The vibrational modes of a triatomic molecule can be determined in specific cases.
A symmetric linear molecule ABA can perform:
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.
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. Trisulfur (S3) is analogous to ozone.
All triatomic molecules may be classified as possessing either a linear, bent, or cyclic geometry.
Xenon difluoride (XeF2) is one of the rare examples of a linear triatomic molecule possessing non-bonded pairs of electrons on the central atom.