Num of vibrational modes in a linear triatomic moleculeis
For polyatomic molecules, the vibrations are complicated. For example, water which is a non-linear tri-atomic molecule has three allowed vibration as shown in Fig 1 [3N-6 = 3 ; N is the no of atoms in molecule]. For carbondioxide, a linear tri-atomic molecule, the number of allowed vibration is 3N-5 = 4. For CO2, there are two bending vibrations; one as shown in the figure in which the atoms moves in plane of the paper and the other in which oxygen atoms moves simultaneously into and out of the plane is not shown here. These two vibrations are identical except in directions.