IUPAC nomenclature of inorganic chemistry

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). 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.

System

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

Naming simple ionic compounds

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

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

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

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

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

Main article: IUPAC nomenclature of inorganic chemistry 2005

See also

• IUPAC nomenclature
• IUPAC nomenclature of organic chemistry
• List of inorganic compounds
• Water of Crystallization

External links

• Bibliography of IUPAC Recommendations on Inorganic Nomenclature (last updated 17 February 2004)
• ChemTeam Highschool Tutorial
• PDF file SUNY Potsdam.edu
• American Chemical Society, Committee on Nomenclature, Terminology & Symbols