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.

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

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

• Symmetric longitudinal vibrations with frequency

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

• Symmetric transversal vibrations with frequency

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

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

Trihydrogen cation

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.