Jump to content

Molecule: Difference between revisions

Edit links
From Wikipedia, the free encyclopedia
Browse history interactively
← Previous editNext edit →
Content deleted Content added
VisualWikitext
m Reverted edits by 117.195.114.137 (talk) to last version by Onegumas
m Molecules are moles, duh.
Line 3: Line 3:
[[File:Atisane3.png|thumb|400px|[[Three-dimensional space|3D]] (left and center) and [[2D geometric model|2D]] (right) representations of the [[terpenoid]] molecule atisane]]
[[File:Atisane3.png|thumb|400px|[[Three-dimensional space|3D]] (left and center) and [[2D geometric model|2D]] (right) representations of the [[terpenoid]] molecule atisane]]


A '''molecule''' ({{IPAc-en|icon|ˈ|m|ɒ|l|ɪ|k|juː|l}}) is an electrically neutral group of two or more [[atom]]s held together by [[covalent bond|covalent]] [[chemical bond]]s.<ref name="iupac">{{GoldBookRef|title=molecule|url=http://goldbook.iupac.org/M04002.html|year=1994}}</ref><ref>{{cite book|author=Pauling, Linus |title=General Chemistry|location=New York|publisher=Dover Publications, Inc.|year=1970|isbn=0-486-65622-5}}</ref><ref>{{cite book| author=Ebbin, Darrell, D. |title=General Chemistry, 3rd Ed.|location=Boston|publisher=Houghton Mifflin Co.|year=1990|isbn=0-395-43302-9}}</ref><ref>{{cite book|author=Brown, T.L. |title=Chemistry – the Central Science, 9th Ed.|location=New Jersey|publisher=Prentice Hall|year=2003|isbn=0-13-066997-0}}</ref><ref>{{cite book|author=Chang, Raymond |title=Chemistry, 6th Ed.|location=New York|publisher=McGraw Hill|year=1998|isbn=0-07-115221-0}}</ref><ref>{{cite book|author=Zumdahl, Steven S. |title= Chemistry, 4th ed.|location= Boston |publisher= Houghton Mifflin|year= 1997|isbn=0-669-41794-7}}</ref> Molecules are distinguished from [[ion]]s by their lack of electrical charge. However, in [[quantum physics]], [[organic chemistry]], and [[biochemistry]], the term ''molecule'' is often used less strictly, also being applied to [[polyatomic ion]]s.
A '''molecule''' ({{IPAc-en|icon|ˈ|m|ɒ|l|ɪ|k|juː|l}}) is a subspecies of the common ground [[mole]]. The relation to the mole is obvious in the molecule's name. |title=molecule|url=http://goldbook.iupac.org/M04002.html|year=1994}}</ref><ref>{{cite book|author=Pauling, Linus |title=General Chemistry|location=New York|publisher=Dover Publications, Inc.|year=1970|isbn=0-486-65622-5}}</ref><ref>{{cite book| author=Ebbin, Darrell, D. |title=General Chemistry, 3rd Ed.|location=Boston|publisher=Houghton Mifflin Co.|year=1990|isbn=0-395-43302-9}}</ref><ref>{{cite book|author=Brown, T.L. |title=Chemistry – the Central Science, 9th Ed.|location=New Jersey|publisher=Prentice Hall|year=2003|isbn=0-13-066997-0}}</ref><ref>{{cite book|author=Chang, Raymond |title=Chemistry, 6th Ed.|location=New York|publisher=McGraw Hill|year=1998|isbn=0-07-115221-0}}</ref><ref>{{cite book|author=Zumdahl, Steven S. |title= Chemistry, 4th ed.|location= Boston |publisher= Houghton Mifflin|year= 1997|isbn=0-669-41794-7}}</ref> Molecules are distinguished from [[ion]]s by their lack of electrical charge. However, in [[quantum physics]], [[organic chemistry]], and [[biochemistry]], the term ''molecule'' is often used less strictly, also being applied to [[polyatomic ion]]s.


In the [[kinetic theory]] of [[gas]]es, the term ''molecule'' is often used for any gaseous particle regardless of its composition. According to this definition [[noble gas]] atoms are considered molecules despite the fact that they are composed of a single non-bonded atom.<ref>{{cite book
In the [[kinetic theory]] of [[gas]]es, the term ''molecule'' is often used for any gaseous particle regardless of its composition. According to this definition [[noble gas]] atoms are considered molecules despite the fact that they are composed of a single non-bonded atom.<ref>{{cite book

Revision as of 01:25, 28 January 2013

For other uses, see Molecules (journal).
3D (left and center) and 2D (right) representations of the terpenoid molecule atisane

A molecule (/[invalid input: 'icon']ˈmɒlɪkjuːl/) is a subspecies of the common ground mole. The relation to the mole is obvious in the molecule's name. |title=molecule|url=http://goldbook.iupac.org/M04002.html%7Cyear=1994}}</ref>[1][2][3][4][5] Molecules are distinguished from ions by their lack of electrical charge. However, in quantum physics, organic chemistry, and biochemistry, the term molecule is often used less strictly, also being applied to polyatomic ions.

