View source for Molecule

{{Short description| Electrically neutral group of two or more atoms}}
{{Other uses}}
{{pp-semi-indef}}
{{More citations needed|date=March 2023}}
{{Use dmy dates|date=February 2016}}
[[File:PTCDA AFM.jpg|thumb|[[Atomic force microscopy]] (AFM) image of a [[Perylenetetracarboxylic dianhydride|PTCDA]] molecule, in which the five six-carbon rings are visible<ref>{{cite journal|doi=10.1038/ncomms8766|pmid=26178193|pmc=4518281|title=Chemical structure imaging of a single molecule by atomic force microscopy at room temperature|journal=Nature Communications|volume=6|page=7766|year=2015|last1=Iwata|first1=Kota|last2=Yamazaki|first2=Shiro|last3=Mutombo|first3=Pingo|last4=Hapala|first4=Prokop|last5=Ondráček|first5=Martin|last6=Jelínek|first6=Pavel|last7=Sugimoto|first7=Yoshiaki|bibcode= 2015NatCo...6.7766I}}</ref>]]
[[File:Pentacene on Ni(111) STM.jpg|thumb|A [[scanning tunneling microscopy]] image of [[pentacene]] molecules, which consist of linear chains of five carbon rings<ref>{{cite journal|doi=10.1039/C4NR07057G|pmid=25619890|title=Pentacene on Ni(111): Room-temperature molecular packing and temperature-activated conversion to graphene|journal=Nanoscale|volume=7|issue=7|pages=3263–9|year=2015|last1=Dinca|first1=L.E.|last2=De Marchi|first2=F.|last3=MacLeod|first3=J.M.|last4=Lipton-Duffin|first4=J.|last5=Gatti|first5=R.|last6=Ma|first6=D.|last7=Perepichka|first7=D.F.|last8=Rosei|first8=F.|author-link7=Dmitrii Perepichka|bibcode= 2015Nanos...7.3263D}}</ref>]]
[[File:TOAT AFM.png|thumb|AFM image of 1,5,9-trioxo-13-azatriangulene and its chemical structure<ref>{{cite journal|doi=10.1038/ncomms11560|pmid=27230940|pmc=4894979|title=Mapping the electrostatic force field of single molecules from high-resolution scanning probe images|journal=Nature Communications|volume=7|article-number=11560|year=2016|last1=Hapala|first1=Prokop|last2=Švec|first2=Martin|last3=Stetsovych|first3=Oleksandr|last4=Van Der Heijden|first4=Nadine J.|last5=Ondráček|first5=Martin|last6=Van Der Lit|first6=Joost|last7=Mutombo|first7=Pingo|last8=Swart|first8=Ingmar|last9=Jelínek|first9=Pavel|bibcode=2016NatCo...711560H}}</ref>]]

A '''molecule''' is a group of two or more [[atom]]s that are held together by [[Force|attractive forces]] known as [[chemical bond]]s; depending on context, the term may or may not include [[ions]] that satisfy this criterion.<ref name="iupac">{{GoldBookRef| title=Molecule|file=M04002|accessdate=23 February 2016}}</ref><ref>{{cite book| author= Ebbin, Darrell D.| title= General Chemistry |edition=3rd| date= 1990| publisher= [[Houghton Mifflin Co.]]| location= Boston| isbn= 978-0-395-43302-7}}</ref><ref>{{cite book| author= Brown, T.L. |author2=Kenneth C. Kemp |author3=Theodore L. Brown |author4=Harold Eugene LeMay |author5=Bruce Edward Bursten |title= Chemistry – the Central Science | url= https://archive.org/details/studentlectureno00theo | url-access= registration |edition=9th| date= 2003| publisher= [[Prentice Hall]]| location= New Jersey| isbn= 978-0-13-066997-1}}</ref><ref>{{cite book| last= Chang| first= Raymond| title= Chemistry | url= https://archive.org/details/chemistry00chan_0| url-access= registration|edition=6th| date= 1998| publisher= [[McGraw Hill]]| location= New York| isbn= 978-0-07-115221-1}}</ref><ref>{{cite book| author= Zumdahl, Steven S.| title= Chemistry |edition=4th| date= 1997| publisher= Houghton Mifflin| location= Boston| isbn= 978-0-669-41794-4}}</ref> In [[quantum physics]], [[organic chemistry]], and [[biochemistry]], the distinction from ions is dropped and ''molecule'' is often used when referring to [[polyatomic ion]]s.

