Attachment molecular degree of freedom
The degree of freedom is the number of independent coordinates that can be written in the motion equation of an object. Monoatomic molecules have three degrees of freedom, diatomic molecules and triatomic molecules are equivalent to rigid bodies without considering vibration, with five degrees of freedom (three stages and two rotations) and six degrees of freedom (three stages and three rotations) respectively. After considering the vibration, diatomic molecules add 1 and triatomic molecules add 2.
(1) monoatomic molecules: He, Ne, Ar and other molecules have only one atom and can be regarded as free particles, so they have three translational degrees of freedom i = t = 3.
(2) Rigid diatomic molecules, such as hydrogen, oxygen, nitrogen, carbon monoxide, carbon monoxide, etc. The distance between two atoms remains the same. Just as two particles are connected by a thin rigid rod without mass (like a dumbbell), three independent coordinates (x, y, z) are needed to determine the spatial position of its center of mass O'; Two independent coordinates (such as α, β) are needed to determine the spatial orientation of the particle line, and the rotation of two particles around the line is meaningless. Therefore, rigid diatomic molecules have not only three translational degrees of freedom, but also two rotational degrees of freedom, and the total * * * has five degrees of freedom i = t+r =3+2 = 5.
(3) Rigid triatomic or polyatomic molecules, such as H2O and ammonia, can be regarded as free rigid bodies as long as the atoms are not arranged in a straight line. * * * has six degrees of freedom, and i = t+r = 3+3 = 6.