Stretching vibrations:

In stretching vibrations, an atoms move along the bond axis so which the bond length increases or decreases at regular intervals. The stretching vibrations again are of two kinds- symmetric and antisymmetric stretching. Within symmetric stretching of a triatomic molecule, both the bonds linked to a general atom either elongate or contract concurrently.  In case of antisymmetric stretching, if one bond is lengthened, the other bond is shortened or vice-versa. The stretching vibrations along with the bending vibrations (discussed below) are displays in Figures, correspondingly for linear and angular molecules of the AB₂ type. The arrows attached to each atom show the direction of its motion during half of the vibration; it reverses in the other half.

Figure: Normal modes of vibrations of a linear triatomic molecule, AB₂

Figure: Normal modes of vibrations of an angular triatomic molecule, AB₂