Editing Molecule (section)

== Molecular formulas ==

=== Chemical formula types ===
{{Main|Chemical formula}}

The [[chemical formula]] for a molecule uses one line of chemical element symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, and ''plus'' (+) and ''minus'' (−) signs. These are limited to one typographic line of symbols, which may include subscripts and superscripts.

A compound's [[empirical formula]] is a very simple type of chemical formula.<ref>{{Cite book|url=https://books.google.com/books?id=6wUmteTIc18C&q=empirical+formula&pg=PA288|title=The Practice of Chemistry|last1=Wink|first1=Donald J.|last2=Fetzer-Gislason|first2=Sharon|last3=McNicholas|first3=Sheila|year=2003|publisher=Macmillan|isbn=978-0-7167-4871-7|language=en|access-date=27 October 2020|archive-date=10 April 2022|archive-url=https://web.archive.org/web/20220410070618/https://books.google.com/books?id=6wUmteTIc18C&q=empirical+formula&pg=PA288|url-status=live}}</ref> It is the simplest [[integer]] [[ratio]] of the chemical elements that constitute it.<ref>{{Cite web|url=http://www.chemteam.info/Mole/EmpiricalFormula.html|title=ChemTeam: Empirical Formula|website=www.chemteam.info|access-date=2017-04-16|archive-date=19 January 2021|archive-url=https://web.archive.org/web/20210119114516/https://www.chemteam.info/Mole/EmpiricalFormula.html|url-status=live}}</ref> For example, water is always composed of a 2:1 ratio of hydrogen to oxygen atoms, and [[ethanol]] (ethyl alcohol) is always composed of carbon, hydrogen, and oxygen in a 2:6:1 ratio. However, this does not determine the kind of molecule uniquely&nbsp;– [[dimethyl ether]] has the same ratios as ethanol, for instance. Molecules with the same [[atom]]s in different arrangements are called [[isomer]]s. Also carbohydrates, for example, have the same ratio (carbon:hydrogen:oxygen= 1:2:1) (and thus the same empirical formula) but different total numbers of atoms in the molecule.

The [[molecular formula]] reflects the exact number of atoms that compose the molecule and so characterizes different molecules. However different isomers can have the same atomic composition while being different molecules.

The empirical formula is often the same as the molecular formula but not always. For example, the molecule [[acetylene]] has molecular formula C<sub>2</sub>H<sub>2</sub>, but the simplest integer ratio of elements is CH.

The [[molecular mass]] can be calculated from the chemical formula and is expressed in conventional [[atomic mass unit]]s equal to 1/12 of the mass of a neutral carbon-12 (<sup>12</sup>[[carbon|C]] [[isotope]]) atom. For [[network solid]]s, the term [[formula unit]] is used in [[stoichiometric]] calculations.
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=== Structural formula ===
[[File:Atisane3.png|thumb|right|upright=1.8|[[Three-dimensional space|3D]] (left and center) and [[2D geometric model|2D]] (right) representations of the [[terpenoid]] molecule atisane]]
{{Main|Structural formula}}

For molecules with a complicated 3-dimensional structure, especially involving atoms bonded to four different substituents, a simple molecular formula or even semi-structural chemical formula may not be enough to completely specify the molecule. In this case, a graphical type of formula called a [[structural formula]] may be needed. Structural formulas may in turn be represented with a one-dimensional chemical name, but such [[chemical nomenclature]] requires many words and terms which are not part of chemical formulas.
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