Triatomic molecule

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

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

$\omega_{s2}=\sqrt{\frac{2k_2M}{m_Am_B}}$

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.

Types

Homonuclear

Homonuclear triatomic molecules contain three of the same kind of atom.

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. ⁴He₃, the helium trimer is only weakly bound by van der Waals force and is in a Efimov state. Trisulfur (S₃) is analogous to ozone.