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There are millions of possible objects that can be described in science, too many to create common names for every one. As a response, a number of systems of systematic names have been created.
These can be as simple as assigning a prefix and a number to each object (in which case they are a sort of catalog reference), or as complex as encoding the complete structure of the object in the name. Many systems combine some information about the named object with an extra sequence number to make it into a unique identifier.
Systematic names often co-exist with earlier common names assigned before the creation of any systematic naming system. For example, many common chemicals are still referred to by their common names, even by chemists.
Chemical names
To give an example of why this is so, consider this example. The names "caffeine" and "3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione" both describe 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 is required. However, 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, the chemical water is always known as such, never as "dihydrogen monoxide."
- 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 CaF2 is calcium fluoride.
- Cations able to take 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).
- Oxyanions (polyatomic anions containing oxygen) are named with -ite or -ate, for a lesser or greater quantity of oxygen. 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-,
- 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".
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 is 2 x -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.
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.
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:
- mono-
- di-
- tri-
- tetra-
- penta-
- hexa-
- hepta-
- octa-
- nona-
- deca-
For example, CuSO4 ยท 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, CO2 is carbon dioxide, and CCl4 is carbon tetrachloride. There are some exceptions to the rule, however. The prefix mono- is not used with the first element; for example, CO2 is carbon dioxide, not "monocarbon dioxide". Sometimes prefixes are shortened when the ending vowel of the prefix "conflicts" with a starting vowel in the compound. This makes the compound easier to speak; for example, CO is "carbon monoxide" (as opposed to "monooxide").
Naming organic compounds
The naming system for organic compounds is complex and still developing, as is knowledge of the compounds themselves. IUPAC nomenclature covers many of these.
Naming acids
Acids are named by the anion they form when dissolved in water. If an acid forms an anion named ___ide, it is named hydro___ic acid. For example, hydrochloric acid forms a chloride anion. Secondly, anions with an -ate suffix are formed from acids with an -ic suffix are dissolved -- chloric acid dissociates to chlorate anions in water. Thirdly, anions with an -ite suffix are formed when acids with an -ous suffix are dissolved in water; for example chlorous acid disassociates into chlorite anions.
See also
External links
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