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Chemistry 401 | Chemical Literature

This guide brings together WVU Libraries resources for the study of chemistry.

What are CAS Registry Numbers?

The Chemical Abstracts Service (CAS) maintains a registry system for all completely identified chemical compounds and substances.  There are over 67 million chemical substances currently registered and one new substance is added to the registry every 9 seconds! 

Each substance is assigned a unique number (referred to as a CAS Registry Number, or CAS RN). A registry number looks like this: 123456-78-9 where the first segment can be from two to seven digits long, followed by two digits, then a single check-digit. The registry number has no structural significance--it is simply used as an identifier.

The rules of chemical nomenclature frequently change and each substance may have several names.  Such as:

  • trade or brand names (Tylenol)
  • generic or common names
  • trivial or semisystematic names (acetaminophen)
  • systematic or IUPAC names. (Acetamide, N-(4-hydroxyphenyl)- AND 4'-Hydroxyacetanilide)

BUT just one CAS registry number, 103-90-2. Even if other names are created for this compound, the registry number will remain unchanged.  Searching by the Chemical Registry number eliminates a lot of confusion about chemical names.

Find CAS Registry Numbers