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Index Databases

If you already have a reference:


Chemistry doesn't have a strong preprint tradition, but these archives provide platforms for the free deposit and dissemination of articles prior to publication.

How many times have you needed to "look up" a chemical?  There are many, many tools out there where you can 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 Nomenclature

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. CAS has also registered tens of millions of biosequences.

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.


Using Registry Numbers as Search Terms

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.

  • CAS' registration policies are complex and highly specific. This is especially true for substances with undefined structures, commercial formulations, polymers, mixtures, inorganic substances (separate RNs are assigned for ions, charge states, hydration states, etc.), drugs (separate RNs for the free base and HCl forms), and for organic salts and stereoisomers.
  • RNs used in non-CAS resources, such as supplier catalogs, MSDS, handbooks, encyclopedias, and databases, may not be correctly assigned, and often are not checked for accuracy or updated to reflect changes. When in doubt, verify a Registry Number in SciFinder.
  • While a de facto standard for chemical identification, CAS RNs are proprietary. Any systematic use of RNs in a third-party database requires a license.

Where to Find Registry Numbers

Many printed and online reference sources about chemicals use CAS Registry Numbers as a standard identifier.

See our guide to:

See the page on Safety, Hazards, Environment for information about sources of Safety Data Sheets (formerly known as MSDS).

For more information about bibliographies and reference managers, see:

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    Last Updated Nov 26, 2018 14582 views this year
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Chemistry Librarian

David Flaxbart's picture
David Flaxbart
University of Texas Libraries

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