Diatomic molecule - wikipedia Diatomic molecule A space - filling model of the diatomic molecule dinitrogen, N Diatomic molecules are molecules composed of only two atoms, of the same or different chemical elements. The prefix di - is of Greek origin, meaning `` two ''. If a diatomic molecule consists of two atoms of the same element, such as hydrogen ( H ) or oxygen ( O ), then it is said to be homonuclear. Otherwise, if a diatomic molecule consists of two different atoms, such as carbon monoxide ( CO ) or nitric oxide ( NO ), the molecule is said to be heteronuclear. A periodic table showing the elements that exist as homonuclear diatomic molecules under typical laboratory conditions. The only chemical elements that form stable homonuclear diatomic molecules at standard temperature and pressure ( STP ) ( or typical laboratory conditions of 1 bar and 25 ° C ) are the gases hydrogen ( H ), nitrogen ( N ), oxygen ( O ), fluorine ( F ), and chlorine ( Cl ). The noble gases ( helium, neon, argon, krypton, xenon, and radon ) are also gases at STP, but they are monatomic. The homonuclear diatomic gases and noble gases together are called `` elemental gases '' or `` molecular gases '', to distinguish them from other gases that are chemical compounds. At slightly elevated temperatures, the halogens bromine ( Br ) and iodine ( I ) also form diatomic gases. All halogens have been observed as diatomic molecules, except for astatine, which is uncertain. The mnemonics BrINClHOF, pronounced `` Brinklehof '', and HONClBrIF, pronounced `` Honkelbrif '', and HOFBrINCl ( pronouced as Hofbrinkle ) have been coined to aid recall of the list of diatomic elements. Other elements form diatomic molecules when evaporated, but these diatomic species repolymerize when cooled. Heating ( `` cracking '' ) elemental phosphorus gives diphosphorus, P. Sulfur vapor is mostly disulfur ( S ). Dilithium ( Li ) is known in the gas phase. Ditungsten ( W ) and dimolybdenum ( Mo ) form with sextuple bonds in the gas phase. The bond in a homonuclear diatomic molecule is non-polar. Dirubidium is diatomic. Contents 1 Heteronuclear molecules 2 Occurrence 3 Molecular geometry 4 Historical significance 5 Excited electronic states 6 Energy levels 6.1 Translational energies 6.2 Rotational energies 6.3 Vibrational energies 6.4 Comparison between rotational and vibrational energy spacings 7 Hund 's cases 8 See also 9 References 10 Further reading 11 External links Heteronuclear molecules All other diatomic molecules are chemical compounds of two different elements. Many elements can combine to form heteronuclear diatomic molecules, depending on temperature and pressure. Common examples include the gases carbon monoxide ( CO ), nitric oxide ( NO ), and hydrogen chloride ( HCl ). Many 1 : 1 binary compounds are not normally considered diatomic because they are polymeric at room temperature, but they form diatomic molecules when evaporated, for example gaseous MgO, SiO, and many others. Occurrence Hundreds of diatomic molecules have been identified in the environment of the Earth, in the laboratory, and in interstellar space. About 99 % of the Earth 's atmosphere is composed of two species of diatomic molecules : nitrogen ( 78 % ) and oxygen ( 21 % ). The natural abundance of hydrogen ( H ) in the Earth 's atmosphere is only of the order of parts per million, but H is the most abundant diatomic molecule in the universe. The interstellar medium is, indeed, dominated by hydrogen atoms. Molecular geometry Main article : Molecular geometry 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 water 's formula to be HO, giving the atomic weight of oxygen as eight 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. 