AskDefine | Define molecule

Dictionary Definition



1 (physics and chemistry) the simplest structural unit of an element or compound
2 (nontechnical usage) a tiny piece of anything [syn: atom, particle, corpuscle, mote, speck]

User Contributed Dictionary



French molécule (1674, Pierre Le Gallois, Conversations tirées de l'Académie de M. l'abbé Bourdelot, contenant diverses recherches et observations physiques) cited in Quemada, Brenard (1965), Datations et documents lexicographiques (tome 3).
Medieval Latin molecula (early XVI cent., Pierre Gassendi), cited in Le Grand Robert de la Langue Française (2e édn) tome 6. ISBN 2-85036-094-5. pp. 522–23. Diminutive of moles


  1. in chemistry, the smallest particle of a specific element or compound that retains the chemical properties of that element or compound; a group of atoms held together by chemical bonds
  2. a tiny amount


Extensive Definition

In chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds . It can also be defined as a unit of two or more atoms held together by covalent bonds. In organic chemistry and biochemistry, the term molecule is used less strictly and also is applied to charged organic molecules and biomolecules. Molecules are distinguished from polyatomic ions in this strict sense.
This definition has evolved as knowledge of the structure of molecules has increased. Earlier definitions were less precise defining molecules as the smallest particles of pure chemical substances that still retain their composition and chemical properties. This definition often breaks down since many substances in ordinary experience, such as rocks, salts, and metals, are composed of atoms or ions, but are not made of molecules.
In the kinetic theory of gases the term molecule is often used for any gaseous particle regardless of their composition. According to this definition noble gases would also be considered molecules despite the fact that they are composed of a single non-bonded atom.


The term "molecule", from the French molécule meaning "extremely minute particle," was coined by French philosopher Rene Descartes in the 1620s. Although the existence of molecules was accepted by many chemists since the early 19th century as a result of Dalton's laws of Definite and Multiple Proportions (1803-1808) and Avogadro's law (1811), there was some resistance among positivists and physicists such as Mach, Boltzmann, Maxwell, and Gibbs, who saw molecules merely as convenient mathematical constructs. The work of Perrin on Brownian motion (1911) is considered to be the final proof of the existence of molecules.
In a molecule, at least two atoms are joined by shared pairs of electrons in a covalent bond. It may consist of atoms of the same chemical element, as with oxygen (O2), or of different elements, as with water (H2O). Atoms and complexes connected by non-covalent bonds such as hydrogen bonds or ionic bonds are generally not considered single molecules.
No typical molecule can be defined for ionic (salts) and covalent crystals (network solids) which are composed of repeating unit cells that extend either in a plane (such as in graphite) or three-dimensionally (such as in diamond or sodium chloride).
The science of molecules is called molecular chemistry or molecular physics, depending on the focus. 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) comprising two or more atoms. Polyatomic ions 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) of electrons and nuclei, such as radicals, molecular ions, Rydberg molecules, transition states, van der Waals complexes, or systems of colliding atoms as in Bose-Einstein condensates.

Molecular size

Most molecules are far too small to be seen with the opened eye, but there are exceptions. DNA, a macromolecule, can reach macroscopic sizes, as can molecules of many polymers. The smallest molecule is the diatomic hydrogen (H2), with an overall length of roughly twice the 74 picometres (0.74 Å) bond length. Molecules commonly used as building blocks for organic synthesis have a dimension of a few Å to several dozen Å. 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 macromolecules or supermolecules.


Effective molecular radius is the size a molecule displays in solution. . The table of permselectivity for different substances contains examples.

Molecular formula

The empirical formula of a molecule is the simplest integer ratio of the chemical elements that constitute the compound. For example, in their pure forms, water is always composed of a 2:1 ratio of hydrogen to oxygen, and ethyl alcohol or ethanol 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 has the same ratio as ethanol, for instance. Molecules with the same atoms in different arrangements are called isomers. The empirical formula is often the same as the molecular formula but not always. For example the molecule acetylene has molecular formula C2H2, but the simplest integer ratio of elements is CH. The molecular formula reflects the exact number of atoms that compose a molecule.
The molecular mass can be calculated from the chemical formula and is expressed in conventional atomic mass units equal to 1/12th of the mass of a neutral carbon-12 (12C isotope) atom. For network solids, the term formula unit is used in stoichiometric calculations.

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 share a chemical formula but normally have very different properties because of their different structures. Stereoisomers, a particular type of isomers, may have very similar physico-chemical properties and at the same time very different biochemical activities.

Molecular spectroscopy

Molecular spectroscopy deals with the response (spectrum) of molecules interacting with probing signals of known energy (or frequency, according to Planck's formula). Scattering theory provides the theoretical background for spectroscopy.
The probing signal used in spectoore can be an electromagnetic wave or a beam of particles (electrons, positrons, etc.) The molecular response can consist of signal absorption (absorption spectroscopy), the emission of another signal (emission spectroscopy), fragmentation, or chemical changes.
Spectroscopy is recognized as a powerful tool in investigating the microscopic properties of molecules, in particular their energy levels. In order to extract maximum microscopic information from experimental results, spectroscopy is often coupled with chemical computations.

