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A molecule is a group of two or more
atom Every atom is composed of a atomic nucleus, nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, l ...
s held together by attractive forces known as
chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of Molecule, molecules and crystals. The bond may result from the Coulomb's law, electrostatic force between oppositely charged ions as in Ionic bonding, ...
s; depending on context, the term may or may not include
ions An ion () is an atom or molecule with a net electric charge, electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be po ...
which satisfy this criterion. In
quantum physics Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including qua ...
,
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...
, and
biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology a ...

biochemistry
, the distinction from ions is dropped and ''molecule'' is often used when referring to
polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a complex (chemistry), metal complex, that can be considered to behave as a single unit and that has a net electrical charge, charge that is no ...
s. A molecule may be
homonuclear Homonuclear molecules, or homonuclear species, are molecules composed of only one Chemical element, element. Homonuclear molecules may consist of various numbers of atoms. The size of the molecule an element can form depends on the element's proper ...
, that is, it consists of atoms of one
chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Unlike chemical compounds, chemical elements canno ...
, e.g. two atoms in the
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...

oxygen
molecule (O2); or it may be heteronuclear, a
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A homonuclear molecule, molecule co ...
composed of more than one element, e.g.
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
(two hydrogen atoms and one oxygen atom; H2O). In the
kinetic theory of gases Kinetic (Ancient Greek: κίνησις “kinesis”, movement or to move) may refer to: * Kinetic theory, describing a gas as particles in random motion * Kinetic energy, the energy of an object that it possesses due to its motion Art and ent ...

kinetic theory of gases
, the term ''molecule'' is often used for any gaseous
particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object which can be described by several physical property, physical or chemical property, chemical ...

particle
regardless of its composition. This relaxes the requirement that a molecule contains two or more atoms, since the
noble gases The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under Standard conditions for temperature and pressure, standard conditions, they are all odorle ...
are individual atoms. Atoms and complexes connected by
non-covalent interactions In chemistry, a non-covalent interaction differs from a covalent bond in that it does not involve the sharing of electrons, but rather involves more dispersed variations of electromagnetic interactions between molecules or within a molecule. The c ...
, such as
hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily Electrostatics, electrostatic force of attraction between a hydrogen (H) atom which is Covalent bond, covalently bound to a more electronegativity, electronegative "donor" atom or group ( ...

hydrogen bond
s or
ionic bond Ionic bonding is a type of chemical bonding that involves the Coulomb's law, electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in ion ...
s, are typically not considered single molecules. 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 Robert Boyle (; 25 January 1627 – 31 December 1691) was an Anglo-Irish natural philosopher, chemist, physicist, Alchemy, alchemist and inventor. Boyle is largely regarded today as the first modern chemist, and therefore one of the foun ...

Robert Boyle
,
Amedeo Avogadro Lorenzo Romano Amedeo Carlo Avogadro, Count of Quaregna and Cerreto (, also , ; 9 August 17769 July 1856) was an Italian people, Italian scientist, most noted for his contribution to molecular theory now known as Avogadro's law, which states th ...

Amedeo Avogadro
,
Jean Perrin Jean Baptiste Perrin (30 September 1870 – 17 April 1942) was a French physicist who, in his studies of the Brownian motion Brownian motion, or pedesis (from grc, πήδησις "leaping"), is the random motion of particles suspended i ...
, and
Linus Pauling Linus Carl Pauling (; February 28, 1901August 19, 1994) was an American chemist, biochemist, chemical engineer, peace activist, author, and educator. He published more than 1,200 papers and books, of which about 850 dealt with scientific topi ...

Linus Pauling
, the study of molecules is today known as
molecular physics Molecular physics is the study of the physical properties of molecules and molecular dynamics. The field overlaps significantly with physical chemistry, chemical physics, and quantum chemistry. It is often considered as a sub-field of atomic, mo ...
or molecular chemistry.


Etymology

According to
Merriam-Webster Merriam-Webster, Inc. is an American company that publishes reference work, reference books and is especially known for its dictionary, dictionaries. It is the oldest dictionary publisher in the United States. In 1831, George Merriam, George a ...
and the
Online Etymology Dictionary The ''Online Etymology Dictionary'' (''Etymonline'') is a free online dictionary, written and compiled by Douglas R. Harper, that describes the origins of English-language words. Description Douglas Harper, an American Civil War historian an ...
, the word "molecule" derives from the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...

