# IUPAC nomenclature of inorganic chemistry

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 compound. 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" simply 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, H$_2$O is most often simply called water in English, though other chemical names do exist.
* Single atom anions are named with an -ide suffix: for example, H$^−$ is hydride.
* 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 CaF$_2$ is calcium fluoride.
* Cations of transition metals able to take multiple charges are labeled with Roman numerals in parentheses to indicate their charge. For example, Cu$^+$ is copper(I), Cu$^{2+}$ 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 Cu$^{2+}$ is cupric. For naming metal complexes see the page on complex (chemistry).
* Oxyanions (polyatomic anions containing oxygen) are named with -ite or -ate, for a lesser or greater quantity of oxygen, respectively. For example, NO$^−$ $_2$ is nitrite, while NO$^−$ $_3$ is nitrate. If four oxyanions are possible, the prefixes hypo- and per- are used: hypochlorite is ClO$^−$, perchlorate is ClO$^−$ $_4$.
* The prefix bi- is a deprecated way of indicating the presence of a single hydrogen ion, as in "sodium bicarbonate" (NaHCO$_3$). The modern method specifically names the hydrogen atom. Thus, NaHCO$_3$ 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, non-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 Fe$_2$O$_3$ 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 (HOClO$_2$) and perchloric acid (HOClO$_3$), and their respective conjugate bases are 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
### Simple ionic compounds

An ionic compound is named by its cation followed by its anion. See polyatomic ion 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(NO$_3$)$_2$ is copper(II) nitrate, because the charge of two nitrate ions (NO$^−$ $_3$) 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 only 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 Archived 2008-10-16 at the Wayback Machine, for more details see selected pages from IUPAC rules for naming inorganic compounds Archived 2016-03-03 at the Wayback Machine.

#### List of common ion names

Monatomic anions:
    Cl$^−$ chloride
    S$^{2−}$ sulfide
    P$^{3−}$ phosphide
Polyatomic ions:
    NH$^+$ $_4$ ammonium
    H $_3$O$^+$ hydronium
    NO$^−$ $_3$ nitrate
    NO$^−$ $_2$ nitrite
    ClO$^−$ hypochlorite
    ClO$^−$ $_2$ chlorite
    ClO$^−$ $_3$ chlorate
    ClO$^−$ $_4$ perchlorate
    SO$^{2−}$ $_3$ sulfite
    SO$^{2−}$ $_4$ sulfate
    S $_2$O$^{2–}$ $_3$ thiosulfate
    HSO$^−$ $_3$ hydrogen sulfite (or bisulfite)
    HCO$^−$ $_3$ hydrogen carbonate (or bicarbonate)
    CO$^{2−}$ $_3$ carbonate
    PO$^{3−}$ $_4$ phosphate
    HPO$^{2−}$ $_4$ hydrogen phosphate
    H $_2$PO$^−$ $_4$ dihydrogen phosphate
    CrO$^{2−}$ $_4$ chromate
    Cr $_2$O$^{2−}$ $_7$ dichromate
    BO$^{3−}$ $_3$ borate
    AsO$^{3−}$ $_4$ arsenate
    C $_2$O$^{2−}$ $_4$ oxalate
    CN$^−$ cyanide
    SCN$^−$ thiocyanate
    MnO$^−$ $_4$ permanganate

### 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):
* mono-
* di-
* tri-
* tetra-
* penta-
* hexa-
* hepta-
* octa-
* nona-
* deca-
For example, CuSO$_4$·5H$_2$O is "copper(II) sulfate pentahydrate".

### 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, H$_2$O (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, SO$_2$ 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

The "a" of the penta- prefix is not dropped before a vowel. As the IUPAC Red Book 2005 page 69 states, "The final vowels of multiplicative prefixes should not be elided (although 'monoxide', rather than 'monooxide', is an allowed exception because of general usage)."
There are a number of exceptions and special cases that violate the above rules. Sometimes the prefix is left off the initial atom: I$_2$O$_5$ is known as iodine pentaoxide, but it should be called diiodine pentaoxide. N$_2$O$_3$ is called nitrogen sesquioxide (sesqui- means 1+1⁄2).
The main oxide of phosphorus is called phosphorus pentaoxide. It should actually be diphosphorus pentaoxide, but it is assumed that there are two phosphorus atoms (P$_2$O$_5$), as they are 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 P$_4$O$_{10}$, not P$_2$O$_5$, yet it is not normally called tetraphosphorus decaoxide.
In writing formulas, ammonia is NH$_3$ even though nitrogen is more electronegative (in line with the convention used by IUPAC as detailed in Table VI of the red book). Likewise, methane is written as CH$_4$ even though carbon is more electronegative (Hill system).

## Nomenclature of Inorganic Chemistry

Nomenclature of Inorganic Chemistry, commonly referred to by chemists as the Red Book, is a collection of recommendations on IUPAC nomenclature, published at irregular intervals by the IUPAC. The last full edition was published in 2005, in both paper and electronic versions.
Published editions :
| Release year | Title | Publisher | ISBN |
| 2005 | Recommendations 2005 (Red Book) | RSC Publishing | 0-85404-438-8 |
| 2001 | Recommendations 2000 (Red Book II) (supplement) | RSC Publishing | 0-85404-487-6 |
| 1990 | Recommendations 1990 (Red Book I) | Blackwell | 0-632-02494-1 |
| 1971 | Definitive Rules 1970 | Butterworth | 0-408-70168-4 |
| 1959 | 1957 Rules | Butterworth |
| 1940/1941 | 1940 Rules | Scientific journals |