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, 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 bound by photon exchange, 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".


External links

molecule in Afrikaans: Molekuul
molecule in Tosk Albanian: Molekül
molecule in Arabic: جزيء
molecule in Aragonese: Molecula
molecule in Asturian: Molécula
molecule in Azerbaijani: Molekul
molecule in Bengali: অণু
molecule in Min Nan: Hun-chú
molecule in Belarusian: Малекула
molecule in Bosnian: Molekula
molecule in Bulgarian: Молекула
molecule in Catalan: Molècula
molecule in Czech: Molekula
molecule in Welsh: Moleciwl
molecule in Danish: Molekyle
molecule in German: Molekül
molecule in Lower Sorbian: Molekul
molecule in Estonian: Molekul
molecule in Modern Greek (1453-): Μόριο
molecule in Spanish: Molécula
molecule in Esperanto: Molekulo
molecule in Basque: Molekula
molecule in Persian: مولکول
molecule in Faroese: Mýl
molecule in French: Molécule
molecule in Irish: Móilín
molecule in Galician: Molécula
molecule in Korean: 분자
molecule in Upper Sorbian: Molekul
molecule in Croatian: Molekula
molecule in Ido: Molekulo
molecule in Indonesian: Molekul
molecule in Icelandic: Sameind
molecule in Italian: Molecola
molecule in Hebrew: מולקולה
molecule in Kannada: ಮಹತ್ಕಣ
molecule in Georgian: მოლეკულა
molecule in Kurdish: Molekul
molecule in Latin: Molecula
molecule in Latvian: Molekula
molecule in Lithuanian: Molekulė
molecule in Lombard: Mulécula
molecule in Hungarian: Molekula
molecule in Macedonian: Молекула
molecule in Malayalam: തന്മാത്ര (രസതന്ത്രം)
molecule in Marathi: रेणू
molecule in Malay (macrolanguage): Molekul
molecule in Dutch: Molecuul
molecule in Japanese: 分子
molecule in Norwegian: Molekyl
molecule in Norwegian Nynorsk: Molekyl
molecule in Narom: Molétchule
molecule in Novial: Molekule
molecule in Occitan (post 1500): Molecula
molecule in Uzbek: Molekula
molecule in Low German: Molekül
molecule in Polish: Cząsteczka
molecule in Portuguese: Molécula
molecule in Romanian: Moleculă
molecule in Quechua: Iñuwa
molecule in Russian: Молекула
molecule in Albanian: Molekula
molecule in Simple English: Molecule
molecule in Slovak: Molekula
molecule in Slovenian: Molekula
molecule in Serbian: Молекул
molecule in Serbo-Croatian: Molekula
molecule in Sundanese: Molekul
molecule in Finnish: Molekyyli
molecule in Swedish: Molekyl
molecule in Tagalog: Molekula
molecule in Tamil: மூலக்கூறு
molecule in Thai: โมเลกุล
molecule in Vietnamese: Phân tử
molecule in Turkish: Molekül
molecule in Ukrainian: Молекула
molecule in Urdu: سالمہ
molecule in Yiddish: מאלעקול
molecule in Contenese: 分子
molecule in Chinese: 分子

Synonyms, Antonyms and Related Words

Kekule formula, ace, acid, acidity, agent, air, alkali, alkalinity, alloisomer, anion, antacid, atom, atomic cluster, atomic particles, base, benzene ring, biochemical, bit, branched chain, brute matter, building block, cation, chain, chemical, chemical element, chromoisomer, closed chain, component, compound, compound radical, constituent, copolymer, cycle, dab, dimer, dole, dot, dram, dribble, driblet, dwarf, earth, electron, element, elementary particle, elementary unit, farthing, fire, fleck, flyspeck, fragment, fundamental particle, gobbet, grain, granule, groat, hair, handful, heavy chemicals, heterocycle, high polymer, homocycle, homopolymer, hydracid, hyle, hypostasis, inorganic chemical, ion, iota, isomer, jot, lattice, little, little bit, lota, macromolecule, material, material world, materiality, matter, meson, metamer, minim, minimum, minutiae, mite, modicum, monad, monomer, mote, natural world, nature, neutralizer, nonacid, nuclear particle, nutshell, organic chemical, ounce, oxyacid, particle, pebble, physical world, pinch, pittance, plenum, point, polymer, proton, pseudoisomer, quark, radical, ray, reagent, ring, scruple, side chain, simple radical, smidgen, smitch, space-lattice, speck, spoonful, spot, straight chain, stuff, substance, substratum, sulfacid, the four elements, thimbleful, tiny bit, tittle, trifling amount, trimer, trivia, unit of being, water, whit
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