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Encyclopedia > Diatomic molecule
A space-filling model of the diatomic molecule dinitrogen, N2.

Diatomic molecules are molecules made only of two atoms, of either the same or different chemical elements. The prefix di- means two in Greek. Image File history File links Size of this preview: 792 Ã— 600 pixelsFull resolution (1100 Ã— 833 pixel, file size: 145 KB, MIME type: image/png) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Size of this preview: 792 Ã— 600 pixelsFull resolution (1100 Ã— 833 pixel, file size: 145 KB, MIME type: image/png) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... This is a calotte model of cyclohexane. ... For other uses, see Atom (disambiguation). ... The periodic table of the chemical elements A chemical element, or element, is a type of atom that is defined by its atomic number; that is, by the number of protons in its nucleus. ...

## Description and occurrence in nature GA_googleFillSlot("encyclopedia_square");

Huber and Herzberg's book, Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules[1], lists hundreds of diatomic molecules, some which have been detected spectroscopically in interstellar space. However, few diatomics are found to occur naturally on Earth outside of laboratories. About 99% of the Earth's atmosphere is composed of diatomic molecules, specifically oxygen O2 (21%) and nitrogen N2 (78%), with the remaining 1% being mostly argon (0.9340%). The natural abundance of hydrogen (H2) in the Earth's atmosphere is only on the order of parts per million, but H2 is, in fact, the most abundant molecule seen in nature, dominating the composition of stars. Classic comprehensive multidisciplinary reference text contains a critical compilation of available data for all diatomic molecules and ions known at present publication- over 900 diatomic species in all; including electronic energies, vibrational and rotational constants, and observed transitions. ... This is a list of molecules that have been detected in the interstellar medium, grouped by the number of component atoms. ... â€œAirâ€ redirects here. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Standard atomic weight 15. ... General Name, symbol, number nitrogen, N, 7 Chemical series nonmetals Group, period, block 15, 2, p Appearance colorless gas Standard atomic weight 14. ... General Name, Symbol, Number argon, Ar, 18 Chemical series noble gases Group, Period, Block 18, 3, p Appearance colorless Standard atomic weight 39. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ...

Elements that consist of diatomic molecules, under typical laboratory conditions of 1 bar and 25 oC, include hydrogen (H2), nitrogen (N2), oxygen (O2), and the halogens: fluorine (F2), chlorine (Cl2), bromine (Br2), iodine (I2), and astatine (At2). Astatine is so rare in nature (its most stable isotope has a half-life of only 8.3 hours) that it is usually not considered in discussions of this subject.[2] Again note that many other diatomics are possible, such as metals heated to their gaseous states. Also, many diatomic molecules are unstable and highly reactive, such as diphosphorus. A few compounds are made of diatomic molecules, including CO and HBr. General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, symbol, number nitrogen, N, 7 Chemical series nonmetals Group, period, block 15, 2, p Appearance colorless gas Standard atomic weight 14. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Standard atomic weight 15. ... The halogens or halogen elements are a series of nonmetal elements from Group 17 (old-style: VII or VIIA; Group 7 IUPAC Style) of the periodic table, comprising fluorine, F, chlorine, Cl, bromine, Br, iodine, I, and astatine, At. ... Distinguished from fluorene and fluorone. ... General Name, symbol, number chlorine, Cl, 17 Chemical series halogens Group, period, block 17, 3, p Appearance yellowish green Standard atomic weight 35. ... â€œBromoâ€ redirects here. ... General Name, Symbol, Number iodine, I, 53 Chemical series halogens Group, Period, Block 17, 5, p Appearance violet-dark gray, lustrous Standard atomic weight 126. ... General Name, Symbol, Number astatine, At, 85 Chemical series halogens Group, Period, Block 17, 6, p Appearance metallic (presumed) Atomic mass (210) gÂ·molâˆ’1 Electron configuration [Xe] 4f14 5d10 6s2 6p5 Electrons per shell 2, 8, 18, 32, 18, 7 Physical properties Phase solid Melting point 575 K (302... For other uses, see Isotope (disambiguation). ... Half-Life For a quantity subject to exponential decay, the half-life is the time required for the quantity to fall to half of its initial value. ... Diphosphorus or P2 is the diatomic form of phosphorus. ... A chemical compound is a chemical substance of two or more different chemically bonded chemical elements, with a fixed ratio determining the composition. ...

If a diatomic molecules consists of two atoms of the same element, such as H2 and O2, then it is said to be homonuclear, but otherwise it is said to heteronuclear, such as with CO or NO. The bond in a homonuclear diatomic molecule is non-polar and fully covalent. Covalent bonding is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or sometimes between atoms and other covalent bonds. ...

