Have you ever met a chemist? I find that chemists are some of the most organized, meticulous people I know. I attribute it to the fact that if they’re not careful they’re apt to blow themselves up in lab (although some of them do have a pyromania and enjoy setting things on fire on purpose).
So true to their organized selves, chemists have assigned a unique number to every chemical substance that is recorded in literature. Every chemical substance! That’s more than 66 million substances. About 15,000 new substances are added each day. (I wonder if they take the weekends off.)
Each substance has a unique 3-part CAS Registry Number. The substance is simply assigned a number in sequence. The number has no chemical significance. The first part has 2 to 7 digits, the second part is is 2 digits, and the last part is a single digit that is a check of the first two parts.
But the reason I even bother mentioning CAS Registry Numbers is that they are used in databases to help search for articles and literature about chemicals. They are listed SciFinder which is also put out by Chemical Abstract Service which is a division of the American Chemical Society. But you can also search for a specific chemical using CAS Registry Numbers in Scopus, MedLine, Biological Abstracts, and Inspec.