CAS registry number (CAS RN)
Definition
A unique numeric identifier assigned by the Chemical Abstracts Service to a chemical substance, such as 7732-18-5 for water.
CAS registry numbers are assigned by CAS, a division of the American Chemical Society. A number has up to ten digits in three parts separated by hyphens: two to seven digits, then two digits, then a single check digit. The number itself carries no chemical meaning, it is simply assigned in sequence.
The check digit is calculated by multiplying each digit, from right to left starting after the check digit, by 1, 2, 3 and so on, adding the results and taking the last digit of the sum. For 7732-18-5: 8x1 + 1x2 + 2x3 + 3x4 + 7x5 + 7x6 = 105, so the check digit is 5. Salts, hydrates and stereoisomers usually have their own CAS numbers.
Related terms
- InChIKeyA fixed length 27 character hashed form of the InChI, used to search and link chemical records, for example LFQSCWFLJHTTHZ-UHFFFAOYSA-N for ethanol.
- InChIThe IUPAC International Chemical Identifier, an open text string that describes a chemical structure in layers.
- SMILESA line notation that writes a chemical structure as a short text string, such as CCO for ethanol.
- IUPAC nameThe systematic name of a chemical built by the nomenclature rules of the International Union of Pure and Applied Chemistry.
- EC numberA seven digit identifier (format 123-456-7) used for substances in the European Union.