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The Hill system is a system of writing chemical formulas such that the number of carbon atoms in a molecule is indicated first, the number of hydrogen atoms next, and then the number of all other chemical elements subsequently, in alphabetical order. When the formula contains no carbon, all the elements, including hydrogen, are listed alphabetically.

By sorting formulas according to the number of atoms of each element present in the formula according to these rules, with differences in earlier elements or numbers being treated as more significant than differences in any later element or number — like sorting text strings into lexicographic order — it is possible to collate chemical formulas into what is known as Hill system order.

The Hill system was first published by Edwin A. Hill of the United States Patent Office in 1900.

Example

The following formulas are written using the Hill system, and listed in Hill order:

1. HBr
2. BrI
3. CH₃I
4. C₂H₅Br
5. HI

References

• Edwin A. Hill (1900). "On A System Of Indexing Chemical Literature; Adopted By The Classification Division Of The U. S. Patent Office". J. Am. Chem. Soc. 22 (8): 478–494. doi:10.1021/ja02046a005.

• Chemical nomenclature