The number of co-ordinates or variables which are required to define a system completely are called degree of freedom.

Degree of freedom is calculated by :-

F = 3N – R

N= no. of particles

                R= Relation between the particles.

For monatomic gas :-                                                                      F = 3

                                                                                                                As N = 1 & R = 0

                                                                                                                So f = 3 x 1- 0 =3.

For diatomic gas:-                                          

                                  F = 5  as N = 2 & R = 1

                                     So F = 3 x2 -1 = 5.

For Triatomic gas :-                  linear molecule    

                                                                      F=7

                                                                   An N = 3 & R =2

                                                                  F= 3 x 3 -2 =7

                                                Non linear molecules

                                                                                F= 6

                                                                                As N = 3 & R =3 ,  F = 3 x3 -3 = 6.