A molecule may be [[homonuclear]], that is, it consists of atoms of one [[chemical element]], e.g. two atoms in the [[oxygen]] molecule (O<sub>2</sub>); or it may be [[heteronuclear]], a [[chemical compound]] composed of more than one element, e.g. [[water (molecule)|water]] (two hydrogen atoms and one oxygen atom; H<sub>2</sub>O). In the [[kinetic theory of gases]], the term ''molecule'' is often used for any gaseous [[particle]] regardless of its composition. This relaxes the requirement that a molecule contains two or more atoms, since the [[noble gases]] are individual atoms.<ref>{{cite book |last=Chandra |first=Sulekh |title=Comprehensive Inorganic Chemistry |date=2005 |publisher=New Age Publishers |isbn=978-81-224-1512-4}}</ref> Atoms and complexes connected by [[non-covalent interactions]], such as [[hydrogen bond]]s or [[ionic bond]]s, are typically not considered single molecules.<ref>{{cite encyclopedia|title=Molecule|encyclopedia=[[Encyclopædia Britannica]]|date=22 January 2016|url=http://global.britannica.com/science/molecule|access-date=23 February 2016|archive-date=3 May 2020|archive-url=https://web.archive.org/web/20200503044729/https://global.britannica.com/science/molecule|url-status=live}}</ref>

Concepts similar to molecules have been discussed since ancient times, but modern investigation into the nature of molecules and their bonds began in the 17th century. Refined over time by scientists such as [[Robert Boyle]], [[Amedeo Avogadro]], [[Jean Baptiste Perrin|Jean Perrin]], and [[Linus Pauling]], the study of molecules is today known as [[molecular physics]] or molecular chemistry.

== Etymology ==
According to [[Merriam-Webster]] and the [[Online Etymology Dictionary]], the word "molecule" derives from the [[Latin]] "[[Mole (unit)|moles]]" or small unit of mass. The word is derived from French ''{{linktext|molécule}}'' (1678), from [[Neo-Latin]] ''{{linktext|molecula}}'', diminutive of Latin ''{{linktext|moles}}'' "mass, barrier". The word, which until the late 18th century was used only in Latin form, became popular after being used in works of philosophy by [[René Descartes|Descartes]].<ref>{{OEtymD|molecule|accessdate=2016-02-22}}</ref><ref>{{cite encyclopedia |title=molecule |dictionary=[[Merriam-Webster]] |url=http://www.merriam-webster.com/dictionary/molecule |access-date=22 February 2016 |archive-url=https://web.archive.org/web/20210224223305/https://www.merriam-webster.com/dictionary/molecule |archive-date=24 February 2021 |url-status=live}}</ref>

== History ==
{{Main|History of molecular theory}}

The definition of the molecule has evolved as knowledge of the structure of molecules has increased. Earlier definitions were less precise, defining molecules as the smallest [[list of particles#Molecules|particles]] of pure [[chemical substance]]s that still retain their [[chemical compound|composition]] and chemical properties.<ref>[http://antoine.frostburg.edu/chem/senese/101/glossary/m.shtml#molecule Molecule Definition] {{Webarchive|url=https://web.archive.org/web/20141013143129/http://antoine.frostburg.edu/chem/senese/101/glossary/m.shtml#molecule|date=13 October 2014}} ([[Frostburg State University]])</ref> This definition often breaks down since many substances in ordinary experience, such as [[rock (geology)|rocks]], [[salt (chemistry)|salts]], and [[metal]]s, are composed of large crystalline networks of [[chemical bond|chemically bonded]] atoms or [[ion]]s, but are not made of discrete molecules.

