How many times have you needed to "look up" a chemical? There are many tools where you can quickly find a chemical name or structure. This is just a selection of some of the better-known ones. Most also contain various types of factual data about chemical substances - properties, safety hazards, etc.
Chemical Abstracts Service has registered millions of chemical substances since 1965. CAS Registry Numbers (RNs) are very useful when searching for information about a specific chemical structure, as well as polymers, mixtures, alloys, and substances whose exact formula is unknown or variable.
A Registry Number (RN) looks like this:
where the first segment can be from two to seven digits long, followed by two digits, then a single check-digit. It is a sequential accession number from the CAS Registry database. The RN carries no chemical or structural meaning in itself. It is simply an identifier for a specific substance that CAS has registered during the process of indexing the literature (or added from another source). The shorter the first segment, the older the registration and the more common (and probably better described) the compound is.
A Registry Number allows you to avoid using chemical names when searching for information about a compound. If you have a RN in hand, use it as a search term in SciFinder, in place of a chemical name. Most non-bibliographic chemical databases also allow searching by Registry Number.
Registry numbers are useful substitutes for names, but they are not perfect.
Many chemical databases and commercial catalogs use CAS Registry Numbers as a standard identifier.
Every discipline, and sometimes individual publishers and journals, have their rules about how bibliographic references and footnotes are to be formatted: these are called styles. Specific styles are often adopted by graduate schools for dissertation writers, and by instructors for student assignments as well. In chemistry, ACS style is the standard; in biochemistry the CSE style is standard, but you should check author instructions for every journal you submit to. The easiest way to cope with the many styles available is to use a reference manager tool like EndNote or Zotero, which can make formatting your references fairly easy. For more detail and explanation about writing for publication, you can turn to style guides.
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