In the kinetic theory of gases, the term molecule is often used for any gaseous particle regardless of its composition. According to this definition noble gas atoms are considered molecules despite the fact that they are composed of a single non-bonded atom.[6]

A molecule may consist of atoms of a single 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.[7]

Molecules as components of matter are common in organic substances (and therefore biochemistry). They also make up most of the oceans and atmosphere. However, the majority of familiar solid substances on Earth, including most of the minerals that make up the crust, mantle, and core of the Earth, contain many chemical bonds, but are not made of identifiable molecules. Also, no typical molecule can be defined for ionic crystals (salts) and covalent crystals (network solids), although these are often composed of repeating unit cells that extend either in a plane (such as in graphene) or three-dimensionally (such as in diamond, quartz, or sodium chloride). The theme of repeated unit-cellular-structure also holds for most condensed phases with metallic bonding, which means that solid metals are also not made of molecules. In glasses (solids that exist in a vitreous disordered state), atoms may also be held together by chemical bonds without presence of any definable molecule, but also without any of the regularity of repeating units that characterises crystals.

Molecular science

The science of molecules is called molecular chemistry or molecular physics, 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) 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 condensate

History and etymology

Main article: History of molecular theory
John Dalton

According to Merriam-Webster and the Online Etymology Dictionary, the word "molecule" derives from the Latin "moles" or small unit of mass.

  • Molecule (1794) – "extremely minute particle," from Fr. molécule (1678)[citation needed], from modern Latin. molecula, diminutive of Latin moles "mass, barrier". A vague meaning at first; the vogue for the word (used until late 18th century only in Latin form) can be traced to the philosophy of Descartes.

Although the existence of molecules has been 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.[citation needed]

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 of pure chemical substances that still retain their composition and chemical properties.[8] This definition often breaks down since many substances in ordinary experience, such as rocks, salts, and metals, are composed of large networks of chemically bonded atoms or ions, but are not made of discrete molecules.

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. 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.

Smallest molecule diameter

The smallest molecule is the diatomic hydrogen (H2), with a bond length of 0.74 Å.[9]

Largest molecule diameter

Mesoporous silica have been produced with a diameter of 1000 Å (100 nm)[10]

Radius

Effective molecular radius is the size a molecule displays in solution.[11][12] The table of permselectivity for different substances contains examples.

Molecular formula

A compound's empirical formula is the simplest integer 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 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 ratios as ethanol, for instance. Molecules with the same atoms in different arrangements are called isomers. 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 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 has molecular formula C2H2, but the simplest integer ratio of elements is CH.

The molecular mass can be calculated from the chemical formula and is expressed in conventional atomic mass units equal to 1/12 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

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 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). Molecules have quantized energy levels that can be analyzed by detecting the molecule's energy exchange through absorbance or emission.[13] Spectroscopy does not generally refer to diffraction studies where particles such as neutrons, electrons, or high energy X-rays interact with a regular arrangement of molecules (as in a crystal).

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, 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".[14] 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[15] and is so loosely bound that it is only likely to be observed at very low temperatures.

See also

Wikimedia Commons has media related to Molecules.

References

  1. ^ Pauling, Linus (1970). General Chemistry. New York: Dover Publications, Inc. ISBN 0-486-65622-5.
  2. ^ Ebbin, Darrell, D. (1990). General Chemistry, 3rd Ed. Boston: Houghton Mifflin Co. ISBN 0-395-43302-9.{{cite book}}: CS1 maint: multiple names: authors list (link)
  3. ^ Brown, T.L. (2003). Chemistry – the Central Science, 9th Ed. New Jersey: Prentice Hall. ISBN 0-13-066997-0.
  4. ^ Chang, Raymond (1998). Chemistry, 6th Ed. New York: McGraw Hill. ISBN 0-07-115221-0.
  5. ^ Zumdahl, Steven S. (1997). Chemistry, 4th ed. Boston: Houghton Mifflin. ISBN 0-669-41794-7.
  6. ^ Chandra, Sulekh. Comprehensive Inorganic Chemistry. New Age Publishers. ISBN 81-224-1512-1.
  7. ^ Molecule, Encyclopaedia Britannica on-line
  8. ^ Molecule Definition (Frostburg State University)
  9. ^ Roger L. DeKock, Harry B. Gray (1989). Chemical structure and bonding. University Science Books. p. 199. ISBN 0-935702-61-X.
  10. ^ http://pubs.acs.org/doi/abs/10.1021/ac303274w
  11. ^ Chang RL, Deen WM, Robertson CR, Brenner BM. (1975). "Permselectivity of the glomerular capillary wall: III. Restricted transport of polyanions". Kidney Int. 8 (4): 212–218. doi:10.1038/ki.1975.104. PMID 1202253.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  12. ^ Chang RL, Ueki IF, Troy JL, Deen WM, Robertson CR, Brenner BM. (1975). "Permselectivity of the glomerular capillary wall to macromolecules. II. Experimental studies in rats using neutral dextran". Biophys J. 15 (9): 887–906. Bibcode:1975BpJ....15..887C. doi:10.1016/S0006-3495(75)85863-2. PMC 1334749. PMID 1182263.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (1997,2006) "spectroscopy". doi:10.1351/goldbook.S05848
  14. ^ Cite error: The named reference iupac was invoked but never defined (see the help page).
  15. ^ Anderson JB (2004). "Comment on "An exact quantum Monte Carlo calculation of the helium-helium intermolecular potential" [J. Chem. Phys. 115, 4546 (2001)]". J Chem Phys. 120 (20): 9886–7. Bibcode:2004JChPh.120.9886A. doi:10.1063/1.1704638. PMID 15268005. {{cite journal}}: Unknown parameter |month= ignored (help)

Template:Composition

Retrieved from "https://wiki.riteme.site/w/index.php?title=Molecule&oldid=535264312"