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 (H2) or oxygen (O2), 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
(H2), nitrogen (N2), oxygen (O2), fluorine (F2), and chlorine
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
At slightly elevated temperatures, the halogens bromine (Br2) and
iodine (I2) also form diatomic gases. All halogens have been
observed as diatomic molecules, except for astatine, which is
The mnemonics BrINClHOF, pronounced "Brinklehof", and
HONClBrIF, pronounced "Honkelbrif", 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, P2. Sulfur vapor is mostly
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
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
Hundreds of diatomic molecules have been identified in the
environment of the Earth, in the laboratory, and in interstellar
space. About 99% of the
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
, 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)
is the total electronic spin quantum number,
is the total electronic angular momentum quantum number along the internuclear axis, and
is the vibrational quantum number.
takes on values 0, 1, 2, …, which are represented by the electronic state symbols
,…. 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
) of diatomic nitrogen (N2), the most abundant gas in the Earth's atmosphere. In the table, the subscripts and superscripts after
give additional quantum mechanical details about the electronic state.
State Energy (
displaystyle T_ 0
, cm−1) See note below
displaystyle X^ 1 Sigma _ g ^ +
displaystyle A^ 3 Sigma _ u ^ +
displaystyle B^ 3 Pi _ g
displaystyle W^ 3 Delta _ u
displaystyle B'^ 3 Sigma _ u ^ -
displaystyle a'^ 1 Sigma _ u ^ -
displaystyle a^ 1 Pi _ g
displaystyle w^ 1 Delta _ u
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 × 10−25 Joule metres, and then multiplying by a further factor of 100 to convert from cm−1 to m−1. 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, N2
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,
. These are classified into distinct sub-band branches, depending on the change in
branch corresponds to
Δ J = + 1
displaystyle Delta J=+1
Δ J = − 1
displaystyle Delta J=-1
, and the
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:
t r a n s
displaystyle E_ trans = frac 1 2 mv^ 2
is the mass of the molecule and
is its velocity. Rotational energies Classically, the kinetic energy of rotation is
r o t
displaystyle E_ rot = frac L^ 2 2I ,
is the angular momentum
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 ( l + 1 )
displaystyle L^ 2 =l(l+1)hbar ^ 2 ,
is a non-negative integer and
is the reduced Planck constant.
Also, for a diatomic molecule the moment of inertia is
I = μ
displaystyle I=mu r_ 0 ^ 2 ,
displaystyle mu ,
is the reduced mass of the molecule and
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 Erot, the rotational energy levels of a diatomic molecule are given by:
r o t
l ( l + 1 )
l = 0 , 1 , 2 , . . .
displaystyle E_ rot = frac l(l+1)hbar ^ 2 2mu 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:
v i b
ℏ ω n = 0 , 1 , 2 , . . . .
displaystyle E_ vib =left(n+ frac 1 2 right)hbar omega n=0,1,2,....,
is an integer
is the reduced Planck constant and
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
^ Hammond, C.R. (2012). "Section 4: Properties of the Elements and
Inorganic Compounds". Handbook of Chemistry and Physics (PDF).
^ Emsley, J. (1989). The Elements. Oxford: Clarendon Press.
^ Whitten, Kenneth W.; Davis, Raymond E.; Peck, M. Larry; Stanley,
George G. (2010). Chemistry (9th ed.). Brooks/Cole, Cengage Learning.
^ Sherman, Alan (1992). Chemistry and Our Changing World. Prentice
Hall. p. 82. ISBN 9780131315419.
^ Huber, K. P.; Herzberg, G. (1979). Molecular Spectra and Molecular
Structure IV. Constants of Diatomic Molecules. New York: Van Nostrand:
^ Langford, Cooper Harold; Beebe, Ralph Alonzo (1995-01-01). The
Development of Chemical Principles. Courier Corporation.
^ 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
^ a b Gilmore, Forrest R.; Laher, Russ R.; Espy, Patrick J. (1992).
"Franck-Condon Factors, r-Centroids, Electronic Transition Moments,
and Einstein Coefficients for Many
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.
Hyperphysics – Rotational Spectra of Rigid Rotor Molecules Hyperphysics – Quantum Harmonic Oscillator 3D Chem – Chemistry, Structures, and 3D Molecules IUMSC – Indiana University Molecular Structure Center
v t e
Coordination number 2
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
Coordination number 8
Coordination number 9
Tricapped trigonal prismatic Capped square antiprismatic
v t e
Diatomic chemical elements
H 2 N 2 O 2 F 2 Cl 2 Br 2 I 2
C 2 P 2 S 2 Li 2
v t e
Molecules detected in outer space
Acetonitrile Cyanobutadiynyl radical E-Cyanomethanimine Cyclopropenone Diacetylene Ethylene Formamide HC4N Ketenimine Methanethiol Methanol Methyl isocyanide Pentynylidyne Propynal Protonated cyanoacetylene
Acetic acid Aminoacetonitrile Cyanoallene Ethanimine Glycolaldehyde Heptatrienyl radical Hexapentaenylidene Methylcyanoacetylene Methyl formate Propenal
Acetamide Cyanohexatriyne Cyanotriacetylene Dimethyl ether Ethanol Methyldiacetylene Octatetraynyl radical Propene Propionitrile
Ten atoms or more
Ethyl methyl ether
Atomic and molecular astrophysics
Diffuse interstellar band
Earliest known life forms
Extraterrestrial liquid water
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