Concepts similar to molecules have been discussed since ancient times, but modern investigation into the nature of molecules and their bonds began in the 17th century. Refined over time by scientists such asRobert Boyle,Amedeo Avogadro,Jean Perrin, andLinus Pauling, the study of molecules is today known asmolecular physics or molecular chemistry.
Etymology
According toMerriam-Webster and theOnline Etymology Dictionary, the word "molecule" derives from theLatin "moles" or small unit of mass. The word is derived from Frenchmolécule (1678), fromNeo-Latinmolecula, diminutive of Latinmoles "mass, barrier". The word, which until the late 18th century was used only in Latin form, became popular after being used in works of philosophy byDescartes.[11][12]
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 smallestparticles of purechemical substances that still retain theircomposition and chemical properties.[13] This definition often breaks down since many substances in ordinary experience, such asrocks,salts, andmetals, are composed of large crystalline networks ofchemically bonded atoms orions, but are not made of discrete molecules.
The modern concept of molecules can be traced back towards pre-scientific and Greek philosophers such asLeucippus andDemocritus who argued that all the universe is composed ofatoms and voids. Circa 450 BCEmpedocles imaginedfundamental elements (fire (),earth (),air (), andwater ()) and "forces" of attraction and repulsion allowing the elements to interact.
A fifth element, the incorruptible quintessenceaether, was considered to be the fundamental building block of the heavenly bodies. The viewpoint of Leucippus and Empedocles, along with the aether, was accepted byAristotle and passed to medieval and renaissance Europe.
In a more concrete manner, however, the concept of aggregates or units of bonded atoms, i.e. "molecules", traces its origins toRobert Boyle's 1661 hypothesis, in his famous treatiseThe Sceptical Chymist, that matter is composed ofclusters of particles and that chemical change results from the rearrangement of the clusters. Boyle argued that matter's basic elements consisted of various sorts and sizes of particles, called "corpuscles", which were capable of arranging themselves into groups. In 1789,William Higgins published views on what he called combinations of "ultimate" particles, which foreshadowed the concept ofvalency bonds. If, for example, according to Higgins, the force between the ultimate particle of oxygen and the ultimate particle of nitrogen were 6, then the strength of the force would be divided accordingly, and similarly for the other combinations of ultimate particles.
Amedeo Avogadro created the word "molecule".[14] His 1811 paper "Essay on Determining the Relative Masses of the Elementary Molecules of Bodies", he essentially states, i.e. according toPartington'sA Short History of Chemistry, that:[15]
The smallest particles of gases are not necessarily simple atoms, but are made up of a certain number of these atoms united by attraction to form a singlemolecule.
In coordination with these concepts, in 1833 the French chemistMarc Antoine Auguste Gaudin presented a clear account of Avogadro's hypothesis,[16] regarding atomic weights, by making use of "volume diagrams", which clearly show both semi-correct molecular geometries, such as a linear water molecule, and correct molecular formulas, such as H2O:
Marc Antoine Auguste Gaudin's volume diagrams of molecules in the gas phase (1833)
In 1917, an unknown American undergraduate chemical engineer namedLinus Pauling was learning theDalton hook-and-eye bonding method, which was the mainstream description of bonds between atoms at the time. Pauling, however, was not satisfied with this method and looked to the newly emerging field of quantum physics for a new method. In 1926, French physicistJean Perrin received the Nobel Prize in physics for proving, conclusively, the existence of molecules. He did this by calculating theAvogadro constant using three different methods, all involving liquid phase systems. First, he used agamboge soap-like emulsion, second by doing experimental work onBrownian motion, and third by confirming Einstein's theory of particle rotation in the liquid phase.[17]
In 1927, the physicistsFritz London andWalter Heitler applied the new quantum mechanics to the deal with the saturable, nondynamic forces of attraction and repulsion, i.e., exchange forces, of the hydrogen molecule. Their valence bond treatment of this problem, in their joint paper,[18] was a landmark in that it brought chemistry under quantum mechanics. Their work was an influence on Pauling, who had just received his doctorate and visited Heitler and London in Zürich on aGuggenheim Fellowship.
