Editing Chemical formula (section)

== Types ==

=== Empirical formula ===
{{anchor|1=Empirical formulae}}
{{main|Empirical formula}}
In [[chemistry]], the [[empirical formula]] of a chemical is a simple expression of the relative number of each type of atom or ratio of the elements in the compound. Empirical formulae are the standard for [[ionic compound]]s, such as {{chem2|CaCl2}}, and for macromolecules, such as {{chem2|SiO2}}. An empirical formula makes no reference to [[isomer]]ism, structure, or absolute number of atoms. The term ''empirical'' refers to the process of [[elemental analysis]], a technique of [[analytical chemistry]] used to determine the relative percent composition of a pure chemical substance by element.

For example, [[hexane]] has a molecular formula of {{chem2|C6H14}}, and (for one of its isomers, n-hexane) a structural formula {{chem2|CH3CH2CH2CH2CH2CH3}}, implying that it has a chain structure of 6 [[carbon]] atoms, and 14 [[hydrogen]] atoms. However, the empirical formula for hexane is {{chem2|C3H7}}. Likewise the empirical formula for [[hydrogen peroxide]], {{chem2|H2O2}}, is simply {{chem2|HO}}, expressing the 1:1 ratio of component elements. [[Formaldehyde]] and [[acetic acid]] have the same empirical formula, {{chem2|CH2O}}. This is also the molecular formula for formaldehyde, but acetic acid has double the number of atoms.

Like the other formula types detailed below, an empirical formula shows the number of elements in a molecule, and determines whether it is a [[binary compound]], [[ternary compound]], [[quaternary compound]], or has even more elements.

=== Molecular formula ===
[[File:Isobutane_numbered_2D.svg|class=skin-invert-image|thumb|right|180px|[[Isobutane]] structural formula<br />Molecular formula: {{chem2|C4H10}}<br />Condensed formula: {{chem2|(CH3)3CH}}]]

{{Image frame
|content=<chem>H-\overset{\displaystyle H \atop |}{\underset{| \atop \displaystyle H}{C}}-\overset{\displaystyle H \atop |}{\underset{| \atop \displaystyle H}{C}}-\overset{\displaystyle H \atop |}{\underset{| \atop \displaystyle H}{C}}-\overset{\displaystyle H \atop |}{\underset{| \atop \displaystyle H}{C}}-H</chem>
|align=right|width=180
|caption=[[n-Butane|''n''-Butane]] structural formula<br />Molecular formula: {{chem2|C4H10}}<br />Condensed formula: {{chem2|CH3CH2CH2CH3}}
}}

Molecular formulae simply indicate the numbers of each type of atom in a molecule of a molecular substance. They are the same as empirical formulae for molecules that only have one atom of a particular type, but otherwise may have larger numbers. An example of the difference is the empirical formula for glucose, which is {{chem2|CH2O}} (''ratio'' 1:2:1), while its molecular formula is {{chem2|C6H12O6}} (''number of atoms'' 6:12:6). For water, both formulae are {{chem2|H2O}}. A molecular formula provides more information about a molecule than its empirical formula, but is more difficult to establish.

=== Structural formula ===
{{Main|Structural formula}}
In addition to indicating the number of atoms of each elementa molecule, a structural formula indicates how the atoms are organized, and shows (or implies) the [[chemical bond]]s between the atoms. There are multiple types of structural formulas focused on different aspects of the molecular structure.

The two diagrams show two molecules which are [[structural isomer]]s of each other, since they both have the same molecular formula {{chem2|C4H10}}, but they have different structural formulas as shown.

=== Condensed formula ===
{{Main|Condensed formula}}
The [[connectivity (graph theory)|connectivity]] of a molecule often has a strong influence on its physical and chemical properties and behavior. Two molecules composed of the same numbers of the same types of atoms (i.e. a pair of [[isomer]]s) might have completely different chemical and/or physical properties if the atoms are connected differently or in different positions. In such cases, a [[structural formula]] is useful, as it illustrates which atoms are bonded to which other ones. From the connectivity, it is often possible to deduce the approximate [[molecular geometry|shape of the molecule]].

A condensed (or semi-structural) formula may represent the types and spatial arrangement of [[Chemical bond|bonds]] in a simple chemical substance, though it does not necessarily specify [[isomer]]s or complex structures. For example, [[ethane]] consists of two carbon atoms single-bonded to each other, with each carbon atom having three hydrogen atoms bonded to it. Its chemical formula can be rendered as {{chem2|CH3CH3}}. In [[ethylene]] there is a double bond between the carbon atoms (and thus each carbon only has two hydrogens), therefore the chemical formula may be written: {{chem2|CH2CH2}}, and the fact that there is a double bond between the carbons is implicit because carbon has a valence of four. However, a more explicit method is to write {{chem2|H2C\dCH2}} or less commonly {{chem2|H2C::CH2}}. The two lines (or two pairs of dots) indicate that a [[double bond]] connects the atoms on either side of them.

