Difference between revisions of "Molecule" - New World Encyclopedia

Revision as of 02:12, 16 May 2006

In chemistry, a molecule is an aggregate of at least two atoms in a definite arrangement held together by special forces.^[1] Generally, a molecule is considered the smallest particle of a pure chemical substance that still retains its composition and chemical properties.^[2] In the molecular sciences, a molecule is a sufficiently stable, electrically neutral entity composed of two or more atoms.^[3] The concept of "monatomic molecule", i.e. a single-atom as found in noble gases, is used almost exclusively in the kinetic theory of gases.^[4] Polyatomic ions may sometimes be usefully thought of as electrically-charged molecules.

3D (left and center) and 2D (right) representations of the terpenoid molecule atisane.

History

Although the concept of molecules was first introduced in 1811 by Avogadro, and was accepted by many chemists as a result of Dalton's laws of Definite and Multiple Proportions (1803-1808), with notable exceptions (Boltzmann, Maxwell, Gibbs), the existence of molecules as anything other than convenient mathematical constructs was still an open debate in the physics community until the work of Perrin (1911), and was strenuously resisted by early positivists such as Mach. The modern theory of molecules makes great use of the many numerical techniques offered by computational chemistry. Dozens of molecules have now been identified in interstellar space by microwave spectroscopy.

Overview

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. 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. A peculiar use of the term molecular is as a synonym to covalent, which arises from the fact that, unlike molecular covalent compounds, ionic compounds do not yield well-defined smallest particles that would be consistent with the definition above. No typical "smallest particle" can be defined for covalent crystals, or network solids, which are composed of repeating unit cells that extend indefinitely either in a plane (such as in graphite) or three-dimensionally (such as in diamond).

Chemical bond

See main article chemical bond

In a molecule, the atoms are joined by shared pairs of electrons in a chemical bond. It may consist of atoms of the same chemical element, as with oxygen (O₂), or of different elements, as with water (H₂O).

Size

Most molecules are much too small to be seen with the naked eye, but there are exceptions. DNA, a macromolecule, can reach macroscopic sizes. The smallest molecule is the hydrogen molecule. The interatomic distance is 0.15 nanometres (1.5 Å). But the size of its electron cloud is difficult to define precisely. Under standard conditions molecules have a dimension of a few to several dozen Å.

Empirical formula

See main article empirical 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.

Chemical formula

See main article chemical formula

The chemical formula reflects the exact number of atoms that compose a molecule.

Molecular mass

See main article molecular mass

The molecular mass can be calculated from the chemical formula and is expressed in conventional units equal to 1/12 from the mass of a ¹²C isotope atom. For network solids, the term formula unit is used in stoichiometric calculations.

Molecular geometry

See main article 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

Main article: 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 spectroscopy 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.

References
ISBN links support NWE through referral fees

↑ Chang, Raymond (1998). Chemistry, 6th Ed.. New York: McGraw Hill. ISBN 0071152210.
↑ Molecule Definition
↑ IUPAC Defintion of Molecule
↑ [1] [2] [3]

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[1] Chang, Raymond (1998). Chemistry, 6th Ed.. New York: McGraw Hill. ISBN 0071152210.

[2] Molecule Definition

[3] IUPAC Defintion of Molecule

[4] [1] [2] [3]

[1]

[2]

[3]

[4]