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, partly due to the belief that atoms of one element would have no chemical affinity toward atoms of the same element, and also partly due to apparent exceptions to Avogadro 's law that were not explained until later in terms of dissociating molecules. 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 prerequisite for the discovery of the periodic law by Dmitri Mendeleev and Lothar Meyer. Excited electronic states Diatomic molecules are normally in their lowest or ground state, which conventionally is also known as the X ( displaystyle X ) state. When a gas of diatomic molecules is bombarded by energetic electrons, some of the molecules may be excited to higher electronic states, as occurs, for example, in the natural aurora ; high - altitude nuclear explosions ; and rocket - borne electron gun experiments. Such excitation can also occur when the gas absorbs light or other electromagnetic radiation. The excited states are unstable and naturally relax back to the ground state. Over various short time scales after the excitation ( typically a fraction of a second, or sometimes longer than a second if the excited state is metastable ), transitions occur from higher to lower electronic states and ultimately to the ground state, and in each transition results a photon is emitted. This emission is known as fluorescence. Successively higher electronic states are conventionally named A ( displaystyle A ), B ( displaystyle B ), C ( displaystyle C ), etc. ( but this convention is not always followed, and sometimes lower case letters and alphabetically out - of - sequence letters are used, as in the example given below ). The excitation energy must be greater than or equal to the energy of the electronic state in order for the excitation to occur. In quantum theory, an electronic state of a diatomic molecule is represented by 2 S + 1 Λ ( v ) ( displaystyle ^ ( 2S + 1 ) Lambda ( v ) ) where S ( displaystyle S ) is the total electronic spin quantum number, Λ ( displaystyle Lambda ) is the total electronic angular momentum quantum number along the internuclear axis, and v ( displaystyle v ) is the vibrational quantum number. Λ ( displaystyle Lambda ) takes on values 0, 1, 2,..., which are represented by the electronic state symbols Σ ( displaystyle Sigma ), Π ( displaystyle Pi ), Δ ( displaystyle Delta ),.... For example, the following table lists the common electronic states ( without vibrational quantum numbers ) along with the energy of the lowest vibrational level ( v = 0 ( displaystyle v = 0 ) ) of diatomic nitrogen ( N ), the most abundant gas in the Earth 's atmosphere. In the table, the subscripts and superscripts after Λ ( displaystyle Lambda ) give additional quantum mechanical details about the electronic state. State Energy ( T 0 ( displaystyle T_ ( 0 ) ), cm ) See note below X 1 Σ g + ( displaystyle X ^ ( 1 ) Sigma _ ( g ) ^ ( + ) ) 0.0 A 3 Σ u + ( displaystyle A ^ ( 3 ) Sigma _ ( u ) ^ ( + ) ) 49754.8 B 3 Π g ( displaystyle B ^ ( 3 ) Pi _ ( g ) ) 59306.8 W 3 Δ u ( displaystyle W ^ ( 3 ) Delta _ ( u ) ) 59380.2 B ′ 3 Σ u − ( displaystyle B ' ^ ( 3 ) Sigma _ ( u ) ^ ( - ) ) 65851.3 a ′ 1 Σ u − ( displaystyle a ' ^ ( 1 ) Sigma _ ( u ) ^ ( - ) ) 67739.3 a 1 Π g ( displaystyle a ^ ( 1 ) Pi _ ( g ) ) 68951.2 w 1 Δ u ( displaystyle w ^ ( 1 ) Delta _ ( u ) ) 71698.4 Note : The `` energy '' units in the above table are actually the reciprocal of the wavelength of a photon emitted in a transition to the lowest energy state. The actual energy can be found by multiplying the given statistic by the product of c ( the speed of light ) and h ( Planck 's constant ), i.e., about 1.99 × 10 Joule metres, and then multiplying by a further factor of 100 to convert from cm to m. The aforementioned fluorescence occurs in distinct regions of the electromagnetic