Latin
" moles" or small unit of mass. The word is derived from French ' (1678), from
New Latin New Latin (also called Neo-Latin or Modern Latin) is the List of revived languages, revival of Literary Latin used in original, scholarly, and scientific works since about 1500. Modern scholarly and technical nomenclature, such as in zoologica ...
', diminutive of Latin ' "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
Descartes
Descartes
.


History

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
particles In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object which can be described by several physical property, physical or chemical property, chemical ...

particles
of pure
chemical substance A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent Chemical element, elements by physical separation m ...
s that still retain their composition and chemical properties. This definition often breaks down since many substances in ordinary experience, such as
rocks In geology Geology () is a branch of natural science concerned with Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they change over tim ...
,
salts In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, stru ...
, and
metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...

metal
s, are composed of large crystalline networks of chemically bonded atoms or
ion An ion () is an atom Every atom is composed of a atomic nucleus, nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has n ...
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 Leucippus (; el, Λεύκιππος, ''Leúkippos''; fl. 5th century BCE) is a pre-Socratic Pre-Socratic philosophy, also known as early Greek philosophy, is ancient Greek philosophy before Socrates. Pre-Socratic philosophers were mostly inte ...
and
Democritus Democritus (; el, Δημόκριτος, ''Dēmókritos'', meaning "chosen of the people"; – ) was an Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient ...

Democritus
who argued that all the universe is composed of atoms and voids. Circa 450 BC
Empedocles Empedocles (; grc-gre, wikt:Ἐμπεδοκλῆς, Ἐμπεδοκλῆς; , 444–443 BC) was a Ancient Greece, Greek pre-Socratic philosopher and a native citizen of Akragas, a Greek city in Sicily. Empedocles' philosophy is best known for ...

Empedocles
imagined fundamental elements (
fire Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction Product (chemistry), products. At a certain point in the combustion reaction, called the ignition ...
(),
earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large list of largest lakes and seas in the Solar System, volumes of water can be found throughout the Solar System, only water distributi ...
(),
air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...
(), and
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
()) and "forces" of attraction and repulsion allowing the elements to interact. A fifth element, the incorruptible quintessence 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 Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical Greece, Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatet ...

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 Robert Boyle (; 25 January 1627 – 31 December 1691) was an Anglo-Irish natural philosopher, chemist, physicist, Alchemy, alchemist and inventor. Boyle is largely regarded today as the first modern chemist, and therefore one of the foun ...

Robert Boyle
's 1661 hypothesis, in his famous treatise ''
The Sceptical Chymist
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 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 Lorenzo Romano Amedeo Carlo Avogadro, Count of Quaregna and Cerreto (, also , ; 9 August 17769 July 1856) was an Italian people, Italian scientist, most noted for his contribution to molecular theory now known as Avogadro's law, which states th ...

Amedeo Avogadro
created the word "molecule". His 1811 paper "Essay on Determining the Relative Masses of the Elementary Molecules of Bodies", he essentially states, i.e. according to
Partington Partington is a town and civil parish in the Metropolitan Borough of Trafford, Greater Manchester, England, south-west of Manchester city centre. Within the boundaries of the Historic counties of England, historic county of Cheshire, it lies ...
's ''A Short History of Chemistry'', that:In coordination with these concepts, in 1833 the French chemist Marc Antoine Auguste Gaudin presented a clear account of Avogadro's hypothesis, 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 H2O: In 1917, an unknown American undergraduate chemical engineer named
Linus Pauling Linus Carl Pauling (; February 28, 1901August 19, 1994) was an American chemist, biochemist, chemical engineer, peace activist, author, and educator. He published more than 1,200 papers and books, of which about 850 dealt with scientific topi ...

Linus Pauling
was learning the Dalton hook-and-eye bonding method, which was the mainstream description of bonds between atoms at the time. Pauling, however, wasn't satisfied with this method and looked to the newly emerging field of quantum physics for a new method. In 1926, French physicist
Jean Perrin Jean Baptiste Perrin (30 September 1870 – 17 April 1942) was a French physicist who, in his studies of the Brownian motion Brownian motion, or pedesis (from grc, πήδησις "leaping"), is the random motion of particles suspended i ...
received the Nobel Prize in physics for proving, conclusively, the existence of molecules. He did this by calculating
Avogadro's number The Avogadro constant, commonly denoted or , is the proportionality factor that relates the number of constituent particles (usually molecule A molecule is a group of two or more atoms held together by attractive forces known as chem ...
using three different methods, all involving liquid phase systems. First, he used a
gamboge Gamboge ( , ) is a partially transparent deep Saffron (colour), saffron to mustard yellow pigment.Other forms and spellings are: cambodia, cambogium, camboge, cambugium, gambaugium, gambogia, gambozia, gamboidea, gambogium, gumbouge, gambouge, ga ...
soap-like emulsion, second by doing experimental work on
Brownian motion Brownian motion, or pedesis (from grc, πήδησις "leaping"), is the random motion of particles suspended in a medium (a liquid or a gas). This pattern of motion typically consists of Randomness, random fluctuations in a particle's po ...

