Editing Chemical formula (section)

== Hill system ==
The '''Hill system''' (or Hill notation) is a system of writing empirical chemical formulae, molecular chemical formulae and components of a condensed formula such that the number of [[carbon]] [[atom]]s in a [[molecule]] is indicated first, the number of [[hydrogen]] atoms next, and then the number of all other [[chemical element]]s subsequently, in [[alphabetical order]] of the [[chemical symbols]]. When the formula contains no carbon,  all the elements, including hydrogen, are listed alphabetically.

By sorting formulae 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&mdash;like sorting text strings into [[lexicographical order]]&mdash;it is possible to [[collation|collate]] chemical formulae into what is known as Hill system order.

The Hill system was first published by [[Edwin A. Hill]] of the [[United States Patent and Trademark Office]] in 1900.<ref>{{cite journal | author = Edwin A. Hill | title = On a system of indexing chemical literature; Adopted by the Classification Division of the U.S. Patent Office | journal = [[J. Am. Chem. Soc.]] | year = 1900 | volume = 22 | issue = 8 | pages = 478–494 | doi = 10.1021/ja02046a005| bibcode = 1900JAChS..22..478H | hdl = 2027/uiug.30112063986233 | url = https://zenodo.org/record/1428916 }}</ref> It is the most commonly used system in chemical databases and printed indexes to sort lists of compounds.<ref name="wiggins">Wiggins, Gary. (1991). ''Chemical Information Sources.'' New York: McGraw Hill. p. 120.</ref>

A list of formulae in Hill system order is arranged alphabetically, as above, with single-letter elements coming before two-letter symbols when the symbols begin with the same letter (so "B" comes before "Be", which comes before "Br").<ref name="wiggins"/>

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

* BrClH<sub>2</sub>Si
* BrI
* CCl<sub>4</sub>
* CH<sub>3</sub>I
* C<sub>2</sub>H<sub>5</sub>Br
* H<sub>2</sub>O<sub>4</sub>S