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Chemistry > Getting Started > CAS Registry Numbers
CAS REGISTRY NUMBERSCAS Registry Numbers (CAS RN) are very useful in that they provide a way to uniquely identify a chemical substance. So if you are trying to find information about a chemical, and you come across the CAS Registry Number, write it down. It will help in the search process.
These numbers are assigned sequentially to the chemical substances when they are entered into the CAS Chemical Registry System. This normally happens when an article, patent, or other publication that is the first to describe the chemical substance is indexed and added to the Chemical Abstracts database.
These numbers have three parts, and are arranged in the following format:
EXAMPLE: ethylene glycol is 107-21-1
- The first group of numbers (x) can vary between 2 and 6 digits;
The second group (y) is always 2 digits, and
- The third (z) is always 1 digit.
You may sometimes see the number written in brackets: [107-21-1]. The number is not part of any hierarchical structure. 107 doesn't mean anything; neither does 21. The 1 at the end is a check digit.
The public form of the CAS Chemical Registry System is known as the CAS Registry file, and you access it via SciFinder Scholar when you do a structure or CAS Registry Number search.
One you know what you're looking for, you'll find Registry Numbers all over the place. Chemical catalogs, the CRC Handbook of Chemistry and Physics, the Merck Index, and many other publications all use these numbers. Beilstein also has them for older chemicals; however, entries for chemicals added to Beilstein since 1994 cannot be searched by CAS Registry Numbers.
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Last updated June 3, 2004
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