The modern concept of molecules can be traced back towards pre-scientific and Greek philosophers such as [[Leucippus]] and [[Democritus]] who argued that all the universe is composed of [[Atomic theory|atoms and voids]]. Circa 450 BC [[Empedocles]] imagined [[Classical element|fundamental elements]] ([[Fire (classical element)|fire]] ([[File:Fire_symbol_(alchemical).svg|20x20px]]), [[Earth (classical element)|earth]] ([[File:Earth_symbol_(alchemical).svg|20x20px]]), [[Air (classical element)|air]] ([[File:Air_symbol_(alchemical).svg|20x20px]]), and [[Water (classical element)|water]] ([[File:Water_symbol_(alchemical).svg|20x20px]])) and "forces" of attraction and repulsion allowing the elements to interact.

A fifth element, the incorruptible quintessence [[Aether (classical element)|aether]], was considered to be the fundamental building block of the heavenly bodies. The viewpoint of Leucippus and Empedocles, along with the aether, was accepted by [[Aristotle]] and passed to medieval and renaissance Europe.

In a more concrete manner, however, the concept of aggregates or units of bonded atoms, i.e. "molecules", traces its origins to [[Robert Boyle]]'s 1661 hypothesis, in his famous treatise ''[[The Sceptical Chymist]]'', that matter is composed of ''clusters of particles'' and that chemical change results from the rearrangement of the clusters. Boyle argued that matter's basic elements consisted of various sorts and sizes of particles, called "corpuscles", which were capable of arranging themselves into groups. In 1789, [[William Higgins (chemist)|William Higgins]] published views on what he called combinations of "ultimate" particles, which foreshadowed the concept of [[valency bonds]]. If, for example, according to Higgins, the force between the ultimate particle of oxygen and the ultimate particle of nitrogen were 6, then the strength of the force would be divided accordingly, and similarly for the other combinations of ultimate particles.

[[Amedeo Avogadro]] created the word "molecule".<ref name="ley196606">{{Cite magazine |last=Ley |first=Willy |date=June 1966 |title=The Re-Designed Solar System |url=https://archive.org/stream/Galaxy_v24n05_1966-06#page/n93/mode/2up |department=For Your Information |magazine=Galaxy Science Fiction |pages=94–106}}</ref> His 1811 paper "Essay on Determining the Relative Masses of the Elementary Molecules of Bodies", he essentially states, i.e. according to [[J. R. Partington|Partington]]'s ''A Short History of Chemistry'', that:<ref>{{cite journal |last1=Avogadro |first1=Amedeo |date=1811 |title=Masses of the Elementary Molecules of Bodies |url=http://web.lemoyne.edu/~giunta/AVOGADRO.HTML |journal=Journal de Physique |volume=73 |pages=58–76 |access-date=25 August 2022 |archive-date=12 May 2019 |archive-url=https://web.archive.org/web/20190512182624/http://web.lemoyne.edu/~giunta/avogadro.html |url-status=live }}</ref>{{Blockquote|The smallest particles of gases are not necessarily simple atoms, but are made up of a certain number of these atoms united by attraction to form a single '''molecule'''.}}In coordination with these concepts, in 1833 the French chemist [[Marc Antoine Auguste Gaudin]] presented a clear account of Avogadro's hypothesis,<ref>{{cite journal |author=Seymour H. Mauskopf |date=1969 |title=The Atomic Structural Theories of Ampère and Gaudin: Molecular Speculation and Avogadro's Hypothesis |journal=Isis |volume=60 |issue=1 |pages=61–74 |doi=10.1086/350449 |jstor=229022 |s2cid=143759556}}</ref> regarding atomic weights, by making use of "volume diagrams", which clearly show both semi-correct molecular geometries, such as a linear water molecule, and correct molecular formulas, such as H<sub>2</sub>O:
[[File:Gaudins-volume-diagrams.jpg|center|thumb|350x350px|Marc Antoine Auguste Gaudin's volume diagrams of molecules in the gas phase (1833)]]
In 1917, an unknown American undergraduate chemical engineer named [[Linus Pauling]] was learning the [[Dalton model|Dalton hook-and-eye bonding method]], which was the mainstream description of bonds between atoms at the time. Pauling, however, was not satisfied with this method and looked to the newly emerging field of quantum physics for a new method. In 1926, French physicist [[Jean Perrin]] received the Nobel Prize in physics for proving, conclusively, the existence of molecules. He did this by calculating the [[Avogadro constant]] using three different methods, all involving liquid phase systems. First, he used a [[gamboge]] soap-like emulsion, second by doing experimental work on [[Brownian motion]], and third by confirming Einstein's theory of particle rotation in the liquid phase.<ref>Perrin, Jean, B. (1926). [https://www.nobelprize.org/prizes/physics/1926/perrin/lecture/ Discontinuous Structure of Matter] {{Webarchive|url=https://web.archive.org/web/20190529115507/https://www.nobelprize.org/prizes/physics/1926/perrin/lecture/ |date=29 May 2019 }}, Nobel Lecture, 11 December.</ref>