spectrum, called `` emission bands '' : each band corresponds to a particular transition from a higher electronic state and vibrational level to a lower electronic state and vibrational level ( typically, many vibrational levels are involved in an excited gas of diatomic molecules ). For example, N A ( displaystyle A ) - X ( displaystyle X ) emission bands ( a.k.a. Vegard - Kaplan bands ) are present in the spectral range from 0.14 to 1.45 μm ( micrometres ). A given band can be spread out over several nanometers in electromagnetic wavelength space, owing to the various transitions that occur in the molecule 's rotational quantum number, J ( displaystyle J ). These are classified into distinct sub-band branches, depending on the change in J ( displaystyle J ). The R ( displaystyle R ) branch corresponds to Δ J = + 1 ( displaystyle Delta J = + 1 ), the P ( displaystyle P ) branch to Δ J = − 1 ( displaystyle Delta J = - 1 ), and the Q ( displaystyle Q ) branch to Δ J = 0 ( displaystyle Delta J = 0 ). Bands are spread out even further by the limited spectral resolution of the spectrometer that is used to measure the spectrum. The spectral resolution depends on the instrument 's point spread function. Energy levels The molecular term symbol is a shorthand expression of the angular momenta that characterize the electronic quantum states of a diatomic molecule, which are eigenstates of the electronic molecular Hamiltonian. It is also convenient, and common, to represent a diatomic molecule as two point masses 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, rotational, and vibrational energies. Translational energies The translational energy of the molecule is given by the kinetic energy expression : E t r a n s = 1 2 m v 2 ( displaystyle E_ ( trans ) = ( \frac ( 1 ) ( 2 ) ) mv ^ ( 2 ) ) where m ( displaystyle m ) is the mass of the molecule and v ( displaystyle v ) is its velocity. Rotational energies Classically, the kinetic energy of rotation is E r o t = L 2 2 I ( displaystyle E_ ( rot ) = ( \frac ( L ^ ( 2 ) ) ( 2I ) ) , ) where L ( displaystyle L , ) is the angular momentum I ( displaystyle 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 L 2 = l ( l + 1 ) ħ 2 ( displaystyle L ^ ( 2 ) = l ( l + 1 ) hbar ^ ( 2 ) , ) where l ( displaystyle l ) is a non-negative integer and ħ ( displaystyle hbar ) is the reduced Planck constant. Also, for a diatomic molecule the moment of inertia is I = μ r 0 2 ( displaystyle I = mu r_ ( 0 ) ^ ( 2 ) , ) where μ ( displaystyle mu , ) is the reduced mass of the molecule and r 0 ( displaystyle r_ ( 0 ) , ) is the average distance between the centers of the two atoms in the molecule. So, substituting the angular momentum and moment of inertia into E, the rotational energy levels of a diatomic molecule are given by : E r o t = l ( l + 1 ) ħ 2 2 μ r 0 2 l = 0, 1, 2,... ( displaystyle E_ ( rot ) = ( \frac ( l ( l + 1 ) hbar ^ ( 2 ) ) ( 2 mu r_ ( 0 ) ^ ( 2 ) ) ) l = 0, 1, 2,... , ) Vibrational energies Another type of motion of a diatomic molecule is for each atom to oscillate -- or vibrate -- along the line connecting the two atoms. The vibrational energy is approximately that of a quantum harmonic oscillator : E v i b = ( n + 1 2 ) ħ ω n = 0, 1, 2,... ( displaystyle E_ ( vib ) = left ( n+ ( \frac ( 1 ) ( 2 ) ) \right ) hbar omega \n = 0, 1, 2,... , ) where n ( displaystyle n ) is an integer ħ ( displaystyle hbar ) is the reduced Planck constant and ω ( displaystyle omega ) is the angular frequency of the vibration. Comparison between rotational and vibrational energy spacings 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. Hund 's cases Main article : Hund 's cases The good quantum numbers for a diatomic molecule, as well as good approximations of rotational energy levels, can be obtained