Dear Student,
For calculating the degree of freedom of a molecule there is a formula, N = 3A - R
where N is the number of degree of freedom of the system
A is number of particles in the system
R is the number of independent relations among the particles.
There are two types of triatomic gases (i) Linear triatomic gas (ii) Non-linear triatomic gas
(i) Linear triatomic gas:
A = 3 and R = 2
N = 3x3 - 2 = 9 - 2
N = 7
(ii) Non-linear triatomic gas
A = 3 and R = 3
N = 3x3 - 3 = 9 - 3
N = 6
Regards,