In 1927, the physicists [[Fritz London]] and [[Walter Heitler]] applied the new quantum mechanics to the deal with the saturable, nondynamic forces of attraction and repulsion, i.e., exchange forces, of the hydrogen molecule. Their valence bond treatment of this problem, in their joint paper,<ref>{{cite journal |last1=Heitler |first1=Walter |last2=London |first2=Fritz |date=1927 |title=Wechselwirkung neutraler Atome und homöopolare Bindung nach der Quantenmechanik |journal=Zeitschrift für Physik |volume=44 |issue=6–7 |pages=455–472 |bibcode=1927ZPhy...44..455H |doi=10.1007/BF01397394 |s2cid=119739102}}</ref> was a landmark in that it brought chemistry under quantum mechanics. Their work was an influence on Pauling, who had just received his doctorate and visited Heitler and London in Zürich on a [[Guggenheim Fellowship]].

Subsequently, in 1931, building on the work of Heitler and London and on theories found in Lewis' famous article, Pauling published his ground-breaking article "The Nature of the Chemical Bond"<ref>{{cite journal |last1=Pauling |first1=Linus |date=1931 |title=The nature of the chemical bond. Application of results obtained from the quantum mechanics and from a theory of paramagnetic susceptibility to the structure of molecules |journal=J. Am. Chem. Soc. |volume=53 |issue=4 |pages=1367–1400 |doi=10.1021/ja01355a027|bibcode=1931JAChS..53.1367P }}</ref> in which he used [[quantum mechanics]] to calculate properties and structures of molecules, such as angles between bonds and rotation about bonds. On these concepts, Pauling developed [[hybridization theory]] to account for bonds in molecules such as CH<sub>4</sub>, in which four sp³ hybridised orbitals are overlapped by [[hydrogen]]'s ''1s'' orbital, yielding four [[Sigma bond|sigma (σ) bonds]]. The four bonds are of the same length and strength, which yields a molecular structure as shown below:
[[File:Ch4_hybridization.svg|center|thumb|200x200px|A schematic presentation of hybrid orbitals overlapping hydrogens's orbitals]]

== Molecular science ==

The science of molecules is called ''molecular chemistry'' or ''[[molecular physics]]'', depending on whether the focus is on chemistry or physics. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of chemical bonds, while molecular physics deals with the laws governing their structure and properties. In practice, however, this distinction is vague. In molecular sciences, a molecule consists of a stable system ([[bound state]]) composed of two or more atoms. [[Polyatomic ion]]s may sometimes be usefully thought of as electrically charged molecules. The term ''unstable molecule'' is used for very [[reactivity (chemistry)|reactive]] species, i.e., short-lived assemblies ([[Resonance (chemistry)|resonances]]) of electrons and [[atomic nucleus|nuclei]], such as [[radical (chemistry)|radicals]], [[Polyatomic ion|molecular ions]], [[Rydberg molecule]]s, [[transition state]]s, [[van der Waals bonding|van der Waals complexes]], or systems of colliding atoms as in [[Bose–Einstein condensate]].

