CAS registry numbers[1] are unique numerical identifiers for chemical elements, compounds, polymers, biological sequences, mixtures and alloys.[2] They are also referred to as CAS numbers, CAS RNs or CAS #s.
Chemical Abstracts Service (CAS), a division of the American Chemical Society, assigns these identifiers to every chemical that has been described in the literature. The intention is to make database searches more convenient, as chemicals often have many names. Almost all molecule databases today allow searching by CAS number.
As of 23:38:46 15 March 2010, there were 52,429,231 organic and inorganic substances and 61,688,865 sequences in the CAS registry.[3] Around 50,000 new numbers are added each week.
CAS also maintains and sells a database of these chemicals, known as the CAS registry. It also contains information about the many different types of substances that have been reported over the years in the scientific literature:
Alloys, Coordination compounds, Minerals, Mixtures, Polymers, Salts and Sequences
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A CAS registry number is separated by hyphens into three parts, the first consisting of up to 7 digits, the second consisting of two digits, and the third consisting of a single digit serving as a check digit. The numbers are assigned in increasing order and do not have any inherent meaning.
The check digit is calculated by taking the last digit times 1, the next digit times 2, the next digit times 3 etc., adding all these up and computing the sum modulo 10. For example, the CAS number of water is 7732-18-5: the checksum 5 is calculated as (8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6) = 105; 105 mod 10 = 5.
Different stereoisomers of a molecule receive different CAS numbers: D-glucose has 50-99-7, L-glucose has 921-60-8, α-D-glucose has 26655-34-5, etc. Occasionally, whole classes of molecules receive a single CAS number: the group of alcohol dehydrogenases has 9031-72-5. An example of a mixture with a CAS number is mustard oil (8007-40-7).
CAS information is copyrighted by the American Chemical Society. Users who wish to incorporate CAS numbers into their own databases should see the details of the CAS usage policy:
A User or Organization may include, without a license and without paying a fee, up to 10,000 CAS Registry Numbers or CASRNs in a catalog, web site, or other product for which there is no charge. The following attribution should be referenced or appear with the use of each CASRN: CAS Registry Number is a Registered Trademark of the American Chemical Society.[4]
To find the CAS number of a compound given its name, formula or structure, the following free resources can be used: