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Homework Help: Degrees of Freedom for a Triatomic Molecule

  1. Dec 7, 2008 #1
    1. The problem statement, all variables and given/known data

    A triatomic molecule consists of 3 atoms arranged along a straight line. The molecules can
    translate and rotate but not vibrate. How many degrees of freedom are associated with this molecule?

    2. Relevant equations

    Molar Heat Capacity of Gases:
    Cv = f/2(R)

    f - degrees of freedom

    3. The attempt at a solution

    I figured that a triatomic gas would have the same translational motion as that of a monoatomic and diatomic molecule and it would have perhaps more axes of rotational motion than diatomic molecules. And from seeing that diatomic molecules have 5 degrees of freedom after ignoring vibrational motion, I am assuming that you can drop vibrational motion for triatomic molecules as well.

    Would I be correct in concluding that a triatomic molecule has 6 degrees of freedom? I came to this conclusion by one more rotational axis. I'm not sure this is correct, however.

    Any help would be greatly appreciated.
    Last edited: Dec 7, 2008
  2. jcsd
  3. Dec 7, 2008 #2

    I don't know if this is correct, but my guess is that there are 5 degrees of rotational and translational freedom. This would be because they are arranged in a straight line.

    If they were arranged in a V-shape (such as H2O gas), the degrees of freedom would be 6, correct? If they are in a straight line it is only 5 because rotation on the axis of the molecules would have to be disregarded. However, since the V-shape offsets one of the molecules, there could be an extra axis of rotation.

    I would appreciate it greatly if someone could affirm this. :smile:
  4. Dec 1, 2010 #3
    Yeah it is 6

    http://arasaraja.blogspot.com" [Broken]

    Last edited by a moderator: May 5, 2017
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