== Prevalence ==
{{Unreferenced section|date=August 2022}}
Molecules as components of matter are common. They also make up most of the oceans and atmosphere. Most organic substances are molecules. The substances of life are molecules, e.g. proteins, the amino acids of which they are composed, the nucleic acids (DNA and RNA), sugars, carbohydrates, fats, and vitamins. The nutrient minerals are generally ionic compounds, thus they are not molecules, e.g. iron sulfate.

However, the majority of familiar solid substances on Earth are made partly or completely of crystals or ionic compounds, which are not made of molecules. These include all of the minerals that make up the substance of the Earth, sand, clay, pebbles, rocks, boulders, [[Crust (geology)|bedrock]], the [[Mantle (geology)|molten interior]], and the [[Earth core|core of the Earth]]. All of these contain many chemical bonds, but are ''not'' made of identifiable molecules.

No typical molecule can be defined for salts nor for [[network solid|covalent crystals]], although these are often composed of repeating [[unit cell]]s that extend either in a [[Plane (mathematics)|plane]], e.g. [[graphene]]; or three-dimensionally e.g. [[diamond]], [[quartz]], [[sodium chloride]]. The theme of repeated unit-cellular-structure also holds for most metals which are condensed phases with [[metallic bond]]ing. Thus solid metals are not made of molecules. In [[glass]]es, which are solids that exist in a vitreous disordered state, the atoms are held together by chemical bonds with no presence of any definable molecule, nor any of the regularity of repeating unit-cellular-structure that characterizes salts, covalent crystals, and metals.

== Bonding ==
Molecules are generally held together by [[covalent bonding]]. Several non-metallic elements exist only as molecules in the environment either in compounds or as homonuclear molecules, not as free atoms: for example, hydrogen.

While some people say a metallic crystal can be considered a single giant molecule held together by [[metallic bonding]],<ref>{{cite book |last1=Harry |first1=B. Gray |title=Chemical Bonds: An Introduction to Atomic and Molecular Structure |pages=210–211 |url=https://authors.library.caltech.edu/105209/15/TR000574_06_chapter-6.pdf |access-date=22 November 2021 |archive-date=31 March 2021 |archive-url=https://web.archive.org/web/20210331062040/https://authors.library.caltech.edu/105209/15/TR000574_06_chapter-6.pdf |url-status=live }}</ref> others point out that metals behave very differently than molecules.<ref>{{cite web |title=How many gold atoms make gold metal? |url=https://phys.org/news/2015-04-gold-atoms-metal.html |website=phys.org |access-date=22 November 2021 |language=en |archive-date=30 October 2020 |archive-url=https://web.archive.org/web/20201030202803/https://phys.org/news/2015-04-gold-atoms-metal.html |url-status=live }}</ref>

=== Covalent ===
[[File:Covalent bond hydrogen.svg|thumb|right|A covalent bond forming H<sub>2</sub> (right) where two [[hydrogen atom]]s share the two electrons]]
{{main|Covalent bonding}}
A covalent bond is a chemical bond that involves the sharing of [[electron pair]]s between atoms. These electron pairs are termed ''shared pairs'' or ''bonding pairs'', and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is termed ''covalent bonding''.<ref>{{cite book| author2= Brad Williamson| author3= Robin J. Heyden| last= Campbell| first= Neil A.| title= Biology: Exploring Life| url= http://www.phschool.com/el_marketing.html| access-date= 2012-02-05| year= 2006| publisher= [[Pearson Prentice Hall]]| location= Boston| isbn= 978-0-13-250882-7| archive-date= 2 November 2014| archive-url= https://web.archive.org/web/20141102041816/http://www.phschool.com/el_marketing.html| url-status= live}}</ref>

=== Ionic ===
{{main|Ionic bonding}} [[File:NaF.gif|thumb|left|[[Sodium]] and [[fluorine]] undergoing a redox reaction to form [[sodium fluoride]]. Sodium loses its outer [[electron]] to give it a stable [[electron configuration]], and this electron enters the fluorine atom [[exothermic]]ally.]]

