Triatomic molecule

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Carbon dioxide

Triatomic molecules are molecules composed of three atoms, of either the same or different chemical elements. Examples include H2O, CO2 (pictured) and HCN.

Molecular vibrations[edit]

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

Symmetric linear molecules[edit]

A symmetric linear molecule ABA can perform:

  • Antisymmetric longitudinal vibrations with frequency
\omega_a=\sqrt{\frac{k_1M}{m_Am_B}}
  • Symmetric longitudinal vibrations with frequency
\omega_{s1}=\sqrt{\frac{k_1}{m_A}}
  • Symmetric transversal vibrations with frequency
\omega_{s2}=\sqrt{\frac{2k_2M}{m_Am_B}}

In the previous formulas, M is the total mass of the molecule, mA and mB 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[edit]

Ozone

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.