Subsequently, in 1931, building on the work of Heitler and London and on theories found in Lewis' famous article, Pauling published his ground-breaking article "The Nature of the Chemical Bond"[19] in which he usedquantum mechanics to calculate properties and structures of molecules, such as angles between bonds and rotation about bonds. On these concepts, Pauling developedhybridization theory to account for bonds in molecules such as CH4, in which four sp³ hybridised orbitals are overlapped byhydrogen's1s orbital, yielding foursigma (σ) bonds. The four bonds are of the same length and strength, which yields a molecular structure as shown below:
A schematic presentation of hybrid orbitals overlapping hydrogens' s orbitals
Molecular science
The science of molecules is calledmolecular chemistry ormolecular 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) composed of two or more atoms.Polyatomic ions may sometimes be usefully thought of as electrically charged molecules. The termunstable molecule is used for veryreactive species, i.e., short-lived assemblies (resonances) of electrons andnuclei, such asradicals,molecular ions,Rydberg molecules,transition states,van der Waals complexes, or systems of colliding atoms as inBose–Einstein condensate.
Molecules as components of matter are common. They also make up most of the oceans and atmosphere. Most organic substances are molecules. The substances of life are molecules, e.g. proteins, the amino acids of which they are composed, the nucleic acids (DNA and RNA), sugars, carbohydrates, fats, and vitamins. The nutrient minerals are generally ionic compounds, thus they are not molecules, e.g. iron sulfate.
However, the majority of familiar solid substances on Earth are made partly or completely of crystals or ionic compounds, which are not made of molecules. These include all of the minerals that make up the substance of the Earth, sand, clay, pebbles, rocks, boulders,bedrock, themolten interior, and thecore of the Earth. All of these contain many chemical bonds, but arenot made of identifiable molecules.
No typical molecule can be defined for salts nor forcovalent crystals, although these are often composed of repeatingunit cells that extend either in aplane, e.g.graphene; or three-dimensionally e.g.diamond,quartz,sodium chloride. The theme of repeated unit-cellular-structure also holds for most metals which are condensed phases withmetallic bonding. Thus solid metals are not made of molecules. Inglasses, which are solids that exist in a vitreous disordered state, the atoms are held together by chemical bonds with no presence of any definable molecule, nor any of the regularity of repeating unit-cellular-structure that characterizes salts, covalent crystals, and metals.
Bonding
Molecules are generally held together bycovalent bonding. Several non-metallic elements exist only as molecules in the environment either in compounds or as homonuclear molecules, not as free atoms: for example, hydrogen.
While some people say a metallic crystal can be considered a single giant molecule held together bymetallic bonding,[20] others point out that metals behave very differently than molecules.[21]
Covalent
A covalent bond forming H2 (right) where twohydrogen atoms share the two electrons
A covalent bond is a chemical bond that involves the sharing ofelectron pairs between atoms. These electron pairs are termedshared pairs orbonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is termedcovalent bonding.[22]
Ionic bonding is a type of chemical bond that involves theelectrostatic attraction between oppositely charged ions, and is the primary interaction occurring inionic compounds. The ions are atoms that have lost one or moreelectrons (termedcations) and atoms that have gained one or more electrons (termedanions).[23] This transfer of electrons is termedelectrovalence in contrast tocovalence. In the simplest case, the cation is ametal atom and the anion is anonmetal atom, but these ions can be of a more complicated nature, e.g. molecular ions like NH4+ or SO42−. At normal temperatures and pressures, ionic bonding mostly creates solids (or occasionally liquids) without separate identifiable molecules, but the vaporization/sublimation of such materials does produce separate molecules where electrons are still transferred fully enough for the bonds to be considered ionic rather than covalent.
Molecular size
Most molecules are far too small to be seen with the naked eye, although molecules of manypolymers can reachmacroscopic sizes, includingbiopolymers such asDNA. Molecules commonly used as building blocks for organic synthesis have a dimension of a fewangstroms (Å) to several dozen Å, or around one billionth of a meter. Single molecules cannot usually be observed bylight (as noted above), but small molecules and even the outlines of individual atoms may be traced in some circumstances by use of anatomic force microscope. Some of the largest molecules aremacromolecules orsupermolecules.
The smallest molecule is thediatomic hydrogen (H2), with a bond length of 0.74 Å.[24]
Thechemical formula for a molecule uses one line of chemical element symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, andplus (+) andminus (−) signs. These are limited to one typographic line of symbols, which may include subscripts and superscripts.