Ionic bonding is a type of chemical bond that involves the [[electrostatic]] attraction between oppositely charged ions, and is the primary interaction occurring in [[ionic compound]]s. The ions are atoms that have lost one or more [[electron]]s (termed ''[[cation]]s'') and atoms that have gained one or more electrons (termed ''[[anion]]s'').<ref>{{Cite book|url=https://books.google.com/books?id=6VdROgeQ5M8C&q=ionic+bonding+-wikipedia&pg=PA7|title=Elements of Metallurgy and Engineering Alloys|last=Campbell|first=Flake C.|year=2008|publisher=[[ASM International]]|isbn=978-1-61503-058-3|language=en|access-date=27 October 2020|archive-date=31 March 2021|archive-url=https://web.archive.org/web/20210331062041/https://books.google.com/books?id=6VdROgeQ5M8C&q=ionic+bonding+-wikipedia&pg=PA7|url-status=live}}</ref> This transfer of electrons is termed ''electrovalence'' in contrast to [[covalent bond|covalence]]. In the simplest case, the cation is a [[metal]] atom and the anion is a [[Nonmetal (chemistry)|nonmetal]] atom, but these ions can be of a more complicated nature, e.g. molecular ions like NH<sub>4</sub><sup>+</sup> or SO<sub>4</sub><sup>2−</sup>. At normal temperatures and pressures, ionic bonding mostly creates solids (or occasionally liquids) without separate identifiable molecules, but the vaporization/sublimation of such materials does produce separate molecules where electrons are still transferred fully enough for the bonds to be considered ionic rather than covalent.
{{clear}}

== Molecular size ==

Most molecules are far too small to be seen with the naked eye, although molecules of many [[polymer]]s can reach [[macroscopic]] sizes, including [[biopolymer]]s such as [[DNA]]. Molecules commonly used as building blocks for organic synthesis have a dimension of a few [[angstrom]]s (Å) to several dozen Å, or around one billionth of a meter. Single molecules cannot usually be observed by [[light]] (as noted above), but small molecules and even the outlines of individual atoms may be traced in some circumstances by use of an [[atomic force microscope]]. Some of the largest molecules are [[macromolecule]]s or [[supermolecule]]s.

The smallest molecule is the [[diatomic]] hydrogen (H<sub>2</sub>), with a bond length of 0.74 Å.<ref>{{cite book| author= Roger L. DeKock| author2= Harry B. Gray| author3= Harry B. Gray| title= Chemical structure and bonding| url= https://books.google.com/books?id=q77rPHP5fWMC&pg=PA199| date= 1989| publisher= University Science Books| isbn= 978-0-935702-61-3| page= 199| access-date= 27 October 2020| archive-date= 31 March 2021| archive-url= https://web.archive.org/web/20210331062042/https://books.google.com/books?id=q77rPHP5fWMC&pg=PA199| url-status= live}}</ref>

Effective molecular radius is the size a molecule displays in solution.<ref>
{{cite journal
 |author=Chang RL |author2=Deen WM |author3=Robertson CR |author4=Brenner BM
 |title=Permselectivity of the glomerular capillary wall: III. Restricted transport of polyanions
 |journal=Kidney Int.
 |volume=8
 |issue=4
 |pages=212–218
 |year=1975
 |pmid=1202253
 |doi=10.1038/ki.1975.104
 |doi-access=free
}}</ref><ref>
{{cite journal
 |author=Chang RL  |author2=Ueki IF |author3=Troy JL |author4=Deen WM |author5=Robertson CR |author6=Brenner BM
 |title=Permselectivity of the glomerular capillary wall to macromolecules. II. Experimental studies in rats using neutral dextran
 |journal=Biophys. J.
 |volume=15
 |issue=9
 |pages=887–906
 |year=1975
 |pmid=1182263
 |doi=10.1016/S0006-3495(75)85863-2
 |pmc=1334749
 |bibcode= 1975BpJ....15..887C
}}</ref>
The [[table of permselectivity for different substances]] contains examples.

