide, or calcium carbonate.

When the metal has more than one possible ionic charge or oxidation number the name becomes ambiguous. In these cases the oxidation number (the same as the charge) of the metal ion is represented by a Roman numeral in parentheses immediately following the metal ion name. For example in uranium(VI) fluoride the oxidation number of uranium is 6. Another example is the iron oxides. FeO is iron(II) oxide and Fe2O3 is iron(III) oxide.

An older system used prefixes and suffixes to indicate the oxidation number, according to the following scheme:

Oxidation state Cations and acids Anions
Lowest hypo- -ous hypo- -ite
  -ous -ite
  -ic -ate
  per- -ic per- -ate
Highest hyper- -ic hyper- -ate

Thus the four oxyacids of chlorine are called hypochlorous acid (HOCl), chlorous acid (HOClO), chloric acid (HOClO2) and perchloric acid (HOClO3), and their respective conjugate bases are the hypochlorite, chlorite, chlorate and perchlorate ions. This system has partially fallen out of use, but survives in the common names of many chemical compounds: the modern literature contains few references to "ferric chloride" (instead calling it "iron(III) chloride"), but names like "potassium permanganate" (instead of "potassium manganate(VII)") and "sulfuric acid" abound.

Traditional naming[edit]

Naming simple ionic compounds[edit]

An ionic compound is named by its cation followed by its anion. See polyatomic ions for a list of possible ions.

For cations that take on multiple charges, the charge is written using Roman numerals in parentheses immediately following the element name. For example, Cu(NO3)2 is copper(II) nitrate, because the charge of two nitrate ions (NO3−1) is 2 × −1 = −2, and since the net charge of the ionic compound must be zero, the Cu ion has a 2+ charge. This compound is therefore copper(II) nitrate. In the case of cations with a 4+ oxidation state, the acceptable format for the Roman numeral 4 is IV and not IIII.

The Roman numerals in fact show the oxidation number, but in simple ionic compounds (i.e., not metal complexes) this will always equal the ionic charge on the metal. For a simple overview see [1], for more details see selected pages from IUPAC rules for naming inorganic compounds.

List of common ion names[edit]

Monatomic anions:

Cl chloride
S2− sulfide
P3− phosphide

Polyatomic ions:

NH4+ ammonium
H3O+ hydronium
NO3 nitrate
NO2 nitrite
ClO hypochlorite
ClO2 chlorite
ClO3 chlorate
ClO4 perchlorate
SO32− sulfite
SO42− sulfate
HSO3 hydrogen sulfite (or bisulfite)
HCO3 hydrogen carbonate (or bicarbonate)
CO32− carbonate
PO43− phosphate
HPO42− hydrogen phosphate
H2PO4 dihydrogen phosphate
CrO42− chromate
Cr2O72− dichromate
BO33− borate
AsO43− arsenate
C2O42− oxalate
CN cyanide
SCN thiocyanate
MnO4 permanganate

Naming hydrates[edit]

Hydrates are ionic compounds that have absorbed water. They are named as the ionic compound followed by a numerical prefix and -hydrate. The numerical prefixes used are listed below (see IUPAC numerical multiplier):

  1. mono-
  2. di-
  3. tri-
  4. tetra-
  5. penta-
  6. hexa-
  7. hepta-
  8. octa-
  9. nona-
  10. deca-

For example, CuSO4 · 5H2O is "copper(II) sulfate pentahydrate".

Naming molecular compounds[edit]

Inorganic molecular compounds are named with a prefix (see list above) before each element. The more electronegative element is written last and with an -ide suffix. For example, H2O (water) can be called dihydrogen monoxide. Organic molecules do not follow this rule. In addition, the prefix mono- is not used with the first element; for example, SO2 is sulfur dioxide, not "monosulfur dioxide". Sometimes prefixes are shortened when the ending vowel of the prefix "conflicts" with a starting vowel in the compound. This makes the name easier to pronounce; for example, CO is "carbon monoxide" (as opposed to "monooxide").

Common Exceptions[edit]

There are a number of exceptions and special cases that violate the above rules. Sometimes the prefix is left off of the initial atom: S2O7 is known as sulfur heptoxide, but it should be called disulfur heptoxide. S2O3 is called sulfur sesquioxide (sesqui means 1½).

The main oxide of phosphorus is called phosphorus pentoxide. It should actually be diphosphorus pentoxide, but everyone knows that there are two phosphorus atoms (P2O5) needed in order to balance the oxidation numbers of the five oxygen atoms. However, people have known for years that the real form of the molecule is P4O10, not P2O5, yet it is not normally called tetraphosphorus decaoxide.

In writing formulas, ammonia is NH3 even though nitrogen is more electronegative. Likewise, methane is written as CH4 even though carbon is more electronegative.

2005 revision of IUPAC's nomenclature for inorganic compounds[edit]

See also[edit]

External links[edit]