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Systematic naming of chemical compounds
Chemical nomenclature is a set of rules to generate systematic names for chemical compounds. The nomenclature used most frequently worldwide is the one created and developed by the International Union of Pure and Applied Chemistry (IUPAC). IUPAC Nomenclature ensures that each compound (and its various isomers) have only one formally accepted name known as the systematic IUPAC name, however, some compounds may have alternative names that are also accepted, known as the recommended IUPAC name which is generally taken from the common name of that compound. Preferably, the name should also represent the structure or chemistry of a compound. For example, the main constituent of white vinegar is , which is commonly called acetic acid and is also its recommended IUPAC name, but its formal, systematic IUPAC name is ethanoic acid. The IUPAC's rules for naming organic and inorganic compounds are contained in two publications, known as the Blue Book and the Red Book, respectively. A third publication, known as the Green Book, recommends the use of symbols for physical quantities (in association with the IUPAP), while a fourth, the Gold Book, defines many technical terms used in chemistry. Similar compendia exist for biochemistry (the White Book, in association with the IUBMB), analytical chemistry (the Orange Book), macromolecular chemistry (the Purple Book), and clinical chemistry (the Silver Book). These "color books" are supplemented by specific recommendations published periodically in the journal Pure and Applied Chemistry.

The main purpose of chemical nomenclature is to disambiguate the spoken or written names of chemical compounds: each name should refer to one compound. Secondarily, each compound should have only one name, although in some cases some alternative names are accepted. Preferably, the name should also represent the structure or chemistry of a compound. This is achieved by the International Chemical Identifier (InChI) nomenclature. However, the American Chemical Society's CAS numbers nomenclature does not represent a compound's structure. The nomenclature used depends on the needs of the user, so no single correct nomenclature exists. Rather, different nomenclatures are appropriate for different circumstances. A common name will successfully identify a chemical compound, given context. Without context, the name should indicate at least the chemical composition. To be more specific, the name may need to represent the three dimensional arrangement of the atoms. This requires adding more rules to the standard IUPAC system (the Chemical Abstracts Service system (CAS system) is the one used most commonly in this context), at the expense of having names which are longer and less familiar. The IUPAC system is often criticized for failing to distinguish relevant compounds (for example, for differing reactivity of sulfur allotropes, which IUPAC does not distinguish). While IUPAC has a human readable advantage over CAS numbering, IUPAC names for some larger, relevant molecules (such as rapamycin) are barely human readable, so common names are used instead.

The nomenclature of alchemy is descriptive, but does not effectively represent the functions mentioned above. Opinions differ about whether this was deliberate on the part of the early practitioners of alchemy or whether it was a consequence of the particular (and often esoteric) theories according to which they worked. While both explanations are probably valid to some extent, it is remarkable that the first "modern" system of chemical nomenclature appeared at the same time as the distinction (by Lavoisier) between elements and compounds, during the late eighteenth century. The French chemist Louis Bernard Guyton de Morveau published his recommendations in 1782, hoping that his "constant method of denomination" would "help the intelligence and relieve the memory". The system was refined in collaboration with Berthollet, de Fourcroy and Lavoisier, and promoted by the latter in a textbook that would survive long after his death by guillotine in 1794. The project was also endorsed by Jöns Jakob Berzelius, who adapted the ideas for the German speaking world. The recommendations of Guyton were only for what would be known now as inorganic compounds. With the massive expansion of organic chemistry during the mid nineteenth century and the greater understanding of the structure of organic compounds, the need for a less ad hoc system of nomenclature was felt just as the theoretical basis became available to make this possible. An international conference was convened in Geneva in 1892 by the national chemical societies, from which the first widely accepted proposals for standardization developed. A commission was established in 1913 by the Council of the International Association of Chemical Societies, but its work was interrupted by World War I. After the war, the task passed to the newly formed International Union of Pure and Applied Chemistry, which first appointed commissions for organic, inorganic, and biochemical nomenclature in 1921 and continues to do so to this day.

Nomenclature has been developed for both organic and inorganic chemistry. There are also designations having to do with structuresee Descriptor (chemistry).

For type I ionic binary compounds, the cation (a metal in most cases) is named first, and the anion (usually a nonmetal) is named second. The cation retains its elemental name (e. g., iron or zinc), but the suffix of the nonmetal changes to ide. For example, the compound |LiBr is made of |Li(+) cations and |Br(−) anions; thus, it is called lithium bromide. The compound |BaO, which is composed of |Ba(2+) cations and |O(2−) anions, is referred to as barium oxide. The oxidation state of each element is unambiguous. When these ions combine into a type I binary compound, their equal but opposite charges are neutralized, so the compound's net charge is zero.

Type III binary compounds are bonded covalently. Covalent bonding occurs between nonmetal elements. Compounds bonded covalently are also known as molecules. For the compound, the first element is named first and with its full elemental name. The second element is named as if it were an anion (base name of the element + ide suffix). Then, prefixes are used to indicate the numbers of each atom present: these prefixes are mono (one), di (two), tri (three), tetra (four), penta (five), hexa (six), hepta (seven), octa (eight), nona (nine), and deca (ten). The prefix mono is never used with the first element. Thus, |NCl3 is termed nitrogen trichloride, |BF3 is termed boron trifluoride, and |P2O5 is termed diphosphorus pentoxide (although the a of the prefix penta should actually not be omitted before a vowel: 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)."). Carbon dioxide is written |CO2; sulfur tetrafluoride is written |SF4. A few compounds, however, have common names that prevail. |H2O, for example, is usually termed water rather than dihydrogen monoxide, and |NH3 is preferentially termed ammonia rather than nitrogen trihydride.

This naming method generally follows established IUPAC organic nomenclature. Hydrides of the main group elements (groups 13–17) are given the base name ending with ane, e. g. borane (|auto=yes|BH3), oxidane (|auto=yes|H2O), phosphane (|auto=yes|PH3) (Although the name phosphine is also in common use, it is not recommended by IUPAC). The compound |auto=yes|PCl3 would thus be named substitutively as trichlorophosphane (with chlorine "substituting"). However, not all such names (or stems) are derived from the element name. For example, |auto=yes|NH3 is termed "azane".

This method of naming has been developed principally for coordination compounds although it can be applied more widely. An example of its application is |[CoCl(NH3)5]Cl2, pentaamminechloridocobalt(III) chloride. Ligands, too, have a special naming convention. Whereas chloride becomes the prefix chloro in substitutive naming, for a ligand it becomes chlorido .