== 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 typically expressed in [[Dalton (unit)|dalton]]s, which are 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.
{{clear}}

=== 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.
{{Clear}}

== Molecular geometry ==
{{Main|Molecular geometry}}

[[File:Cyanostar STM.png|thumb|left|upright|Structure and [[Scanning tunneling microscopy|STM]] image of a "cyanostar" [[dendrimer]] molecule<ref>{{cite journal|doi=10.1039/C4CC03725A|pmid=25080328|title=Anion-induced dimerization of 5-fold symmetric cyanostars in 3D crystalline solids and 2D self-assembled crystals|journal=Chemical Communications|volume=50|issue=69|pages=9827–30|year=2014|last1=Hirsch|first1=Brandon E.|last2=Lee|first2=Semin|last3=Qiao|first3=Bo|last4=Chen|first4=Chun-Hsing|last5=McDonald|first5=Kevin P.|last6=Tait|first6=Steven L.|last7=Flood|first7=Amar H.|s2cid=12439952 |url=https://zenodo.org/record/889879|access-date=20 April 2018|archive-date=31 March 2021|archive-url=https://web.archive.org/web/20210331062049/https://zenodo.org/record/889879|url-status=live}}</ref>]]

Molecules have fixed [[mechanical equilibrium|equilibrium]] geometries—bond lengths and angles— about which they continuously oscillate through vibrational and rotational motions. A pure substance is composed of molecules with the same average geometrical structure. The chemical formula and the structure of a molecule are the two important factors that determine its properties, particularly its [[reactivity (chemistry)|reactivity]]. [[Isomers]] share a chemical formula but normally have very different properties because of their different structures. [[Stereoisomer]]s, a particular type of isomer, may have very similar physico-chemical properties and at the same time different [[biochemistry|biochemical]] activities.

== Molecular spectroscopy ==
{{Main|Spectroscopy}}

[[File:Dehydrogenation of H2TPP by STM.jpg|thumb|upright=1.3|Hydrogen can be removed from individual [[Tetraphenylporphyrin|H<sub>2</sub>TPP]] molecules by applying excess voltage to the tip of a [[scanning tunneling microscope]] (STM, a); this removal alters the current–voltage (I–V) curves of TPP molecules, measured using the same STM tip, from [[diode]] like (red curve in b) to [[resistor]] like (green curve). Image (c) shows a row of TPP, H<sub>2</sub>TPP and TPP molecules. While scanning image (d), excess voltage was applied to H<sub>2</sub>TPP at the black dot, which instantly removed hydrogen, as shown in the bottom part of (d) and in the rescan image (e). Such manipulations can be used in [[single-molecule electronics]].<ref>{{cite journal|doi=10.1038/srep08350|pmid=25666850|pmc=4322354|title=N and p type character of single molecule diodes|journal=Scientific Reports|volume=5|page=8350|year=2015|bibcode= 2015NatSR...5.8350Z|last1=Zoldan|first1=V. C.|last2=Faccio|first2=R|last3=Pasa|first3=A.A.}}</ref>]]

'''Molecular spectroscopy''' deals with the response ([[frequency spectrum|spectrum]]) of molecules interacting with probing signals of known [[energy]] (or [[frequency]], according to the [[Planck relation]]). Molecules have quantized energy levels that can be analyzed by detecting the molecule's energy exchange through [[absorbance]] or [[Emission (electromagnetic radiation)|emission]].<ref name="iupac2">{{GoldBookRef|title=Spectroscopy|file=S05848|accessdate=23 February 2016}}</ref>
Spectroscopy does not generally refer to [[diffraction]] studies where particles such as [[neutron]]s, electrons, or high energy [[X-ray]]s interact with a regular arrangement of molecules (as in a crystal).