Brownian motion
, and third by confirming Einstein's theory of particle rotation in the liquid phase. In 1927, the physicists
Fritz London Fritz Wolfgang London (March 7, 1900 – March 30, 1954) was a German physicist and professor at Duke University. His fundamental contributions to the theories of chemical bonding and of intermolecular forces (London dispersion forces) are today c ...
and
Walter Heitler Walter Heinrich Heitler (; 2 January 1904 – 15 November 1981) was a German physicist who made contributions to quantum electrodynamics and quantum field theory. He brought chemistry under quantum mechanics through his theory of Valence bond t ...
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, 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 Guggenheim Fellowships are grants that have been awarded annually since by the John Simon Guggenheim Memorial Foundation to those "who have demonstrated exceptional capacity for productive scholarship or exceptional creative ability in the a ...
. 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" in which he used
quantum mechanics Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including qua ...
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 CH4, in which four sp³ hybridised orbitals are overlapped by
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the chemical ...

hydrogen
's ''1s'' orbital, yielding four
sigma (σ) bonds
sigma (σ) bonds
. The four bonds are of the same length and strength, which yields a molecular structure as shown below:


Molecular science

The science of molecules is called ''molecular chemistry'' or ''
molecular physics Molecular physics is the study of the physical properties of molecules and molecular dynamics. The field overlaps significantly with physical chemistry, chemical physics, and quantum chemistry. It is often considered as a sub-field of atomic, mo ...
'', 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 Bound or bounds may refer to: Mathematics * Bound variable * Upper and lower bounds, observed limits of mathematical functions Physics * Bound state, a particle that has a tendency to remain localized in one or more regions of space Geography ...
) composed of two or more atoms.
Polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a complex (chemistry), metal complex, that can be considered to behave as a single unit and that has a net electrical charge, charge that is no ...
s may sometimes be usefully thought of as electrically charged molecules. The term ''unstable molecule'' is used for very reactive species, i.e., short-lived assemblies (
resonances Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied Periodic function, periodic force (or a Fourier analysis, Fourier component of it) is equal or close to a natural frequency of the system ...
) of electrons and nuclei, such as radicals, molecular
ion An ion () is an atom Every atom is composed of a atomic nucleus, nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has n ...
s, Rydberg molecules,
transition state In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, struc ...

transition state
s, van der Waals complexes, or systems of colliding atoms as in
Bose–Einstein condensate In condensed matter physics, a Bose–Einstein condensate (BEC) is a state of matter that is typically formed when a gas of bosons at very low Density, densities is cooled to temperatures very close to absolute zero#Relation with Bose–Einste ...
.


Prevalence

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,
bedrock In geology, bedrock is solid Rock (geology), rock that lies under loose material (regolith) within the crust (geology), crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface mater ...
, the molten interior, and the 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 covalent crystals, although these are often composed of repeating
unit cell In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...

unit cell
s that extend either in a
plane Plane(s) most often refers to: * Aero- or airplane An airplane or aeroplane (informally plane) is a fixed-wing aircraft that is propelled forward by thrust from a jet engine, Propeller (aircraft), propeller, or rocket engine. Airplanes co ...
, e.g.
graphene Graphene () is an allotrope of carbon consisting of a Single-layer materials, single layer of atoms arranged in a hexagonal lattice nanostructure.
graphene
; or three-dimensionally e.g.
diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystal, crystalline material ...

diamond
,
quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra ...

quartz
,
sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salt (chemistry), salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of ...
. The theme of repeated unit-cellular-structure also holds for most metals which are condensed phases with
metallic bond Metallic bonding is a type of chemical bonding that arises from the electrostatic attractive force between conduction electrons (in the form of an electron cloud of delocalized electrons) and positively charged metal ions. It may be describ ...
ing. Thus solid metals are not made of molecules. In
glass Glass is a non-Crystallinity, crystalline, often transparency and translucency, transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most ...

