Wikipedia:Manual of Style/Chemistry

This page aims to be brief, touching on the general aspects of chemistry-related articles. In-depth guides are found in linked pages.

Scope
Many chemical topics are suitable for inclusion; the central criterion is that the article meets the general notability guidelines. Important topics include compounds, reactions, methods of analysis, instrumentation/apparatus, techniques, significant chemists, branches of chemistry, chemical theories, and principles which have received significant coverage in independent reliable sources.

Editing policy
The style of editing embraced by the Wikipedia chemistry project is collaborative and consensus-driven. Edits to existing articles are typically incremental, which allow changes to be evaluated by other editors. Long-standing or mature articles should not be rewritten in their entirety because such large-scale changes inhibit discussion and often marginalize seemingly small but significant improvements that have been hammered out by previous editors. If an editor feels that a mature article warrants a major revision, it is both customary and considerate for the revising editor to announce their intentions on the relevant talk page and to heed the consensus of the responses. Typically, responses to such announcements can take days to accumulate, so major revisions require a sense of pace and patience.

Being affiliated with or bound by no scientific organizations, the content in Wikipedia-Chemistry is not constrained by recommendations or rules, but seeks to objectively describe knowledge. For example, the International Union of Pure and Applied Chemistry (IUPAC) provides recommendations and definitions for nomenclature and terminology. Wikipedia editors strive to be mindful of IUPAC's advice but do not follow this advice rigidly, especially when the advice deviates from mainstream usage (see comments below on nomenclature).

Article curation and creation
The thousands of Wikipedia articles about chemistry benefit from continuing improvements that address clarity and content. Very long articles are difficult to read and maintain, so at some stage, long articles are often split (subdivided into two articles). Being a consensus driven process, plans for splitting are ordinarily discussed on the talk page for the parent article.

Category creation
Articles are placed in one or more categories to help readers and editors locate similar topics. When categories become too large to be readily searched, it is common to create subcategories. For example, Category:Alcohols has subcategories Category:Tertiary alcohols, Category:Secondary alcohols, and Category:Primary alcohols. Prior to creating multiple categories, such plans are ordinarily discussed on the talk page for the parent article since they affect many articles.

Attributions to people and places
In general, descriptions of chemical knowledge do not mention who did the work or where the discoveries were made, in part because such information is available in the citations. This approach simplifies the presentation and helps readers focus on facts and explanations. The obvious exceptions to this guideline are articles or sections of articles on biographies and history. Even in regular articles, scientific advances are sometimes attributed to noteworthy individuals and institutions, especially when this information illuminates the content or enlivens the prose. Attribution to individuals and institutions is subject to guidelines on conflict of interest.

Nomenclature
Per WP:ENGVAR, the type of English used to write an article does not matter, but it should be consistent. The following exceptions apply for chemical names, when the article itself is primarily about chemistry:
 * "sulfur" (and related "sulfuric", etc.), "caesium", and "aluminium" should be spelled this way regardless of the English variation used in the article, being the IUPAC names for these elements;
 * "phosphine" is preferred over "phosphane", being predominantly used in the chemical literature.

Systematic nomenclature, while being precise, can be cumbersome. Commonly accepted trivial or alternative names are preferred over systematic names. In particular, IUPAC recommends the use of non-systematic names for some organic compounds, and these recommendations should be followed in article titles. Examples include acetic acid over ethanoic acid, toluene over methylbenzene, lysine over 2,6-diaminohexanoic acid.

Stock nomenclature (e.g. iron(III) chloride): there should be no space between the words and the oxidation state in parentheses (between "iron" and (III)). The oxidation numbers are stated only for cations, not for anions. Where the oxidation number is obvious (i.e. group 1 or group 2 metals), it is not included. Compounds with a substantial degree of covalency do not use Stock nomenclature.

Isotopes should be labelled by their mass number, e.g. 14C and 18F. Deuterium (2H) and tritium (3H) may be labelled "D" (or "2D") and "T" (or "3T"), respectively. Deuterated solvents for NMR use are customarily described variously as: methanol-d4 for CD3OD, DMSO-d6 for CD3SOCD3, etc. These established systems are all acceptable, but should remain consistent within an article.

For organic radicals denoted by "R", indices used for numbering must be superscript: R1−CH2−R2 (not R1−CH2−R2).

Symbols
Chemical symbols shall be Roman (CaCO3) and may not be italicized by any means, neither wiki syntax nor &lt;math> tag. Notation of atomic shells, subshells and orbitals (1s, 2p, 3d, ...) also shall be Roman. Numerals (1, 2, 3, ...) shall invariably be Roman.

Skeletal formulas
In prose, carbon–carbon bond uses an en dash (&amp;ndash;), as is normal for such compound terms. However, for skeletal formulas, the mathematical symbols minus, equals, and identity are used to represent the bonds: This style is selected because in most professional fonts only the minus sign $\langle−\rangle$ matches the double bond $\langle=\rangle$ and triple bond $\langle≡\rangle$ in width, so an en dash would look out of place in a skeletal formula.
 * Single bond: C−C (C&amp;minus;C or C C)
 * Double bond: C=C (equal sign)
 * Triple bond: C≡C

Etymology
Many chemical terms have interesting etymology that merits description. Since focus of the articles and the readership is on technical aspects, sections on etymology are ordinarily placed near the end of the article.

