Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Triatomic molecule

Triatomic molecules are formed by three atoms. Examples include H2O, CO2, HCN etc.

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
 * $$\omega_a=\sqrt{\frac{k_1M}{m_Am_B}}$$


 * Symmetric longitudinal vibrations with frequency
 * $$\omega_{s1}=\sqrt{\frac{k_1}{m_A}}$$


 * Symmetric transversal vibrations with frequency
 * $$\omega_{s2}=\sqrt{\frac{2k_2M}{m_A}}$$

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
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.