@@ Line 1: / Line 1: @@
-A '''molecule''' is the smallest [[list of particles#Molecules|particle]] of a pure [[chemical substance]] that still retains its [[chemical compound|chemical composition]] and properties.  The science of molecules is called ''molecular [[chemistry]]'' or ''[[molecular physics]]'', depending on the particular aspect of focus. Molecular chemistry is concerned with the laws governing the interaction between molecules that results in the formation and breakage of [[chemical bond]]s, while molecular physics is concerned with the laws governing their structure and properties. In practice, however, this distinction is rather vague.
+In [[chemistry]], a '''molecule''' is an aggregate of at least two atoms in a definite arrangement held together by special forces.<ref>{{cite book|author=Chang, Raymond |title=Chemistry, 6th Ed.|location=New York | publisher=McGraw Hill|year=1998|id=ISBN 0071152210}}</ref> Generally, a molecule is considered the smallest [[list of particles#Molecules|particle]] of a pure [[chemical substance]] that still retains its [[chemical compound|composition]] and chemical properties.<ref>[http://antoine.frostburg.edu/chem/senese/101/glossary/m.shtml#molecule Molecule Definition]</ref> In the [[molecular science]]s, a molecule is a sufficiently stable, [[electric charge|electrically]] neutral [[entity]] composed of two or more [[atom]]s.<ref>[http://www.iupac.org/goldbook/M04002.pdf IUPAC Defintion of Molecule]</ref> The concept of "[[monatomic|monatomic molecule]]", i.e. a single-atom as found in [[noble gas]]es, is used almost exclusively in the [[kinetic theory]] of gases.<ref>[http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Thermal/kinTheoryGas.html] [http://www.usd.edu/phys/courses/phys_111sf/ch_10/10_notes.htm] [http://www.google.com/search?q=monatomic-molecule+kinetic-theory+%2B%22.edu%22]</ref> [[Polyatomic ion]]s may sometimes be usefully thought of as electrically-charged molecules.
-According to the strict definition, molecules can consist of one [[atom]] (as in [[noble gas]]es) or more atoms bonded together. The concept of ''[[monatomic]] (single-atom)'' molecule is used almost exclusively in the [[kinetic theory]] of gases.  In molecular sciences a molecule consists of a stable system ([[bound state]]) comprising two or more [[atom]]s.  The term ''unstable molecule'' is used for very [[reactivity|reactive]] species, i.e. short-lived assemblies ([[resonance]]s) of [[electron]]s and [[atomic nucleus | nuclei]], such as [[radical (chemistry) | radicals]], molecular [[ion]]s, [[Rydberg molecule]]s (see [[Rydberg atom]]s), [[transition state]]s, [[Van der Waals bonding|Van der Waals complex]]es, or systems of colliding atoms as in [[Bose-Einstein condensate]]s.  A peculiar use of the term ''molecular'' is as a synonym to ''[[covalent bond|covalent]]'', which arises from the fact that, unlike covalent compounds, [[ionic compound]]s do not yield well-defined ''smallest particles'' that would be consistent with the definition above.
+[[Image:Atisane3.png|600px|thumb|right|[[3D geometric model|3D]] (left and center) and [[2D geometric model|2D]] (right) representations of the [[terpenoid]] molecule [[atisane]].]]
-Although the concept of molecule was first introduced in [[1811]] by [[Amadeo Avogadro|Avogadro]], the existence of molecules was still an open debate in the chemistry community till the work of [[Jean Perrin | Perrin]] ([[1911]]). The modern theory of molecules makes great use of the many numerical techniques offered by [[computational chemistry]].
+== History ==
+Although the concept of molecules was first introduced in [[1811]] by [[Amadeo Avogadro|Avogadro]], and was accepted by many chemists as a result of [[John Dalton|Dalton's]] laws of Definite and Multiple Proportions (1803-1808), with notable exceptions ([[Ludwig Boltzmann|Boltzmann]], [[James Clerk Maxwell|Maxwell]], [[Willard Gibbs|Gibbs]]), the existence of molecules as anything other than convenient mathematical constructs was still an open debate in the physics community until the work of [[Jean Perrin|Perrin]] ([[1911]]), and was strenuously resisted by early [[logical positivism|positivists]] such as [[Ernst Mach|Mach]]. The modern theory of molecules makes great use of the many numerical techniques offered by [[computational chemistry]]. Dozens of molecules have now been identified in [[interstellar medium|interstellar space]] by [[rotational spectroscopy|microwave spectroscopy]].
-For a list of molecules see the [[List of compounds]].
+== Overview ==
