Molecule: Difference between revisions
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In [[science]], a '''molecule''' is a group of [[atom]]s in a definite arrangement held together by [[chemical bond]]s.<ref>{{cite book|author=Pauling, Linus |title=General Chemistry|location=New York | publisher=Dover Publications, Inc.|year=1970|id=ISBN 0-486-65622-5}}<br> {{cite book| author=Ebbin, Darrell, D. |title=General Chemistry, 3th Ed.|location=Boston | publisher=Houghton Mifflin Co.|year=1990|id=ISBN 0-395-43302-9}}<br>{{cite book|author=Brown, T.L. |title=Chemistry – the Central Science, 9th Ed.|location=New Jersey | publisher=Prentice Hall|year=2003|id=ISBN 0-13-066997-0}}<br>{{cite book|author=Chang, Raymond |title=Chemistry, 6th Ed.|location=New York | publisher=McGraw Hill|year=1998|id=ISBN 0-07-115221-0}}<br>{{cite book|author=Zumdahl, Steven S. |title= Chemistry, 4th ed.|location= Boston |publisher= Houghton Mifflin|year= 1997|id=ISBN 0-669-41794-7}}</ref> [[ |
In [[science]], a '''molecule''' is 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.<ref>[[Compendium of Chemical Terminology]], [http://goldbook.iupac.org/M04002.html molecule]<br>{{cite book|author=Pauling, Linus |title=General Chemistry|location=New York | publisher=Dover Publications, Inc.|year=1970|id=ISBN 0-486-65622-5}}<br> {{cite book| author=Ebbin, Darrell, D. |title=General Chemistry, 3th Ed.|location=Boston | publisher=Houghton Mifflin Co.|year=1990|id=ISBN 0-395-43302-9}}<br>{{cite book|author=Brown, T.L. |title=Chemistry – the Central Science, 9th Ed.|location=New Jersey | publisher=Prentice Hall|year=2003|id=ISBN 0-13-066997-0}}<br>{{cite book|author=Chang, Raymond |title=Chemistry, 6th Ed.|location=New York | publisher=McGraw Hill|year=1998|id=ISBN 0-07-115221-0}}<br>{{cite book|author=Zumdahl, Steven S. |title= Chemistry, 4th ed.|location= Boston |publisher= Houghton Mifflin|year= 1997|id=ISBN 0-669-41794-7}}</ref> Only in the [[kinetic theory]] of gases the term ''molecule'' is often used for any gaseous particle regardless of their composition.<ref>E.g. see [http://www.usd.edu/phys/courses/phys_111sf/ch_10/10_notes.htm]</ref> |
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Molecules and atoms are 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] - [[Frostburg State University]] (Department of Chemistry)</ref> |
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In the [[molecular science]]s, a "molecule" is a sufficiently stable, [[electric charge|electrically]] neutral [[entity]] composed of two or more [[atom]]s.<ref>[[Compendium of Chemical Terminology]], [http://goldbook.iupac.org/M04002.html molecule]</ref> The concept of a single-atom or [[monatomic|monatomic molecule]], as found in [[noble gas]]es, is used almost exclusively in the [[kinetic theory]] of gases, where the fundamental gas particles are conventionally termed "molecules" regardless of their composition. <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]</ref> |
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== History == |
== History == |
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{{main|History of the molecule}} |
{{main|History of the molecule}} |
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Although the concept of molecules was first introduced in [[1811]] by [[Amadeo Avogadro|Avogadro]], and was accepted by many [[chemist]]s 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 word ''molecule'' was coined by the Scottish botanist [[Robert Brown (botanist)|Robert Brown]] in his famous paper<ref>R. Brown, Edinb. New Philos. J. 5, 358 (1828).</ref>, as a derivation of [[Leeuwenhoek]]'s term [[animalcules]]. 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]]. |
Although the concept of molecules was first introduced in [[1811]] by [[Amadeo Avogadro|Avogadro]], and was accepted by many [[chemist]]s 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 word ''molecule'' was coined by the Scottish botanist [[Robert Brown (botanist)|Robert Brown]] in his famous paper<ref>R. Brown, Edinb. New Philos. J. 5, 358 (1828).</ref>, as a derivation of [[Leeuwenhoek]]'s term [[animalcules]]. 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]]. |
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== Overview == |
== Overview == |
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In a molecule, at least two atoms are joined by shared pairs of [[electron]]s in a [[covalent 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). Atoms and complexes connected by non-covalent bonds such as [[hydrogen bond]]s or [[ionic bond]]s are generally not considered single molecules. |
In a molecule, at least two atoms are joined by shared pairs of [[electron]]s in a [[covalent 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). Atoms and complexes connected by non-covalent bonds such as [[hydrogen bond]]s or [[ionic bond]]s are generally not considered single molecules. |
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Revision as of 02:15, 1 May 2007
In science, a molecule is a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by strong chemical bonds.[1] Only in the kinetic theory of gases the term molecule is often used for any gaseous particle regardless of their composition.[2]
Molecules and atoms are the smallest particles of pure chemical substances that still retain their composition and chemical properties.[3]
Many substances in ordinary experience, such as rocks, crystals, and metals, are composed of atoms or ions, but are not made of molecules.
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 word molecule was coined by the Scottish botanist Robert Brown in his famous paper[4], as a derivation of Leeuwenhoek's term animalcules. 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
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 naked eye, but there are exceptions. DNA, a macromolecule, can reach macroscopic sizes, as can molecules of many polymers. The smallest of all molecules is the hydrogen ion molecule H2+, comprised of two protons bonded together by the sharing of one electron.[citation needed] The next largest molecule is the hydrogen molecule H2, with a length roughly twice the 74 picometres (0.74 Å) distance between the two hydrogen nuclei; but as with all molecules, however, the exact size of its electron cloud is difficult to define precisely. 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 supermolecules.
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 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.
See also
- Covalent bond
- Diatomic molecule
- Molecular geometry
- Molecular Hamiltonian
- Molecular orbital
- Nonpolar molecule
- Polar molecule
- For a list of molecules see the List of compounds
- List of molecules in interstellar space
References
- ^ Compendium of Chemical Terminology, molecule
Pauling, Linus (1970). General Chemistry. New York: Dover Publications, Inc. ISBN 0-486-65622-5.
Ebbin, Darrell, D. (1990). General Chemistry, 3th Ed. Boston: Houghton Mifflin Co. ISBN 0-395-43302-9.{{cite book}}: CS1 maint: multiple names: authors list (link)
Brown, T.L. (2003). Chemistry – the Central Science, 9th Ed. New Jersey: Prentice Hall. ISBN 0-13-066997-0.
Chang, Raymond (1998). Chemistry, 6th Ed. New York: McGraw Hill. ISBN 0-07-115221-0.
Zumdahl, Steven S. (1997). Chemistry, 4th ed. Boston: Houghton Mifflin. ISBN 0-669-41794-7. - ^ E.g. see [1]
- ^ Molecule Definition - Frostburg State University (Department of Chemistry)
- ^ R. Brown, Edinb. New Philos. J. 5, 358 (1828).
External links
- Molecule of the Month - School of Chemistry, University of Bristol
- Antibody Molecule - The National Health Museum
- Data Book of Molecules - Home Page for Learning Environmental Chemistry