## Historical significance

Diatomic elements played an important role in the elucidation of the concepts of element, atom, and molecule in the 19th century, because some of the most common elements, such as hydrogen, oxygen, and nitrogen, occur as diatomic molecules. John Dalton's original atomic hypothesis assumed that all elements were monatomic and that the atoms in compounds would normally have the simplest atomic ratios with respect to one another. For example, Dalton assumed that water's formula was HO, giving the atomic weight of oxygen as 8 times that of hydrogen, instead of the modern value of about 16. As a consequence, confusion existed regarding atomic weights and molecular formulas for about half a century. John Dalton John Dalton (September 6, 1766 â€“ July 27, 1844) was an English chemist and physicist, born at Eaglesfield, near Cockermouth in Cumberland. ...

As early as 1805, Gay-Lussac and von Humboldt showed that water is formed of two volumes of hydrogen and one volume of oxygen, and by 1811 Amedeo Avogadro had arrived at the correct interpretation of water's composition, based on what is now called Avogadro's law and the assumption of diatomic elemental molecules. However, these results were mostly ignored until 1860. Part of this rejection was due to the belief that atoms of one element would have no chemical affinity towards atoms of the same element, and part was due to apparent exceptions to Avogadro's law that were not explained until later in terms of dissociating molecules. Joseph-Louis Gay-Lussac (December 6, 1778&#8211;May 10, 1850) was a French chemist and physicist. ... Alexander von Humboldt (age 89): an 1859 portrait by artist Julius Schrader, showing Mount Chimborazo in the background. ... Portrait of Amedeo Avogadro Lorenzo Romano Amedeo Carlo Avogadro, Count of Quaregna and Cerreto (August 9, 1776â€“July 9, 1856) was an Italian chemist, most noted for his contributions to the theory of molarity and molecular weight. ... This article does not cite any references or sources. ... Chemical affinity results from electronic properties by which dissimilar substances are capable of forming chemical compounds. ...

At the 1860 Karlsruhe Congress on atomic weights, Cannizzaro resurrected Avogadro's ideas and used them to produce a consistent table of atomic weights, which mostly agree with modern values. These weights were an important pre-requisite for the discovery of the periodic law by Dmitri Mendeleev and Lothar Meyer.[3] Formulas of acetic acid given by KekulÃ© in 1861 The Karlsruhe Congress was an international meeting of chemists held in Karlsruhe, Germany from September 3, 1860 to September 5, 1860. ... Stanislao Cannizzaro (July 13, 1826 - May 10, 1910) was an Italian chemist. ... In the beginning People have known about basic chemical elements such as gold, silver, and copper from antiquity, as these can all be discovered in nature in native form and are relatively simple to mine with primitive tools. ... Portrait of Dimitri Mendeleyev by Ilya Repin Dimitri Mendeleev (Russian: , Dimitriy Ivanovich Mendeleyev  ) (8 February [O.S. 27 January] 1834 in Tobolsk â€“ 2 February [O.S. 20 January] 1907 in Saint Petersburg), was a Russian chemist. ... Julius Lothar Meyer (19 August 1830 - 11 April 1895) was born in Varel, at that time belonging to the duchy of Oldenburg, now part of Germany. ...

## Energy levels

It is convenient, and common, to represent a diatomic molecule as two point masses (the two atoms) connected by a massless spring. The energies involved in the various motions of the molecule can then be broken down into three categories.

• The translational energies
• The rotational energies
• The vibrational energies

### Translational energies

The translational energy of the molecule is simply given by the kinetic energy expression:

$E_{trans}=frac{1}{2}mv^2$

where m is the mass of the molecule and v is its velocity.

### Rotational energies

Classically, the kinetic energy of rotation is The kinetic energy of an object is the extra energy which it possesses due to its motion. ...

$E_{rot} = frac{L^2}{2 I} ,$
where
$L ,$ is the angular momentum
$I ,$ is the moment of inertia of the molecule

For microscopic, atomic-level systems like a molecule, angular momentum can only have specific discrete values given by This gyroscope remains upright while spinning due to its angular momentum. ... Moment of inertia, also called mass moment of inertia and, sometimes, the angular mass, (SI units kg mÂ², Former British units slug ft2), is the rotational analog of mass. ...

$L^2 = l(l+1) hbar^2 ,$
where l is a positive integer and $hbar$ is Planck's reduced constant.

