Triatomic molecule

{{Short description|Molecules made of three atoms}} thumb|right|Carbon dioxide '''Triatomic molecules''' are molecules composed of three atoms, of either the same or different chemical elements. Examples include H₂O, CO₂ (pictured), HCN, O₃ (ozone) and NO₂.

==Molecular vibrations==

The vibrational modes of a triatomic molecule can be determined in specific cases.

=== Symmetric linear molecules === A symmetric linear molecule ABA can perform: * Antisymmetric longitudinal vibrations with frequency :<math>\omega_a=\sqrt{\frac{k_1M}{m_Am_B}}</math> * Symmetric longitudinal vibrations with frequency :<math>\omega_{s1}=\sqrt{\frac{k_1}{m_A}}</math> * Symmetric transversal vibrations with frequency :<math>\omega_{s2}=\sqrt{\frac{2k_2M}{m_Am_B}}</math> In the previous formulas, ''M'' is the total mass of the molecule, ''m_A'' and ''m_B'' are the masses of the elements A and B, ''k''₁ and ''k''₂ are the spring constants of the molecule along its axis and perpendicular to it.

==Types==

===Homonuclear=== thumb|right|Ozone, O₃ thumb|right|Trihydrogen cation, H₃⁺

Homonuclear triatomic molecules contain three of the same kind of atom. That molecule will be an allotrope of that element.

Ozone, O₃ is an example of a triatomic molecule with all atoms the same. Triatomic hydrogen, H₃, is unstable and breaks up spontaneously. H₃⁺, the trihydrogen cation is stable by itself and is symmetric. ⁴He₃, the helium trimer is only weakly bound by van der Waals force and is in an Efimov state.<ref name=Kunitski15>{{cite journal |last1=Kunitski |first1=M. |last2=Zeller |first2=S. |last3=Voigtsberger |first3=J. |last4=Kalinin |first4=A. |last5=Schmidt |first5=L. P. H. |last6=Schoffler |first6=M. |last7=Czasch |first7=A. |last8=Schollkopf |first8=W. |last9=Grisenti |first9=R. E. |last10=Jahnke |first10=T. |last11=Blume |first11=D. |last12=Dorner |first12=R. |title=Observation of the Efimov state of the helium trimer |journal=Science |date=30 April 2015 |volume=348 |issue=6234 |pages=551–555 |doi=10.1126/science.aaa5601 |pmid=25931554 |arxiv=1512.02036 |bibcode=2015Sci...348..551K|s2cid=102090236 }}</ref> Trisulfur (S₃) is analogous to ozone.

==Geometry== All triatomic molecules may be classified as possessing either a linear, bent, or cyclic geometry.{{expansion needed|date=June 2022}}

===Linear=== Linear triatomic molecules owe their geometry to their '''''sp''''' or '''''sp³d''''' hybridised central atoms. Well-known linear triatomic molecules include carbon dioxide (CO₂) and hydrogen cyanide (HCN).

Xenon difluoride (XeF₂) is one of the rare examples of a linear triatomic molecule possessing non-bonded pairs of electrons on the central atom.

===Bent===

===Cyclic===

==References== {{Reflist}}

==External links== *{{Commons category-inline|Triatomic molecules}}

{{Triatomic elements}} {{Molecules detected in outer space}}

Category:Molecular vibration Category:Triatomic molecules