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In [[chemistry]], a '''molecule''' is defined as a sufficiently stable [[electric charge|electrically]] neutral group of at least two [[atom]]s in a definite arrangement held together by strong [[chemical bond]]s.[http://goldbook.iupac.org/M04002.html]Pauling, Linus General Chemistry, Dover Publications, Inc.ISBN 0-486-65622-General Chemistry, 3th Ed., Houghton Mifflin Co.ISBN 0-395-43302-9, Brown, T.L., Chemistry – the Central Science, 9th Ed.Prentice Hall, ISBN 0-13-066997-0, Chang, Raymond, Chemistry, 6th Ed., McGraw Hill, ISBN 0-07-115221-0, Zumdahl, Steven S., Chemistry, 4th ed., Houghton Mifflin|year, ISBN 0-669-41794-7.  In [[organic chemistry]] and [[biochemistry]], the term ''molecule'' is used less strictly and also is applied to charged [[organic compound|organic molecules]] and [[biomolecule]]s.

In [[chemistry]], a '''molecule''' is defined as a sufficiently stable [[electric charge|electrically]] neutral group of at least two [[atom]]s in a definite arrangement held together by strong [[chemical bond]]s.[https://goldbook.iupac.org/M04002.html]Pauling, Linus General Chemistry, Dover Publications, Inc.ISBN 0-486-65622-General Chemistry, 3th Ed., Houghton Mifflin Co.ISBN 0-395-43302-9, Brown, T.L., Chemistry – the Central Science, 9th Ed.Prentice Hall, ISBN 0-13-066997-0, Chang, Raymond, Chemistry, 6th Ed., McGraw Hill, ISBN 0-07-115221-0, Zumdahl, Steven S., Chemistry, 4th ed., Houghton Mifflin|year, ISBN 0-669-41794-7.  In [[organic chemistry]] and [[biochemistry]], the term ''molecule'' is used less strictly and also is applied to charged [[organic compound|organic molecules]] and [[biomolecule]]s.

Molecules are distinguished from [[polyatomic ion]]s in the strict sense.

Molecules are distinguished from [[polyatomic ion]]s in the 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 [[list of particles#Molecules|particles]] of pure [[chemical substance]]s that still retain their [[chemical compound|composition]] and chemical properties. [http://antoine.frostburg.edu/chem/senese/101/glossary/m.shtml#molecule Molecule Definition]  (Frostburg State University). This definition often breaks down since many substances in ordinary experience, such as [[rock (geology)|rock]]s, [[salt]]s, and [[metal]]s, are composed of atoms or [[ion]]s, but are not made of molecules.

This definition 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. [https://antoine.frostburg.edu/chem/senese/101/glossary/m.shtml#molecule Molecule Definition]  (Frostburg State University). This definition often breaks down since many substances in ordinary experience, such as [[rock (geology)|rock]]s, [[salt]]s, and [[metal]]s, are composed of atoms or [[ion]]s, but are not made of molecules.

In the [[kinetic theory]] of [[gas]]es the term ''molecule'' is often used for any gaseous particle regardless of their composition.E.g. see [http://www.usd.edu/phys/courses/phys_111sf/ch_10/10_notes.htm] According to this definition [[noble gas]]es would also be considered ''molecules'' despite the fact that they are composed of a single non-bonded atom.

In the [[kinetic theory]] of [[gas]]es the term ''molecule'' is often used for any gaseous particle regardless of their composition.E.g. see [https://www.usd.edu/phys/courses/phys_111sf/ch_10/10_notes.htm] According to this definition [[noble gas]]es would also be considered ''molecules'' despite the fact that they are composed of a single non-bonded atom.

== History ==

== History ==

Although the existence of molecules was accepted by many chemists since the early 19th century as a result of [[John Dalton|Dalton's]] laws of Definite and Multiple Proportions (1803-1808) and [[Avogadro's law]] (1811), there was some resistance among [[logical positivism|positivists]] and physicists such as [[Ernst Mach|Mach]], [[Ludwig Boltzmann|Boltzmann]], [[James Clerk Maxwell|Maxwell]], and [[Willard Gibbs|Gibbs]], who saw molecules merely as convenient mathematical constructs. The work of [[Jean Perrin|Perrin]] on Brownian motion ([[1911]]) is considered to be the final proof of the existence of molecules.

Although the existence of molecules was accepted by many chemists since the early 19th century as a result of [[John Dalton|Dalton's]] laws of Definite and Multiple Proportions (1803-1808) and [[Avogadro's law]] (1811), there was some resistance among [[logical positivism|positivists]] and physicists such as [[Ernst Mach|Mach]], [[Ludwig Boltzmann|Boltzmann]], [[James Clerk Maxwell|Maxwell]], and [[Willard Gibbs|Gibbs]], who saw molecules merely as convenient mathematical constructs. The work of [[Jean Perrin|Perrin]] on Brownian motion (1911) is considered to be the final proof of the existence of molecules.

== Overview ==

== Overview ==

The study of molecules by [[molecular physics]] and [[theoretical chemistry]] is largely based on [[quantum mechanic]]s and is essential for the understanding of the [[chemical bond]]. The simplest molecules used as model systems since the early days of [[quantum chemistry]] are the [[hydrogen molecule]] and its cation, H₂⁺. Among these two, the cation has the advantage of being a one-electron system, 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]].

The study of molecules by [[molecular physics]] and [[theoretical chemistry]] is largely based on [[quantum mechanic]]s and is essential for the understanding of the [[chemical bond]]. The simplest molecules used as model systems since the early days of [[quantum chemistry]] are the [[hydrogen molecule]] and its cation, H₂⁺. Among these two, the cation has the advantage of being a one-electron system, 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]], He₂, which has one vibrational [[bound state]] but is so loosely bound that it is only likely to be observed at very low temperatures.

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]], He₂, which has one vibrational [[bound state]] but is so loosely bound that it is only likely to be observed at very low temperatures.

 

==External links==

==External links==

*[http://www.chm.bris.ac.uk/motm/motm.htm Molecule of the Month] - School of Chemistry, University of Bristol

*[https://www.chm.bris.ac.uk/motm/motm.htm Molecule of the Month] - School of Chemistry, University of Bristol

*[http://www.accessexcellence.org/RC/VL/GG/antiBD_mol.html Antibody Molecule] - The National Health Museum

*[https://www.accessexcellence.org/RC/VL/GG/antiBD_mol.html Antibody Molecule] - The National Health Museum

*[http://www.ecosci.jp/ec.html Data Book of Molecules] - Home Page for Learning Environmental Chemistry

*[https://www.ecosci.jp/ec.html Data Book of Molecules] - Home Page for Learning Environmental Chemistry

[[Category: General Reference]]

[[Category: General Reference]]