Editing Chemical formula (section)

== Overview ==
A chemical formula identifies each constituent [[chemical element|element]] by its [[chemical symbol]] and indicates the proportionate number of atoms of each element. In empirical formulae, these proportions begin with a key element and then assign numbers of atoms of the other elements in the compound, by ratios to the key element. For molecular compounds, these ratio numbers can all be expressed as whole numbers. For example, the empirical formula of [[ethanol]] may be written {{chem2|C2H6O}} because the molecules of ethanol all contain two carbon atoms, six hydrogen atoms, and one oxygen atom. Some types of ionic compounds, however, cannot be written with entirely whole-number empirical formulae. An example is [[boron carbide]], whose formula of {{chem2|CB_{''n''} }} is a variable non-whole number ratio with n ranging from over 4 to more than 6.5.

When the chemical compound of the formula consists of simple [[molecule]]s, chemical formulae often employ ways to suggest the structure of the molecule. These types of formulae are variously known as ''molecular formulae'' and ''[[structural formula|condensed formulae]]''. A molecular formula enumerates the number of atoms to reflect those in the molecule, so that the molecular formula for [[glucose]] is {{chem2|C6H12O6}} rather than the glucose empirical formula, which is {{chem2|CH2O}}. However, except for very simple substances, molecular chemical formulae lack needed structural information, and are ambiguous.

For simple molecules, a condensed (or semi-structural) formula is a type of chemical formula that may fully imply a correct structural formula. For example, ethanol may be represented by the condensed chemical formula {{chem2|CH3CH2OH}}, and [[dimethyl ether]] by the condensed formula {{chem2|CH3OCH3}}. These two molecules have the same empirical and molecular formulae ({{chem2|C2H6O}}), but may be differentiated by the condensed formulae shown, which are sufficient to represent the full structure of these simple [[organic compound]]s.

Condensed chemical formulae may also be used to represent [[ionic compound]]s that do not exist as discrete molecules, but nonetheless do contain covalently bound clusters within them. These [[polyatomic ion]]s are groups of atoms that are covalently bound together and have an overall ionic charge, such as the [[sulfate]] {{chem2|[SO4]^{2-} }} ion. Each polyatomic ion in a compound is written individually in order to illustrate the separate groupings. For example, the compound [[dichlorine hexoxide]] has an empirical formula {{chem2|ClO3}}, and molecular formula {{chem2|Cl2O6}}, but in liquid or solid forms, this compound is more correctly shown by an ionic condensed formula {{chem2|[ClO2]+[ClO4]-}}, which illustrates that this compound consists of {{chem2|[ClO2]+}} ions and {{chem2|[ClO4]-}} ions. In such cases, the condensed formula only need be complex enough to show at least one of each ionic species.

Chemical formulae as described here are distinct from the far more complex chemical systematic names that are used in various systems of [[chemical nomenclature]]. For example, one systematic name for glucose is (2''R'',3''S'',4''R'',5''R'')-2,3,4,5,6-pentahydroxyhexanal. This name, interpreted by the rules behind it, fully specifies glucose's structural formula, but the name is not a chemical formula as usually understood, and uses terms and words not used in chemical formulae. Such names, unlike basic formulae, may be able to represent full structural formulae without graphs.