Chemical compounds

This section covers:

For compounds of biological importance, see:

Terms to watch out for:

natural, organic

See all terms

Caution! Chemical compound names follow complex rules, and should be written and edited with great care. Some basic rules for expressing compound names are shown in this section, but writers and editors should refer to the International Union of Pure and Applied Chemistry (IUPAC) and other standards for chemistry terminology to check the accuracy of all names; see Chemistry terminology.

Chemical formulas

Chemical compounds (ie a molecule that contains atoms of more than 1 element) are represented by formulas that show the number of the different atoms in the molecule. Show the number of atoms of each element in the molecule in the right-hand subscript position:

NH4     Al2O3

In general texts, it is best to write out simple chemical compound names in full, wherever possible (eg carbon dioxide, methanol, sodium acetate); however, in technical reports and scientific journal papers, it might be appropriate to use the symbol (especially in tables and figures). If abbreviations for chemical symbols are used, consider identifying them in full at first use in text, and in notes under tables or figures:

sodium chloride (NaCl)     sulfuric acid (H2SO4)

Use spaces between ions and neutral segments of a chemical compound when written in full, but keep chemical formulas without spaces:

sodium chloride (NaCl)     magnesium potassium fluoride (MgKF)

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Trivial versus systematic names

Chemical compounds can be named by their trivial (common), systematic or semisystematic names:

acetone [trivial name], propan-2-one [systematic name]

water [trivial name], hydrogen oxide [systematic name]

benzene carboxylic acid [systematic name], benzoic acid [semisystematic name]

Terms to watch out for:

benzene, benzine

benzene: a single species of hydrocarbon molecule (which, confusingly, was originally called benzine!)

benzine: a mixture of hydrocarbons obtained in the distillation of petrol

See all terms

Use IUPAC names for simple organic compounds (eg alcohols):

methanol   not   methyl alcohol

ethanol   not   ethyl alcohol

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Capitals in chemical names

Do not use initial capitals for chemical names unless they are at the beginning of a sentence or part of a title:

methyl propyl ether [in running text]

Methyl propyl ether [at the beginning of a sentence, or in a title if the style for titles is sentence case]

Methyl Propyl Ether [in a title if the style for titles is initial capitals]

See also Locants, descriptors and isomers.

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Prefixes in chemical names

Set solid prefixes that denote multiplication or are integral parts of the name:

cyclohexane     dinitrogen oxide     isopentane

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Locants, descriptors and isomers

Locants are numbers or letters that indicate the positions of functional groups in a molecular structure. The numbers or letters are placed immediately before the part of the name that they relate to and are set off with hyphens:

but-1-ene   not   1-butene     3,5-decadien-1-yne

The prime symbol (see Single prime symbol) indicates the position of the functional groups in multi-ring compounds:

3'-methylphthalanilic acid

Letters are italicised:

(Z)-2-butene     (2E,5Z)-2,5-decadiene

Also use italics for substituted chemical element symbols:

N-methylethanamine     S-ethylcysteine

When element symbols are used with a type of reaction to form an adjective or noun, roman type can be used to distinguish this use from the use of element symbols as substituted atoms:

N-acetylated     O-substitution

Note: Some organisations and journals do not follow this style rule.

Use italics for cis- and trans- isomer names, and all other positional, stereochemical, configurational, symmetry and descriptive prefixes, whether with the full name or an abbreviation:

9-cis-retinal     trans-1,2-dichlorocyclohexane     meso-3,4-dichlorohexane     o-methylphenol      p,p'-DDT

R (rectus/right-handed) and S (sinister/left-handed) stereocentre designations are given in italics and brackets:

(R)-2-bromobutane     (S)-2,3-dihydroxypropanol

Use small capitals for D (dexter/on the right) and L (laevus/on the left) optical isomers (however, take care when using this notation as it has been largely superseded by the R, S system):

-glucose     -alanine

Ignore italicised elements and locants when capitalising at the beginning of sentences or in headings:

9-cis-Retinal is …     o-Methylphenol can be …     N-methylethanamine was …

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Chemical bonds

It is not necessary to show chemical bonds in formulas unless they are specifically required for discussion:


If it is necessary to indicate bonds in running text, use unspaced en dashes:

the angle of the C–C–C bonds

How to insert an en dash:
In Windows (eg Microsoft Word), use Ctrl+- on the numerical keypad; use Alt+0150 on the numerical keypad; or insert from the symbol set (special characters). On a Mac, use Option+-. Word can also be set up to convert 2 hyphens (--) to an en dash (–) using the autocorrect options.

When necessary, show double and triple bonds as follows:

CH3C(=O)OH     CH3C≡CH

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A polymer is a structure composed of macromolecules. For common names of polymers, the prefix poly is set in roman type, lower case, and solid with the rest of the name, which may or may not be in brackets:

polyacrylamide     poly(vinyl chloride) [Poly(vinyl chloride) at the start of a sentence]

Other descriptors are given in lower case and italics (eg graft, blend, co, per):

poly(3-hexylthiophene)-blend-polystyrene     poly(styrene-co-isoprene)

IUPAC has a good guide: A brief guide to polymer nomenclature.

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