Also, for a diatomic molecule the moment of inertia is A commemoration plaque for Max Planck on his discovery of Plancks constant, in front of Humboldt University, Berlin. ...

$I = mu r_{0}^2 ,$
where
$mu ,$ is the reduced mass of the molecule and
$r_{0} ,$ is the average distance between the two atoms in the molecule.

So, substituting the angular momentum and moment of inertia into Erot, the rotational energy levels of a diatomic molecule are given by: Reduced mass is an algebraic term of the form that simplifies an equation of the form The reduced mass is typically used as a relationship between two system elements in parallel, such as resistors; whether these be in the electrical, thermal, hydraulic, or mechanical domains. ...

 $E_{rot} = frac{l(l+1) hbar^2}{2 mu r_{0}^2} l=0,1,2,... ,$

### Vibrational energies

Another way a diatomic molecule can move is to have each atom oscillate - or vibrate - along a line (the bond) connecting the two atoms. The vibrational energy is approximately that of a quantum harmonic oscillator: Look up vibration in Wiktionary, the free dictionary. ... The quantum harmonic oscillator is the quantum mechanical analogue of the classical harmonic oscillator. ...

 $E_{vib} = left(n+frac{1}{2} right)hf n=0,1,2,... ,$
where
n is an integer
h is Planck's constant and
f is the frequency of the vibration.

A commemoration plaque for Max Planck on his discovery of Plancks constant, in front of Humboldt University, Berlin. ... FreQuency is a music video game developed by Harmonix and published by SCEI. It was released in November 2001. ...

### Comparison between rotational and vibrational energy spacings

The lowest rotational energy level of a diatomic molecule occurs for l = 0 and gives Erot = 0. For O2, the next highest quantum level (l = 1) has an energy of roughly:

 $E_{rot,1} ,$ $= frac{hbar^2}{2 m_{O_{2}} r_{0}^2} ,$ $approx frac{left(1.05 times 10^{-34} mathrm{Jcdot s} right)^2}{2 left(27 times 10^{-27} mathrm{kg} right) left(10^{-10} mathrm{m} right)^2} ,$ $approx 2 times 10^{-23} mathrm{J} ,$

This spacing between the lowest two rotational energy levels of O2 is comparable to that of a photon in the microwave region of the electromagnetic spectrum. In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. ... This article is about the type of Electromagnetic radiation. ...

The lowest vibrational energy level occurs for n = 0, and a typical vibration frequency is 5 x 1013 Hz. Doing a calculation similar to that above gives:

$E_{vib,0} approx 3 times 10^{-21} mathrm{J} ,$.

So the spacing, and the energy of a typical spectroscopic transition, between vibrational energy levels is about 100 times greater than that of a typical transition between rotational energy levels.

ERROR: ASTATINE IS NOT DIATOMIC. (See HALOGENS page on Wikipedia)

Astatine is NOT diatomic

• Huber, K. P. and Herzberg, G. (1979). Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules. New York: Van Nostrand: Reinhold.
• Tipler, Paul (1998). Physics For Scientists and Engineers : Vol. 1 (4th ed.). W. H. Freeman. ISBN 1-57259-491-8.

## Notes and references

1. ^ Huber, K. P. and Herzberg, G. (1979). Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules. New York: Van Nostrand: Reinhold.
2. ^ For details on astatine, see Emsley, J. (1989). The Elements. Oxford: Clarendon Press, 22-23.
3. ^ Ihde, Aaron J. (1961). "The Karlsruhe Congress: A centennial retrospective.". Journal of Chemical Education 38: 83-86. Retrieved on 2007-08-24.

Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era. ... is the 236th day of the year (237th in leap years) in the Gregorian calendar. ...

Results from FactBites:

 Molecule - Wikipedia, the free encyclopedia (877 words) A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. Although the concept of molecule was first introduced in 1811 by Avogadro, the existence of molecules was still an open debate in the chemistry community until the work of Perrin (1911). Molecular spectroscopy is the study of the response (spectrum) of a molecule to a signal characterized by its tunable frequency (or, according to Planck formula, its energy).
 Diatomic - Wikipedia, the free encyclopedia (154 words) Diatomic molecules are molecules formed of exactly two atoms, of the same or different chemical elements. Diatomic elements are those that almost exclusively exist as diatomic molecules, known as homonuclear diatomic molecules, in their natural elemental state, not chemically bonded with other elements. The diatomic elements are hydrogen, nitrogen, oxygen, and the halogens: fluorine, chlorine, bromine, iodine, and astatine.
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