Triatomic molecule

Short description: Molecules made of three atoms

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]\displaystyle{ \omega_a=\sqrt{\frac{k_1M}{m_Am_B}} }[/math]

Symmetric longitudinal vibrations with frequency

[math]\displaystyle{ \omega_{s1}=\sqrt{\frac{k_1}{m_A}} }[/math]

Symmetric transversal vibrations with frequency

[math]\displaystyle{ \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, k₁ and k₂ are the spring constants of the molecule along its axis and perpendicular to it.

Types

Homonuclear

Ozone, O₃

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.^[1] Trisulfur (S₃) is analogous to ozone.

Geometry

All triatomic molecules may be classified as possessing either a linear, bent, or cyclic geometry.Template:Expansion needed

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

↑ Kunitski, M.; Zeller, S.; Voigtsberger, J.; Kalinin, A.; Schmidt, L. P. H.; Schoffler, M.; Czasch, A.; Schollkopf, W. et al. (30 April 2015). "Observation of the Efimov state of the helium trimer". Science 348 (6234): 551–555. doi:10.1126/science.aaa5601. PMID 25931554. Bibcode: 2015Sci...348..551K.