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I was introduced to a formula for finding the DOF of a molecule which was 3N-k and i was told, it was just for translation and rotational degree of freedom. Here N is the no. Of atoms in that molecule, and k is the no. Of independent Criteria(like Bonds). For example, if its a diatomic molecule, then DOF=3(2)-1 which is 5. But by this, when i find for Triatomic Linear molecule DOF=3(3)-2 which is 7. And Visualising the molecule i could find only 5 DOF. So I raised up the question, but I didn't find a satisfactory response. If there are any considerations or something thats missing, please help.

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  • $\begingroup$ Refer to this question $\endgroup$
    – lakhi
    Sep 3 '18 at 8:19
  • $\begingroup$ There must be some undisclosed assumptions underlying your formula. I prefer the explanation at en.wikipedia.org/wiki/… $\endgroup$
    – my2cts
    Apr 25 '20 at 16:54
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Source:https://en.wikipedia.org/wiki/Degrees_of_freedom_(physics_and_chemistry)#Counting_the_Minimum_Number_of_Coordinates_to_Specify_a_Position

Source:https://en.wikipedia.org/wiki/Degrees_of_freedom_(physics_and_chemistry)#Counting_the_Minimum_Number_of_Coordinates_to_Specify_a_Position

Cleary you can see all linear molecules at moderate temperature has 5 DOF

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Everything is right, IN Triatomic linear molecule (like:co2) has two vibration mode in normal room temperature. So, F=3+2+2=7, But many books not explain it well manner, That is why you get Cv value =7/2RT. (Mind: linear triatomic molecules has 4 vibrational mode (v=3(3)-5=4),but we attained two modes of vibration normal room temperature,so total freedom is 7)

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