Triatomic molecules aremolecules composed of threeatoms, of either the same or differentchemical elements. Examples includeH₂O,CO₂ (pictured),HCN, O₃ (ozone) andNO₂.

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

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 andm_B are the masses of the elements A and B,k₁ andk₂ are the spring constants of the molecule along its axis and perpendicular to it.

Homonuclear triatomic molecules contain three of the same kind of atom. That molecule will be anallotrope of thatelement.

Ozone, O₃ is an example of a triatomic molecule with all atoms the same.Triatomic hydrogen, H₃, is unstable and breaks up spontaneously. H₃⁺, thetrihydrogen cation is stable by itself and is symmetric.⁴He₃, thehelium trimer is only weakly bound byvan der Waals force and is in anEfimov state.^[1]Trisulfur (S₃) is analogous to ozone.

All triatomic molecules may be classified as possessing either alinear,bent, orcyclic geometry.^{[further explanation needed]}

Linear triatomic molecules owe their geometry to theirsp orsp³d hybridised central atoms. Well-known linear triatomic molecules includecarbon dioxide (CO₂) andhydrogen 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.

• Media related toTriatomic molecules at Wikimedia Commons

• Molecular vibration
• Triatomic molecules

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