A compound'sempirical formula is a very simple type of chemical formula.[27] It is the simplestintegerratio of the chemical elements that constitute it.[28] For example, water is always composed of a 2:1 ratio of hydrogen to oxygen atoms, andethanol (ethyl alcohol) 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 sameatoms in different arrangements are calledisomers. 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.
Themolecular 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 moleculeacetylene has molecular formula C2H2, but the simplest integer ratio of elements is CH.
For molecules with a complicated 3-dimensional structure, especially involving atoms bonded to four different substituents, a simple molecular formula or even semi-structural chemical formula may not be enough to completely specify the molecule. In this case, a graphical type of formula called astructural formula may be needed. Structural formulas may in turn be represented with a one-dimensional chemical name, but suchchemical nomenclature requires many words and terms which are not part of chemical formulas.
Molecules have fixedequilibrium 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 itsreactivity.Isomers share a chemical formula but normally have very different properties because of their different structures.Stereoisomers, a particular type of isomer, may have very similar physico-chemical properties and at the same time differentbiochemical activities.
Hydrogen can be removed from individualH2TPP molecules by applying excess voltage to the tip of ascanning tunneling microscope (STM, a); this removal alters the current-voltage (I-V) curves of TPP molecules, measured using the same STM tip, fromdiode like (red curve in b) toresistor like (green curve). Image (c) shows a row of TPP, H2TPP and TPP molecules. While scanning image (d), excess voltage was applied to H2TPP at the black dot, which instantly removed hydrogen, as shown in the bottom part of (d) and in the rescan image (e). Such manipulations can be used insingle-molecule electronics.[30]
Molecular spectroscopy deals with the response (spectrum) of molecules interacting with probing signals of knownenergy (orfrequency, according to thePlanck relation). Molecules have quantized energy levels that can be analyzed by detecting the molecule's energy exchange throughabsorbance oremission.[31]Spectroscopy does not generally refer todiffraction studies where particles such asneutrons, electrons, or high energyX-rays interact with a regular arrangement of molecules (as in a crystal).
Microwave spectroscopy commonly measures changes in the rotation of molecules, and can be used to identify molecules in outer space.Infrared spectroscopy measures the vibration of molecules, including stretching, bending or twisting motions. It is commonly used to identify the kinds of bonds orfunctional groups in molecules. Changes in the arrangements of electrons yield absorption or emission lines in ultraviolet, visible ornear infrared light, and result in colour.Nuclear resonance spectroscopy measures the environment of particular nuclei in the molecule, and can be used to characterise the numbers of atoms in different positions in a molecule.
Theoretical aspects
The study of molecules by molecular physics andtheoretical chemistry is largely based onquantum mechanics and is essential for the understanding of the chemical bond. The simplest of molecules is thehydrogen molecule-ion, H2+, and the simplest of all the chemical bonds is theone-electron bond. H2+ is composed of two positively chargedprotons and one negatively chargedelectron, which means that theSchrödinger 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 ofcomputational chemistry.
When trying to define rigorously whether an arrangement of atoms issufficiently stable to be considered a molecule, IUPAC suggests that it "must correspond to a depression on thepotential energy surface that is deep enough to confine at least one vibrational state".[4] 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 theheliumdimer,He2, which has one vibrational bound state[32] and is so loosely bound that it is only likely to be observed at very low temperatures.
Whether or not an arrangement of atoms issufficiently stable to be considered a molecule is inherently an operational definition. Philosophically, therefore, a molecule is not a fundamental entity (in contrast, for instance, to anelementary particle); rather, the concept of a molecule is the chemist's way of making a useful statement about the strengths of atomic-scale interactions in the world that we observe.
^Seymour H. Mauskopf (1969). "The Atomic Structural Theories of Ampère and Gaudin: Molecular Speculation and Avogadro's Hypothesis".Isis.60 (1):61–74.doi:10.1086/350449.JSTOR229022.S2CID143759556.
^Pauling, Linus (1931). "The nature of the chemical bond. Application of results obtained from the quantum mechanics and from a theory of paramagnetic susceptibility to the structure of molecules".J. Am. Chem. Soc.53 (4):1367–1400.Bibcode:1931JAChS..53.1367P.doi:10.1021/ja01355a027.
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