Molecule

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For other uses, see Molecule (disambiguation).

Atomic force microscopy (AFM) image of a PTCDA molecule, in which the five six-carbon rings are visible.[1]
scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings.[2]
AFM image of 1,5,9-trioxo-13-azatriangulene and its chemical structure.[3]

A molecule is a group of two or more

quantum physics, organic chemistry, and biochemistry, the distinction from ions is dropped and molecule is often used when referring to polyatomic ions
.

A molecule may be

ionic bonds, are typically not considered single molecules.[10]

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 as Robert Boyle, Amedeo Avogadro, Jean Perrin, and Linus Pauling, the study of molecules is today known as molecular physics or molecular chemistry.

Etymology

According to Merriam-Webster and the Online Etymology Dictionary, the word "molecule" derives from the Latin "moles" or small unit of mass. The word is derived from French molécule (1678), from Neo-Latin molecula, diminutive of Latin moles "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 by Descartes.[11][12]

History

Main article: History of molecular theory

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.[13] This definition often breaks down since many substances in ordinary experience, such as rocks, salts, and metals, are composed of large crystalline networks of chemically bonded atoms or ions, but are not made of discrete molecules.

The modern concept of molecules can be traced back towards pre-scientific and Greek philosophers such as

atoms and voids. Circa 450 BC Empedocles imagined fundamental elements (fire (), earth (), air (), and water (
)) and "forces" of attraction and repulsion allowing the elements to interact.

A fifth element, the incorruptible quintessence aether, was considered to be the fundamental building block of the heavenly bodies. The viewpoint of Leucippus and Empedocles, along with the aether, was accepted by Aristotle 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 to

valency 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 to Partington's A 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 single molecule.

In coordination with these concepts, in 1833 the French chemist Marc 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 named

Jean Perrin received the Nobel Prize in physics for proving, conclusively, the existence of molecules. He did this by calculating the Avogadro constant using three different methods, all involving liquid phase systems. First, he used a gamboge soap-like emulsion, second by doing experimental work on Brownian motion, and third by confirming Einstein's theory of particle rotation in the liquid phase.[17]

In 1927, the physicists Fritz London and Walter 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 a Guggenheim 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"

hybridization theory to account for bonds in molecules such as CH4, in which four sp³ hybridised orbitals are overlapped by hydrogen's 1s orbital, yielding four sigma (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 called molecular chemistry or

van der Waals complexes, or systems of colliding atoms as in Bose–Einstein condensate
.

Prevalence

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,

core of the Earth
. All of these contain many chemical bonds, but are not made of identifiable molecules.

No typical molecule can be defined for salts nor for

metallic bonding. Thus solid metals are not made of molecules. In glasses
, 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 by

covalent 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 by metallic bonding,[20] others point out that metals behave very differently than molecules.[21]

Covalent

A covalent bond forming H2 (right) where two hydrogen atoms share the two electrons
Main article:
Covalent bonding

A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are termed shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is termed covalent bonding.[22]

Ionic

Main article:
exothermically
.

Ionic bonding is a type of chemical bond that involves the

nonmetal
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 many

atomic force microscope. Some of the largest molecules are macromolecules or supermolecules
.

The smallest molecule is the

diatomic hydrogen (H2), with a bond length of 0.74 Å.[24]

Effective molecular radius is the size a molecule displays in solution.[25][26] The table of permselectivity for different substances contains examples.

Molecular formulas

Chemical formula types

Main article: Chemical formula

The chemical formula for a molecule uses one line of chemical element symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, and plus (+) and minus (−) signs. These are limited to one typographic line of symbols, which may include subscripts and superscripts.

A compound's empirical formula is a very simple type of chemical formula.[27] It is the simplest integer ratio of the chemical elements that constitute it.[28] For example, water is always composed of a 2:1 ratio of hydrogen to oxygen atoms, and ethanol (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 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

stoichiometric
calculations.

Structural formula

3D (left and center) and 2D (right) representations of the terpenoid molecule atisane
Main article: Structural formula

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 a structural formula may be needed. Structural formulas may in turn be represented with a one-dimensional chemical name, but such chemical nomenclature requires many words and terms which are not part of chemical formulas.

Molecular geometry

Main article: Molecular geometry
STM image of a "cyanostar" dendrimer molecule.[29]

Molecules have fixed

Stereoisomers, a particular type of isomer, may have very similar physico-chemical properties and at the same time different biochemical
activities.

Molecular spectroscopy

Main article: Spectroscopy
single-molecule electronics.[30]

Molecular spectroscopy deals with the response (

emission.[31]
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).

near 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 and

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

dimer, 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 is sufficiently 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 an elementary 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.

See also

References

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  10. ^ "Molecule". Encyclopædia Britannica. 22 January 2016. Archived from the original on 3 May 2020. Retrieved 23 February 2016.
  11. ^ Harper, Douglas. "molecule". Online Etymology Dictionary. Retrieved 22 February 2016.
  12. ^ "molecule". Merriam-Webster. Archived from the original on 24 February 2021. Retrieved 22 February 2016.
  13. ^ Molecule Definition Archived 13 October 2014 at the Wayback Machine (Frostburg State University)
  14. ^ Ley, Willy (June 1966). "The Re-Designed Solar System". For Your Information. Galaxy Science Fiction. pp. 94–106.
  15. ^ Avogadro, Amedeo (1811). "Masses of the Elementary Molecules of Bodies". Journal de Physique. 73: 58–76. Archived from the original on 12 May 2019. Retrieved 25 August 2022.
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  17. ^ Perrin, Jean, B. (1926). Discontinuous Structure of Matter Archived 29 May 2019 at the Wayback Machine, Nobel Lecture, 11 December.
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  20. ^ Harry, B. Gray. Chemical Bonds: An Introduction to Atomic and Molecular Structure (PDF). pp. 210–211. Archived (PDF) from the original on 31 March 2021. Retrieved 22 November 2021.
  21. ^ "How many gold atoms make gold metal?". phys.org. Archived from the original on 30 October 2020. Retrieved 22 November 2021.
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External links
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