by modeling the molecule using Hund 's cases. See also Symmetry of diatomic molecules AXE method Octatomic element Covalent bond Industrial gas References Jump up ^ Hammond, C.R. ( 2012 ). `` Section 4 : Properties of the Elements and Inorganic Compounds ''. Handbook of Chemistry and Physics ( PDF ). Jump up ^ Emsley, J. ( 1989 ). The Elements. Oxford : Clarendon Press. pp. 22 -- 23. Jump up ^ Whitten, Kenneth W. ; Davis, Raymond E. ; Peck, M. Larry ; Stanley, George G. ( 2010 ). Chemistry ( 9th ed. ). Brooks / Cole, Cengage Learning. pp. 337 -- 338. Jump up ^ Sherman, Alan ( 1992 ). Chemistry and Our Changing World. Prentice Hall. p. 82. ISBN 9780131315419. Jump up ^ Huber, K.P. ; Herzberg, G. ( 1979 ). Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules. New York : Van Nostrand : Reinhold. Jump up ^ Langford, Cooper Harold ; Beebe, Ralph Alonzo ( 1995 - 01 - 01 ). The Development of Chemical Principles. Courier Corporation. ISBN 9780486683591. Jump up ^ Ihde, Aaron J. ( 1961 ). `` The Karlsruhe Congress : A centennial retrospective ''. Journal of Chemical Education. 38 ( 2 ) : 83 -- 86. Bibcode : 1961JChEd... 38... 83I. doi : 10.1021 / ed038p83. Retrieved 2007 - 08 - 24. ^ Jump up to : Gilmore, Forrest R. ; Laher, Russ R. ; Espy, Patrick J. ( 1992 ). `` Franck - Condon Factors, r - Centroids, Electronic Transition Moments, and Einstein Coefficients for Many Nitrogen and Oxygen Band Systems ''. Journal of Physical and Chemical Reference Data. 21 ( 5 ) : 1005 -- 1107. Bibcode : 1992JPCRD... 21.1005 G. doi : 10.1063 / 1.555910. Jump up ^ Laher, Russ R. ; Gilmore, Forrest R. ( 1991 ). `` Improved Fits for the Vibrational and Rotational Constants of Many States of Nitrogen and Oxygen ''. Journal of Physical and Chemical Reference Data. 20 ( 4 ) : 685 -- 712. Bibcode : 1991JPCRD... 20... 685L. doi : 10.1063 / 1.555892. Jump up ^ Levine, Ira N. ( 1975 ), Molecular Spectroscopy, John Wiley & Sons, pp. 508 -- 9, ISBN 0 - 471 - 53128 - 6 Further reading Huber, K.P. ; 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. External links Hyperphysics -- Rotational Spectra of Rigid Rotor Molecules Hyperphysics -- Quantum Harmonic Oscillator 3D Chem -- Chemistry, Structures, and 3D Molecules IUMSC -- Indiana University Molecular Structure Center Molecular geometry Coordination number 2 Linear Bent Coordination number 3 Trigonal planar Trigonal pyramidal T - shaped Coordination number 4 Tetrahedral Square planar Seesaw Coordination number 5 Trigonal bipyramidal Square pyramidal Pentagonal planar Coordination number 6 Octahedral Trigonal prismatic Pentagonal pyramidal Distorted octahedral Coordination number 7 Pentagonal bipyramidal Coordination number 8 Square antiprismatic Coordination number 9 Tricapped trigonal prismatic Capped square antiprismatic Diatomic chemical elements Normal Cl Br Others Li Rb Molecules detected in outer space Molecules Diatomic Aluminium monochloride Aluminium monofluoride Aluminium monoxide Argonium Carbon monophosphide Carbon monosulfide Carbon monoxide Carborundum Cyanogen radical Diatomic carbon Fluoromethylidynium Hydrogen chloride Hydrogen fluoride Hydrogen ( molecular ) Hydroxyl radical Iron ( II ) oxide Magnesium monohydride cation Methylidyne radical Nitric oxide Nitrogen ( molecular ) Nitrogen monohydride Nitrogen sulfide Oxygen ( molecular ) Phosphorus monoxide Phosphorus mononitride Potassium chloride Silicon carbide Silicon mononitride Silicon monoxide Silicon monosulfide Sodium chloride Sodium iodide Sulfur monohydride Sulfur monoxide Titanium oxide Triatomic Aluminium hydroxide Aluminium isocyanide Amino radical Carbon dioxide Carbonyl sulfide CCP radical Chloronium Diazenylium Dicarbon monoxide Disilicon carbide Ethynyl radical Formyl radical Hydrogen cyanide ( HCN ) Hydrogen isocyanide ( HNC ) Hydrogen sulfide Hydroperoxyl Iron cyanide Isoformyl Magnesium cyanide Magnesium isocyanide Methylene radical Nitrous oxide Nitroxyl Ozone Phosphaethyne Potassium cyanide Protonated molecular hydrogen Sodium cyanide Sodium hydroxide Silicon carbonitride c - Silicon dicarbide SiNC Sulfur dioxide Thioformyl Thioxoethenylidene Titanium dioxide Tricarbon Water Four atoms Acetylene Ammonia Cyanic acid Cyanoethynyl Cyclopropynylidyne Formaldehyde Fulminic acid HCCN Hydrogen peroxide Hydromagnesium isocyanide Isocyanic acid Isothiocyanic acid Ketenyl Methylene amidogen Methyl radical Propynylidyne Protonated carbon dioxide Protonated hydrogen cyanide Silicon tricarbide Thioformaldehyde Tricarbon monoxide Tricarbon sulfide Thiocyanic acid Five atoms Ammonium ion Butadiynyl Carbodiimide Cyanamide Cyanoacetylene Cyanoformaldehyde Cyanomethyl Cyclopropenylidene Formic acid Isocyanoacetylene Ketene Methane Methoxy radical Methylenimine Propadienylidene Protonated formaldehyde Protonated formaldehyde Silane Silicon - carbide cluster Six atoms Acetonitrile Cyanobutadiynyl radical E-Cyanomethanimine Cyclopropenone Diacetylene Ethylene Formamide HC N Ketenimine Methanethiol Methanol Methyl isocyanide Pentynylidyne Propynal Protonated cyanoacetylene Seven atoms Acetaldehyde Acrylonitrile Vinyl cyanide Cyanodiacetylene Ethylene oxide Hexatriynyl radical Methylacetylene Methylamine Methyl isocyanate Vinyl alcohol Eight atoms Acetic acid Aminoacetonitrile Cyanoallene Ethanimine Glycolaldehyde Heptatrienyl radical Hexapentaenylidene Methylcyanoacetylene Methyl formate Propenal Nine atoms Acetamide Cyanohexatriyne Cyanotriacetylene Dimethyl ether Ethanol Methyldiacetylene Octatetraynyl radical Propene Propionitrile Ten atoms or more Acetone Benzene Benzonitrile Buckminsterfullerene ( C fullerene, buckyball ) C fullerene Cyanodecapentayne Cyanopentaacetylene Cyanotetra - acetylene Ethylene glycol Ethyl formate Methyl acetate Methyl - cyano - diacetylene Methyltriacetylene Propanal n - Propyl cyanide Pyrimidine Deuterated molecules Ammonia Ammonium ion Formaldehyde Formyl radical Heavy water Hydrogen cyanide Hydrogen deuteride Hydrogen isocyanide Methylacetylene Trihydrogen cation Unconfirmed Anthracene Dihydroxyacetone Ethyl methyl ether Glycine Graphene H NCO Linear C Naphthalene cation Phosphine Pyrene Silylidine Related Abiogenesis Astrobiology Astrochemistry Atomic and molecular astrophysics Chemical formula Circumstellar envelope Cosmic dust Cosmic ray Cosmochemistry Diffuse interstellar band Earliest known life forms Extraterrestrial life Extraterrestrial liquid water Forbidden mechanism Helium hydride ion Homochirality Intergalactic dust Interplanetary medium Interstellar medium Photodissociation region Iron -- sulfur world theory Kerogen Molecules in stars Nexus for Exoplanet System Science Organic compound Outer space PAH world hypothesis Panspermia Polycyclic aromatic hydrocarbon ( PAH ) RNA world hypothesis Spectroscopy Tholin Category : Astrochemistry Portal : Astrobiology Retrieved from `` https://en.wikipedia.org/w/index.php?title=Diatomic_molecule&oldid=853093859 '' Categories : Diatomic molecules General chemistry Molecular geometry Stereochemistry Hidden categories : Use dmy dates from July 2013 All articles with unsourced statements Articles with unsourced statements from March 2018 Talk Contents About Wikipedia Bosanski Català Dansk Ελληνικά Español فارسی Français 한국어 Bahasa Indonesia Italiano עברית Kreyòl ayisyen മലയാളം Nederlands 日本 語 Norsk ਪੰਜਾਬੀ Português Русский Simple English Slovenščina Türkçe Українська 中文 16 more Edit links This page was last edited on 2 August 2018, at 11 : 39 ( UTC ). 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Question: which element exists as a diatomic molecule at stp
Answer: hydrogen (H2)