[[Microwave spectroscopy]] commonly measures changes in the rotation of molecules, and can be used to identify molecules in outer space. [[Infrared spectroscopy]] measures the vibration of molecules, including stretching, bending or twisting motions. It is commonly used to identify the kinds of bonds or [[functional group]]s in molecules. Changes in the arrangements of electrons yield absorption or emission lines in ultraviolet, visible or [[near infrared]] light, and result in colour. [[Nuclear magnetic resonance spectroscopy|Nuclear resonance spectroscopy]] measures the environment of particular nuclei in the molecule, and can be used to characterise the numbers of atoms in different positions in a molecule.

== Theoretical aspects ==

The study of molecules by molecular physics and [[theoretical chemistry]] is largely based on [[quantum mechanics]] and is essential for the understanding of the chemical bond. The simplest of molecules is the [[hydrogen molecule-ion]], H<sub>2</sub><sup>+</sup>, and the simplest of all the chemical bonds is the [[one-electron bond]]. H<sub>2</sub><sup>+</sup> is composed of two positively charged [[proton]]s and one negatively charged [[electron]], which means that the [[Schrödinger equation]] for the system can be solved more easily due to the lack of electron–electron repulsion. With the development of fast digital computers, approximate solutions for more complicated molecules became possible and are one of the main aspects of [[computational chemistry]].

When trying to define rigorously whether an arrangement of atoms is ''sufficiently stable'' to be considered a molecule, IUPAC suggests that it "must correspond to a depression on the [[potential energy surface]] that is deep enough to confine at least one vibrational state".<ref name="iupac" /> This definition does not depend on the nature of the interaction between the atoms, but only on the strength of the interaction. In fact, it includes weakly bound species that would not traditionally be considered molecules, such as the [[helium]] [[Dimer (chemistry)|dimer]], [[helium dimer|He<sub>2</sub>]], which has one vibrational bound state<ref>{{cite journal |author=Anderson JB |title=Comment on "An exact quantum Monte Carlo calculation of the helium-helium intermolecular potential" [J. Chem. Phys. 115, 4546 (2001)] |journal=J Chem Phys |volume=120 |issue=20 |pages=9886–7 |date=May 2004 |pmid=15268005 |doi=10.1063/1.1704638 |bibcode= 2004JChPh.120.9886A|doi-access=free }}</ref> and is so loosely bound that it is only likely to be observed at very low temperatures.

Whether or not an arrangement of atoms is ''sufficiently stable'' to be considered a molecule is inherently an operational definition. Philosophically, therefore, a molecule is not a fundamental entity (in contrast, for instance, to an [[elementary particle]]); rather, the concept of a molecule is the chemist's way of making a useful statement about the strengths of atomic-scale interactions in the world that we observe.
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== See also ==
{{div col|colwidth=23em}}
* [[Atom]]
* [[Chemical polarity]]
* [[Chemical structure]]
* [[Covalent bond]]
* [[Diatomic molecule]]
* [[List of compounds]]
* [[List of interstellar and circumstellar molecules]]
* [[Molecular biology]]
* [[Molecular design software]]
* [[Molecular engineering]]
* [[Molecular geometry]]
* [[Molecular Hamiltonian]]
* [[Molecular ion]]
* [[Molecular modelling]]
* [[Molecular promiscuity]]
* [[Molecular orbital]]
* [[Non-covalent bonding]]
* [[Periodic systems of small molecules]]
* [[Small molecule]]
* [[Comparison of software for molecular mechanics modeling]]
* [[Van der Waals molecule]]
* [[World Wide Molecular Matrix]]
{{div col end}}

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== References ==
{{reflist}}

== External links ==
{{Sister project links|collapsible=true|c=Category:Molecules|voy=no|wikt=molecule|v=no|n=no|q=Molecule|s=Molecule|b=no|species=no|d=Q11369}}
* [http://www.chm.bris.ac.uk/motm/motm.htm Molecule of the Month{{snds}}School of Chemistry, University of Bristol]

{{Molecules detected in outer space}}
{{Particles}}
{{Branches of chemistry}}
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[[Category:Molecular physics| ]]
[[Category:Molecules| ]]
[[Category:Chemistry]]
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