+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 bond]]s, 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 [[atom]]s. The term ''unstable molecule'' is used for very [[reactivity|reactive]] species, i.e., short-lived assemblies ([[resonance]]s) of [[electron]]s and [[atomic nucleus|nuclei]], such as [[radical (chemistry)|radicals]], molecular [[ion]]s, [[Rydberg molecule]]s, [[transition state]]s, [[Van der Waals bonding|Van der Waals complex]]es, or systems of colliding atoms as in [[Bose-Einstein condensate]]s. A peculiar use of the term ''molecular'' is as a synonym to ''[[covalent bond|covalent]]'', which arises from the fact that, unlike molecular covalent compounds, [[ionic compound]]s do not yield well-defined ''smallest particles'' that would be consistent with the definition above. No typical "smallest particle" can be defined for covalent [[crystal]]s, or [[network solids]], which are composed of repeating [[unit cell]]s that extend indefinitely either in a [[Plane (mathematics)|plane]] (such as in [[graphite]]) or three-dimensionally (such as in [[diamond]]).
-<center>[[Image:Atisane3.png|center|framed|''Figure 1.'' [[3D geometric model|3D]] (left and center) and [[2D geometric model|2D]] (right) representations of the [[terpenoid]], [[atisane]]. In the 3D model on the left, [[carbon]] atoms are represented by gray spheres, white spheres represent the [[hydrogen]] atoms and the cylinders represent the bonds.  The model is enveloped in a "mesh" representation of the molecular surface, colored by areas of positive (red) and negative (blue) [[electric charge]].  In the 3D model (center), the light-blue spheres represent carbon atoms, the white spheres are hydrogen atoms, and the cylinders in between the atoms correspond to single-bonds.]]</center>
 == Chemical bond ==
+:''See main article [[chemical bond]]''
-:''see main article [[chemical bond]]''
+In a molecule, the atoms are joined by shared pairs of [[electron]]s in a '''chemical bond'''. It may consist of atoms of the same [[chemical element]], as with [[oxygen]] (O<sub>2</sub>), or of different elements, as with [[water (molecule)|water]] (H<sub>2</sub>O).
-In a molecule the atoms are joined by shared pairs of [[electron]]s in a '''chemical bond'''. It may consist of atoms of the same [[chemical element]], as with [[oxygen]] (O<sub>2</sub>), or of different elements, as with [[water (molecule)|water]] (H<sub>2</sub>O).
+== Size ==
+Most molecules are much too small to be seen with the naked eye, but there are exceptions. [[DNA]], a [[macromolecule]], can reach [[macroscopic]] sizes.  The smallest molecule is the [[hydrogen]] molecule. The interatomic distance is 0.15 [[nanometre]]s (1.5 [[Angstrom|Å]]). But the size of its [[electron cloud]] is difficult to define precisely. Under [[standard conditions]] molecules have a dimension of a few to several dozen Å.
-== Size ==
+== Empirical formula ==
+:''See main article [[empirical formula]]''
-Most molecules are much too small to be seen with the naked eye, but there are exceptions. [[DNA]], a [[macromolecule]], can reach [[macroscopic]] sizes.
+The '''empirical formula''' of a molecule is the '''simplest''' [[integer]] [[ratio]] of the [[chemical element]]s 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 [[atom]]s in different arrangements are called [[isomer]]s. 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 smallest molecule is the [[hydrogen]] molecule.  The interatomic distance is 1.5 [[Angstrom|Å]].  But the size of its [[electron cloud]] is difficult to define precisely.  Under [[normal condition]]s molecules have a dimension of a few to a few dozen Å.
+== Chemical formula ==
+:''See main article [[chemical formula]]''
-== Empirical formula ==
+The '''chemical formula''' reflects the exact number of atoms that compose a molecule.
-:''see main article [[empirical formula]]''
+== Molecular mass ==
+:''See main article [[molecular mass]]''
-The '''empirical formula''' of a molecule is the [[integer]] [[ratio]] of the [[element]]s 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 [[atom]]s in different arrangements are called [[isomer]]s.
+The '''[[molecular mass]]''' can be calculated from the chemical formula and is expressed in conventional units equal to 1/12 from the mass of a <sup>12</sup>[[carbon|C]] [[isotope]] atom. For [[network solids]], the term [[formula unit]] is used in [[stoichiometric]] calculations.
-== Chemical formula ==
+== Molecular geometry ==
+:''See main article [[molecular geometry]]''
-:''see main article [[chemical formula]]''
