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Chemistry : How to Search

CA RN Number

Chemical Abstracts Service has assigned a number to each chemical, simplifying your search. 

CAS registry numbers appear as three numbers separated by hyphens.  For example, Tetrachlorodibenzodioxin:  1746-01-6

Look up RN numbers using:


A variety of structure searching mechanisms are available in print indexes.  Structure drawing tools are available for online resources.

Searching by Chemical Name

Nomenclature systems vary. Names can be complex, making searching prone to problems with typos, variant naming systems, etc. Most chemical reference works will have a name index. Review the material in the front of the book or on the help screens to understand which system they are using.

Try these resources to search by chemical names:

ChemID Plus Advanced will provide synonyms, CAS registry numbers, properties, and database links for various chemicals.

Search NIST Chemistry Web Book by chemical name.

Chemical naming Resources:

IUPAC Compendium of Chemical Terminology - the Gold Book

Searching by Molecular Formula

Chemistry reference works and online resources generally include a molecular formula index or search feature.

Most resources order the elements in organic molecules by carbons, then hydrogens, then other elements in alphabetic order. Secondary arrangement is by the number of each element. See for example: C12-H4-Cl4-O2. All of the C12s come after the C11s, then H4s comes after H3s…

Because there are often several molecules with the same molecular formula, scan the list of names for your chemical.