Structure drawing
Appropriate formats are PNG and SVG. ACS settings have been adopted as the convention. Images should be legible at 450 pixels wide, to avoid interference from the chembox on the right.

Hydrogens should be implied (hidden), except for the benefit of the target audience. The use of Me to denote methyl may be confusing. The use of Et, Pr, etc., is discouraged. When Ph is used to denote phenyl and X for halogen or any atom, it should be clearly defined within the image.

Sample images
Images of chemical compounds or their solutions are useful to readers. Most useful are colored compounds but even colorless/white samples/solutions can be useful. The provenance of these images is however impossible to verify. The following images provide some guidance.

All images must follow the image use policy, be of the chemical subject of the article, and should be uploaded to Wikimedia Commons whenever licensing allows.

Safety
The majority of compounds are described by a long list of potential hazards as well as H- and P-phrases. Wikipedia does not aspire to be an MSDS. The hazards associated with a chemical compound should ordinarily be described in the Chembox (via GHSPictograms, GHSSignalWord, NFPA, or MainHazard parameters; further elaborated in H- and P-phrases). The information in the Chembox is sufficient for most compounds. News reports of routine accidents, even though they may be tragic, are usually not relevant.

Three main points:
 * If the hazards are relatively obvious (e.g. hexafluorophosphoric acid is a strong acid, and should not be stored with bases and reactive metals), do not create a separate sub-section here.
 * The description of hazards should avoid speculation. This recommendation is partly an extension of Wikipedia NPOV policy, but not entirely. There is no need to include a section which merely states "all chemical compounds should be treated with the utmost precaution"; such a section tells the reader nothing. If hazards are unknown, there is nothing for Wikipedia to say.
 * The description of hazards should avoid hyperbole. The role of Wikipedia is to give balanced and accurate information, to allow its readers to reach their own conclusions.

Descriptions of hazards should, as far as possible, be based on published, peer-reviewed sources (which should, of course, be cited at the appropriate point in the article). A list of resources for chemical safety information is given in the external links section of these guidelines.

In general a safety section should only be used when it adds something to an article, and should be based (where at all possible) on peer-reviewed or otherwise highly reliable sources.

Toxicology
Depending on the extent and nature of the information, toxicological content may be incorporated into the Safety section or it may be separate. If the compound is a drug, follow WikiProject Drugs' recommendations.

Current events
From WP:NOT:

"Journalism. Wikipedia should not offer first-hand news reports on breaking stories. Wikipedia is not a primary source. However, our sister project Wikinews does exactly that, and is intended to be a primary source. Wikipedia does have many encyclopedia articles on topics of historical significance that are currently in the news, and can be updated with recently verified information."

Accidents and incidents occur all the time. While their scale and magnitude may merit inclusion in Wikipedia on grounds of notability, that such an accident has occurred is not sufficient justification for inclusion in the context of an article about chemicals. Wikipedia does not attempt to dispense advise on what to do in the event of a (...) incident, either. (See WP:NOTGUIDE) Historic accidents and incidents may only be contextualized in the discussion on the specific hazards of certain chemicals, without serving as case studies in itself. To reiterate, if such accidents are sufficiently notable, they should have their own article (e.g. discussion in Bhopal disaster, not in methyl isocyanate).

References and external links
Claims and statements in articles should mainly be supported by references to textbooks, monographs, and related book series. In many cases, especially for historic purposes, the primary literature (journals) are used. Intermediate between journal articles, which are often highly specialized, and books, which are not always available, are review series (e.g. Chemical Reviews, Advances in Enzymology, etc.). Patents, which are considered self-published and primary sources for Wikipedia's purposes, are sometimes cited, especially for historic purposes, but they are less useful because they are not vetted on scholarly basis, they are sometimes not very readable, and often they are not very available.

Compounds
All articles on chemicals, real or hypothetical, should have a Chembox. Formulae should be readily available, variables like n, x, or y are permissible for substances of variable composition such as polymers. For compounds of defined composition, the molar masses should be available as well. The article should cover these aspects as appropriate:

Sectioning

 * Introductory paragraph (WP:lede)
 * Properties (optional), the physical appearance of the material (at STP) Often this content is included in the lede.  Unless notable, properties listed in the ChemBox are not repeated.
 * Occurrence. Usually occurrence refers natural products, compounds or ions that exist naturally.
 * Structure. The structures of many organic compounds are obvious, but inorganic and metal-containing compounds often adopt nonintuitive structures.  When possible, their structures should be described, ideally based on crystallographic or related techniques.
 * Preparation. It is our tradition to cite the first preparation of a compound.  For commercially important compounds, it is often useful to distinguish reactions used in industrial production separately from "laboratory routes".  For natural products, the biosynthesis is described.
 * Uses. Although compounds can be used for diverse and weird ways, this section emphasizes, hierarchically, substantial commercial applications and laboratory uses.
 * Reactions. All chemical compounds undergo many reactions, so these reactions are expected to have applicability.
 * History. Often this section would be superfluous to mention of the first preparation, but some compounds, e.g., Teflon, have rich history.
 * Safety
 * Toxicology. Often this section is blended with safety.  Ideally, the LD50 would be mentioned.
 * Suppliers should not be listed unless the compound is rare and only available from one or two suppliers
 * References

Scope
Ordinarily, compounds that differ in terms of their solvent of crystallization or hydration are described in a single article. The discussion that led to this consensus is here. For example, several different hydrates are known for copper(II) sulfate as well as the anhydrous form. All of these compounds are discussed in a single article, copper(II) sulfate.

