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.

The vibrational 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 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.

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.

All triatomic molecules may be classified as possessing either a linear, bent, or cyclic geometry.^{[further explanation needed]}

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.

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