INFRA RED ABSORPTIONS Different covalent bonds have different strengths due to the masses of different atoms at either end of the bond.  As a result, they vibrate at different frequencies.  The frequency of vibration can be found by detecting when the molecules absorb electro-magnetic radiation. Various types of vibration are possible.  Bending and stretching are two examples and are found in water molecules.  Each occurs at a different frequency. Symmetric       Bending     Asymmetric stretching      stretching Bending in a carbon dioxide molecule A water molecule absorbing radiation