+Molecules have fixed [[equilibrium]] geometries&mdash;bond lengths and angles&mdash; 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. [[Stereoisomer]]s, a particular type of isomers, may have very similar physico-chemical properties and at the same time very different [[biochemistry|biochemical]] activities.
-The '''chemical 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 units equal to 1/12 from the mass of a <sup>12</sup>[[carbon|C]] [[isotope]] atom.
+== Molecular spectroscopy ==
+{{main|Spectroscopy}}
-== Molecular geometry ==
+'''Molecular spectroscopy''' deals with the response ([[frequency spectrum|spectrum]]) of molecules interacting with probing signals of known [[energy]] (or [[frequency]], according to [[Planck's constant|Planck's formula]]). [[Scattering theory]] provides the theoretical background for spectroscopy.
+The probing signal used in spectroscopy can be an [[electromagnetic wave]] or a beam of [[Elementary particle|particle]]s ([[electron]]s, [[positron]]s, etc.) The molecular response can consist of signal absorption ([[absorption spectroscopy]]), the emission of another signal ([[emission spectroscopy]]), fragmentation, or chemical changes.
-:''see main article [[molecular geometry]]''
+Spectroscopy is recognized as a powerful tool in investigating the [[microscopic]] properties of molecules, in particular their [[energy level]]s. In order to extract maximum microscopic information from experimental results, spectroscopy is often coupled with [[computational chemistry|chemical computations]].
-Molecules have fixed equilibrium geometries&mdash;bond lengths and angles&mdash;. A pure substance is composed of molecules with the same 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. [[Stereoisomer]]s, a particular type of isomers, may have very similar physico-chemical properties and at the same time very different [[biochemistry|biochemical]] activities.
+==References==
+<div class="references-small">
+<references />
+</div>
-== Molecular spectroscopy ==
+== See also ==
+* [[Covalent bond]]
+* [[Diatomic molecule]]
+* [[Molecular geometry]]
+* [[Molecular orbital]]
+* [[Nonpolar molecule]]
+* [[Polar molecule]]
-:''see main article [[spectroscopy]]''
+=== Related lists ===
+* For a list of molecules see the [[List of compounds]]
+* [[List of molecules in interstellar space]]
-'''Molecular spectroscopy''' is the study of the response ([[frequency spectrum |spectrum]]) of a molecule to a signal characterized by its tunable [[frequency]] (or, according to [[Planck's constant | Planck formula]], its energy).  This signal is usually an [[electromagnetic wave]] or a beam of [[electron]]s, but new molecular spectroscopies are under development like the [[positron]] spectroscopy.  The molecular response can be the absorption of the signal ([[absorption spectroscopy]]), the emission of another signal ([[emission spectroscopy]]), its fragmentation or a change of its chemical nature.
+{{composite}}
-Spectroscopy is recognized as the most powerful tool in the investigation of the microscopic properties of molecules, and, in particular, their [[energy level]]s.  Nowadays, in order to extract the maximum microscopic information from the experimental results, spectroscopical studies are very often coupled with [[computational chemistry | computational chemical]] investigations.  The theoretical background of spectroscopy is the [[scattering theory]].
+[[Category:Matter]]
+[[Category:Molecular physics| ]]
-== Related articles ==
+<!-- interwiki —>
-* [[covalent bond]]
+[[af:Molekule]]
-* [[diatomic]]
+[[als:Molekül]]
-* [[list of molecules in interstellar space]]
+[[ar:جزيء]]
-* [[molecular geometry]]
+[[bg:Молекула]]
-* [[molecular orbital]]
+[[ca:Molècula]]
-* [[nonpolar|nonpolar molecules]]
+[[cs:Molekula]]
-* [[polar molecules]]
+[[da:Molekyle]]
+[[de:Molekül]]
+[[et:Molekul]]
+[[el:Μόριο]]
+[[es:Molécula]]
+[[eo:Molekulo]]
+[[fo:Mýl]]
+[[fr:Molécule]]
+[[gl:Molécula]]
+[[ko:분자]]
+[[hr:Molekula]]
+[[io:Molekulo]]
+[[id:Molekul]]
+[[is:Sameind]]
+[[it:Molecola]]
+[[he:מולקולה]]
+[[lv:Molekula]]
+[[lt:Molekulė]]
+[[hu:Molekula]]
+[[mk:Молекула]]
+[[nl:Molecuul]]
+[[ja:分子]]
+[[no:Molekyl]]
+[[nn:Molekyl]]
+[[nds:Molekül]]
+[[pl:Cząsteczka]]
+[[pt:Molécula]]
+[[ru:Молекула]]
+[[simple:Molecule]]
+[[sk:Molekula]]
+[[sl:Molekula]]
+[[sr:Молекул]]
+[[sh:Molekula]]
+[[su:Molekul]]
+[[fi:Molekyyli]]
+[[sv:Molekyl]]
+[[tl:Molekula]]
+[[th:โมเลกุล]]
+[[vi:Phân tử]]
+[[tr:Molekül]]
+[[uk:Молекула]]
+[[zh:分子]]
 [[Category:Chemistry]]
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-<!-- interwiki —>
+{{credit|52473716}}
-{{credit|23444932}}
 [[category:Physical sciences]]