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 A covalent bond is a chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of Molecule, molecules and crystals. The bond may result from the Coulomb's law, electrostatic force between oppos ...
. 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 Metallic bonding is a type of chemical bonding that arises from the electrostatic attractive force between conduction electrons (in the form of an electron cloud of delocalized electrons) and positively charged metal ions. It may be describ ...

metallic bonding
, others point out that metals behave very differently than molecules.


Covalent

A covalent bond is a chemical bond that involves the sharing of
electron pair In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, struct ...
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''.


Ionic

Ionic bonding is a type of chemical bond that involves the
electrostatic Electrostatics is a branch of physics that studies electric charges at rest ( static electricity). Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word f ...
attraction between oppositely charged ions, and is the primary interaction occurring in
ionic compound In chemistry, an ionic compound is a chemical compound composed of ions held together by Coulomb's law, electrostatic forces termed ionic bonding. The compound is neutral overall, but consists of positively charged ions called cations and negativ ...
s. The ions are atoms that have lost one or more
electron The electron ( or ) is a subatomic particle with a negative one elementary charge, elementary electric charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are generally thought t ...

electron
s (termed
cation An ion () is an atom or molecule with a net electric charge, electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be po ...
s) and atoms that have gained one or more electrons (termed
anion An ion () is an atom or molecule with a net electric charge, electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be po ...
s). This transfer of electrons is termed ''electrovalence'' in contrast to covalence. In the simplest case, the cation is a
metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...

metal
atom and the anion is a
nonmetal In chemistry, a nonmetal is a chemical element that generally lacks a predominance of metallic properties; they range from colorless gases (like hydrogen) to shiny solids (like carbon, as graphite). The electrons in nonmetals behave differentl ...
atom, but these ions can be of a more complicated nature, e.g. molecular ions like NH4+ or SO42−. 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.


Molecular size

Most molecules are far too small to be seen with the naked eye, although molecules of many
polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many Repeat unit, repeating subunits. Due to t ...

polymer
s can reach
macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical instruments. It is the opposite of microscopic scale, microscopic. Overview When applied to ph ...
sizes, including
biopolymer Biopolymers are natural polymers produced by the cells of Organism, living organisms. Like other polymers, biopolymers consist of monomeric units that are Covalent bond, covalently bonded in chains to form larger molecules. There are three main c ...
s such as
DNA
DNA
. Molecules commonly used as building blocks for organic synthesis have a dimension of a few
angstrom The angstromEntry "angstrom" in the Oxford online dictionary. Retrieved on 2019-03-02 from https://en.oxforddictionaries.com/definition/angstrom.Entry "angstrom" in the Merriam-Webster online dictionary. Retrieved on 2019-03-02 from https://www.m ...

angstrom
s (Å) to several dozen Å, or around one billionth of a meter. Single molecules cannot usually be observed by
light Light or visible light is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequency, fr ...

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 Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very-high-resolution type of scanning probe microscopy Scan may refer to: Acronyms * Schedules for Clinical Assessment in Neuropsychiatry (SCAN), a psychiatric diagnostic ...
. Some of the largest molecules are
macromolecule A macromolecule is a very large molecule important to biophysical processes, such as a protein or nucleic acid. It is composed of thousands of covalent bond, covalently bonded atoms. Many macromolecules are polymers of smaller molecules called ...
s or supermolecules. The smallest molecule is the
diatomic Diatomic molecules () are molecules composed of only two atoms, of the same or different chemical elements. If a diatomic molecule consists of two atoms of the same element, such as hydrogen () or oxygen (), then it is said to be homonuclear mol ...
hydrogen (H2), with a bond length of 0.74 Å. Effective molecular radius is the size a molecule displays in solution. The table of permselectivity for different substances contains examples.


Molecular formulas


Chemical formula types

The
chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
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 In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, struc ...
is a very simple type of chemical formula. It is the simplest
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign (−1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language of ...
ratio In mathematics, a ratio shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...

ratio
of the chemical elements that constitute it. For example, water is always composed of a 2:1 ratio of hydrogen to oxygen atoms, and
ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...