A [[triple bond]] may be expressed with three lines ({{chem2|HC\tCH}}) or three pairs of dots ({{chem2|HC:::CH}}), and if there may be ambiguity, a single line or pair of dots may be used to indicate a single bond.

Molecules with multiple [[functional group]]s that are the same may be expressed by enclosing the repeated group in [[parenthesis|round brackets]]. For example, [[isobutane]] may be written {{chem2|(CH3)3CH}}. This condensed structural formula implies a different connectivity from other molecules that can be formed using the same atoms in the same proportions ([[isomer]]s). The formula {{chem2|(CH3)3CH}} implies a central carbon atom connected to one hydrogen atom and three [[methyl group]]s ({{chem2|CH3}}). The same number of atoms of each element (10 hydrogens and 4 carbons, or {{chem2|C4H10}}) may be used to make a straight chain molecule, ''n''-[[butane]]: {{chem2|CH3CH2CH2CH3}}.

=== Chemical names in answer to limitations of chemical formulae ===<!-- what why here??? -->
{{main|Chemical nomenclature}}
The alkene called [[but-2-ene]] has two isomers, which the chemical formula {{chem2|CH3CH\dCHCH3}} does not identify. The relative position of the two methyl groups must be indicated by additional notation denoting whether the methyl groups are on the same side of the double bond (''cis'' or ''Z'') or on the opposite sides from each other (''trans'' or ''E'').<ref>{{Cite book|last=Burrows, Andrew.|title=Chemistry³ : introducing inorganic, organic and physical chemistry|isbn=978-0-19-969185-2|edition=Second|location=Oxford|oclc=818450212|date=2013-03-21 |publisher=Oxford University Press}}</ref>

As noted above, in order to represent the full structural formulae of many complex organic and inorganic compounds, [[chemical nomenclature]] may be needed which goes well beyond the available resources used above in simple condensed formulae. See [[IUPAC nomenclature of organic chemistry]] and [[IUPAC nomenclature of inorganic chemistry|IUPAC nomenclature of inorganic chemistry 2005]] for examples. In addition, linear naming systems such as [[International Chemical Identifier]] (InChI) allow a computer to construct a structural formula, and [[simplified molecular-input line-entry system]] (SMILES) allows a more human-readable ASCII input. However, all these nomenclature systems go beyond the standards of chemical formulae, and technically are chemical naming systems, not formula systems.<ref>{{Cite web |last=Miles |first=Linda |title=LibGuides: CHE 120 - Introduction to Organic Chemistry - Textbook: Chapter 1 - Organic Chemistry Review / Hydrocarbons |url=https://guides.hostos.cuny.edu/che120/chapter1 |access-date=2024-07-13 |website=guides.hostos.cuny.edu |language=en}}</ref> 

=== Polymers in condensed formulae ===

For [[polymer]]s in condensed chemical formulae, parentheses are placed around the repeating unit. For example, a [[hydrocarbon]] molecule that is described as {{chem2|CH3(CH2)50CH3}}, is a molecule with fifty repeating units. If the number of repeating units is unknown or variable, the letter ''n'' may be used to indicate this formula: {{chem2|CH3(CH2)_{''n''}CH3}}.

=== Ions in condensed formulae ===
For [[ion]]s, the charge on a particular atom may be denoted with a right-hand superscript. For example, {{chem2|Na+}}, or {{chem2|Cu(2+)}}. The total charge on a charged molecule or a [[polyatomic ion]] may also be shown in this way, such as for [[hydronium]], {{chem2|H3O+}}, or [[sulfate]], {{chem2|SO4(2-)}}. Here + and − are used in place of +1 and −1, respectively.

For more complex ions, brackets [ ] are often used to enclose the ionic formula, as in {{chem2|[B12H12](2-)}}, which is found in compounds such as [[caesium dodecaborate]], {{chem2|Cs2[B12H12]}}. Parentheses ( ) can be nested inside brackets to indicate a repeating unit, as in [[Hexamminecobalt(III) chloride]], {{chem2|[Co(NH3)6](3+)Cl3-}}. Here, {{chem2|(NH3)6}} indicates that the ion contains six [[Metal ammine complex|ammine group]]s ({{chem2|NH3}}) bonded to [[cobalt]], and [ ] encloses the entire formula of the ion with charge +3. {{Elucidate|date=November 2012}}

This is strictly optional; a chemical formula is valid with or without ionization information, and Hexamminecobalt(III) chloride may be written as {{chem2|[Co(NH3)6](3+)Cl3-}} or {{chem2|[Co(NH3)6]Cl3}}. Brackets, like parentheses, behave in chemistry as they do in mathematics, grouping terms together{{snd}}they are not specifically employed only for ionization states. In the latter case here, the parentheses indicate 6 groups all of the same shape, bonded to another group of size 1 (the cobalt atom), and then the entire bundle, as a group, is bonded to 3 chlorine atoms. In the former case, it is clearer that the bond connecting the chlorines is [[ionic bonding|ionic]], rather than [[covalent bond|covalent]].