Triatomic molecule

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.

Carbon dioxide

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

${\displaystyle \omega _{a}={\sqrt {\frac {k_{1}M}{m_{A}m_{B}}}}}$

• Symmetric longitudinal vibrations with frequency

${\displaystyle \omega _{s1}={\sqrt {\frac {k_{1}}{m_{A}}}}}$

• Symmetric transversal vibrations with frequency

${\displaystyle \omega _{s2}={\sqrt {\frac {2k_{2}M}{m_{A}m_{B}}}}}$

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.^{[further explanation 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