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 –
dimethyl ether Dimethyl ether (DME; also known as methoxymethane) is the organic compound with the formula CH3OCH3, (sometimes ambiguously simplified to C2H6O as it is an isomer of ethanol). The simplest ether, it is a colorless gas that is a useful precursor ...
has the same ratios as ethanol, for instance. Molecules with the same
atom Every atom is composed of a atomic nucleus, nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, l ...
s in different arrangements are called
isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. Isomerism is existence or possibil ...

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 In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, str ...
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 Acetylene (Chemical nomenclature, systematic name: ethyne) is the chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is ...

acetylene
has molecular formula C2H2, but the simplest integer ratio of elements is CH. The
molecular mass The molecular mass (''m'') is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotope Isotopes are two or more ty ...
can be calculated from the chemical formula and is expressed in conventional
atomic mass unit The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a body. It was traditionally believed to be related to the physical quantity, quantity ...
s equal to 1/12 of the mass of a neutral carbon-12 (12
C
C
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbe ...
) atom. For network solids, the term
formula unit In chemistry, a formula unit is the empirical formula of any Ionic compound, ionic or covalent network solid compound used as an independent entity for stoichiometric calculations. It is the lowest whole number ratio of ions represented in an ioni ...
is used in
stoichiometric Stoichiometry refers to the relationship between the quantities of reactants and Product (chemistry), products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of ...
calculations.


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 The structural formula of a chemical compound is a graphic representation of the molecular structure (determined by structural chemistry methods), showing how the atoms are possibly arranged in the real three-dimensional space. The chemical bondi ...
may be needed. Structural formulas may in turn be represented with a one-dimensional chemical name, but such
chemical nomenclature A chemical nomenclature is a set of rules to generate systematic names for chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) containing atoms from mor ...
requires many words and terms which are not part of chemical formulas.


Molecular geometry

Molecules have fixed 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.
Isomers In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, st ...

Isomers
share a chemical formula but normally have very different properties because of their different structures.
Stereoisomer In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...
s, a particular type of isomer, may have very similar physico-chemical properties and at the same time different
biochemical Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...

biochemical
activities.


Molecular spectroscopy

Molecular spectroscopy deals with the response (
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a Continuum (measurement), continuum. The word was first used scientifically in optics to describ ...
) of molecules interacting with probing signals of known
energy In physics, energy (from Ancient Greek: wikt:ἐνέργεια#Ancient_Greek, ἐνέργεια, ''enérgeia'', “activity”) is the physical quantity, quantitative physical property, property that is #Energy transfer, transferred to a phy ...

energy
(or
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in Hertz (unit), hertz (H ...

frequency
, according to
Planck's formula
Planck's formula
). Molecules have quantized energy levels that can be analyzed by detecting the molecule's energy exchange through
absorbance Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative lo ...
or emission. Spectroscopy does not generally refer to
diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of Umbra, penumbra and antumbra, geometrical shadow of the obstacle/aperture. The diffracting object or ape ...

diffraction
studies where particles such as
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the atomic nucleus, nuclei of atoms. Since protons and ...

neutron
s, electrons, or high energy
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 Picometre, picometers to 10 Nanometre, nanometers, corresponding to frequency, ...

X-ray
s interact with a regular arrangement of molecules (as in a crystal).
Microwave spectroscopy Microwave spectroscopy is the spectroscopy method that employs microwaves, i.e. electromagnetic radiation at GHz frequencies, for the study of matter. History The ammonia molecule NH3 is shaped like a pyramid 0.38 Å in height, with an equilatera ...
commonly measures changes in the rotation of molecules, and can be used to identify molecules in outer space.
Infrared spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption spectroscopy, absorption, emission spectrum, emission, or reflection (physics), reflection. ...

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 In organic chemistry, a functional group is a substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions re ...
s in molecules. Changes in the arrangements of electrons yield absorption or emission lines in ultraviolet, visible or
near infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry mom ...
light, and result in colour. 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 Theoretical chemistry is the branch of chemistry which develops Scientific theory, theoretical generalizations that are part of the theoretical arsenal of modern chemistry: for example, the concepts of chemical bonding, chemical reaction, Valenc ...
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, H2+, and the simplest of all the chemical bonds is the one-electron bond. H2+ is composed of two positively charged protons and one negatively charged
electron The electron ( or ) is a subatomic particle with a negative one elementary charge, elementary electric charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are generally thought t ...

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". 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, He2, which has one vibrational bound state 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.


See also

* 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


References


External links


Molecule of the MonthSchool of Chemistry, University of Bristol
{{Authority control Molecular physics, Molecules, Chemistry Matter