CAS registry numbers are unique numerical identifiers of chemical compounds. They are also referred to as CAS numbers or CAS RNs. The Chemical Abstract Service, a division of the American Chemical Society, assigns these identifiers to every chemical that's described in the literature. About 20 million compounds have received a CAS number so far, with about 4,000 new ones being added each day. 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.
A CAS registry number is separated by hyphens into three parts, the first consisting of up to 6 digits, the second consisting of two digits, and the third consisting of a single digit serving as a checksum. For example, the CAS number of water is 7732-18-5. The numbers are assigned in increasing order and do not have any inherent meaning. Different isomers of a molecule receive different CAS numbers: D-glucose has 50-99-7 and L-glucose has 921-60-8.
To find the CAS number of a compound given its name, formula or structure, the search option of NIST's Chemistry WebBook at
can be used.