Compound classes
These articles belong to one of these categories: Where a compound class does not have sufficient detail to merit a full article, it should be merged to the parent article (usually that of the acid). Articles named after a parent compound should generally appear at the plural of the parent compound.
 * monoatomic ions (chloride, bromide; oxide, sulfide)
 * polyatomic ions (nitrates, perchlorates, triflates, tetrafluoroborates)
 * functional groups (alcohols, aldehydes, acids, nitriles)
 * "backbone" moieties, both organic and inorganic
 * classes of organic compounds (including many biochemicals): steroids, aldohexoses, terpenes
 * classes of inorganic and coordination compounds: metal oxo compounds, metal carbonyl compounds, metal clusters
 * elements are handled under a separate Wikiproject with guidelines, see WikiProject Elements/Guidelines

Aspects to be covered include:
 * Nomenclature
 * Structure and bonding, including illustrative bond distance and angles.
 * Properties
 * Characterization, discussing spectroscopic tools and illustrative data
 * Applications, in order of the scale or impact of the application
 * Occurrence, usually involving natural occurrence as in nature or the mineral kingdom
 * Preparation, ordinarily distinguishing between technical methods and those used in the laboratory
 * Reactions, if extensive, these entries should be aggregated thematically
 * History
 * Safety

Reactions
Simple chemical reactions can be typed out in text. Reactions should be indented using a colon, and not centered. Separate the number of molecules from the molecule symbol by a space (i.e. 3 H2 instead of 3H2). For example:
 * 2 Na + 2 H2O → 2 NaOH + H2

Reactions in the form of images should also be indented using a colon:
 * Grignard Reaction Scheme.png

Although many organometallic reagents have complex structures involving solvation or clusters, these reagents are ordinarily depicted in simplified structures (RMgX with two-coordinate Mg, BuLi with one-coordinate Li, etc.). To facilitate sharing of drawings between different language wikis, reagents above and below arrows should consist of formulas, not words. Experimental conditions are ordinarily omitted from equations. The prose can comment on conditions, yields, and other details. Equations are ordinarily not numbered.

Line equations
Do not include phase definitions unless they are absolutely essential (an example is given below). Modern textbooks and journals do not use them, so Wikipedia should not pioneer a new way of describing chemical reactions. They confuse readers trying to understand basics, since stoichiometry is conflated with phase information.

Avoid the use of chem and math markup notations: the plain-text character set is adequate for most chemical equations; the change in size and font form is jarring to the reader. The ce tag is a deprecated synonym for chem; see Help:Displaying a formula. Ionic equations are preferred to the molecular form. State symbols are omitted unless they are relevant (e.g. thermochemistry, to illustrate precipitation for chemical separation). "Heat" should not be a reaction product; stating ΔrH, or giving its sign is preferable (note that the use of phase information is essential for evaluating the thermodynamics):

C2H5OH(g) + 3 O2(g) → 2 CO2(g) + 3 H2O(l) (ΔrH = −1409 kJ/mol)

instead of:

C2H5OH + 3 O2  →   2 CO2  + 3 H2O  + heat

A comma, period or other punctuation is not required at the end of a line equation.

Compounds and atoms in reactions should not be wikilinked. These links should be in the surrounding text.

Sectioning
Each reaction article should include the following sections:
 * A brief overview
 * This section should include a broad description of the reaction in both text and reaction scheme. References to review articles are preferred.  References to the discovery of the reaction is also welcome.


 * Reaction mechanism
 * Reaction mechanisms often include mention of stereochemistry, order of reaction, and, for inorganics, electron count and configuration. Arrow pushing is helpful in organic chemistry.


 * Scope (optional)
 * Side reactions and exceptions might be included. If the outcome of the reaction is affected by small changes, such descriptions should go in this section.


 * See also (optional)
 * In this section, not related reactions that are not mentioned in the above text.


 * References
 * This entire section should only be the following text:
 * ==References==

Additional notes:
 * Avoid language in schemes, which makes the images more versatile. Descriptions belong in the caption, which are readily edited.
 * Reaction schemes are best aligned left without borders as follows:
 * Horner-Wadsworth-Emmons Reaction Example.png

Result being:


 * Horner-Wadsworth-Emmons Reaction Example.png

(Taken from the Horner–Wadsworth–Emmons reaction, which is a good example of a quality organic reaction article.)
 * Follow the reaction category hierarchy and categorize the article appropriately.
 * Upload all images to Wikimedia Commons, so other language Wikipedias can use the same images.