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Question atom / molecular movement

  1. Aug 24, 2004 #1

    first of all I'd like to say hello as I'm new to this forum.

    Here are my questions:
    If a single atom would be held in an empty room (vacuum/no gravity) would it move or would it stand still? If it does move what would the movement would look like?

    The same question goes for molecules e.g. O2

    If there was a bunch of O2 molecules would their movement be influenced by intermolecular forces?

    Thanks in advance!
  2. jcsd
  3. Aug 24, 2004 #2

    Classically, there would be one chance in an infinity that the single atom would move with the exact same velocity than the empty room itself. So my answer is yes, it would move, it would move in a straight line at a constant speed.

    Same for a molecule, except that it would also rotate, and vibrate.

    As for a bunch, we call this a gas, they would each move as above, except that once in a while, they will be a collision. Depending on the density of the particles, you can either ignore the collisions between each, consider them elastic, or inelastic. In the extreme case, they would ionize each other. Different mathematics/theories apply in each case.
  4. Aug 25, 2004 #3
    thanks gonzolo!

    what are the reasons for the molecule rotating and vibrating?

    when having a gas are collisions the only reactions between the molecules or would e.g. van der walls forces also play a role in the movement?
  5. Aug 27, 2004 #4
    The simplest molecule is in the shape of a dumbell : two atoms seperated by a bond. The bond can be considered as a spring. But unlike a true spring though, there is nothing to make it stop vibrating, so it does so forever. Each bond in a multi atom molecule vibrates. Any stated distance between atoms of a molecule is an average. I understand no vibration would essentially mean absolute 0 temperature.

    On the other hand, a single atom doesn't have much to vibrate against.

    As for rotation, well again, chances are it will be rotating relative to your empty room. Any multi-atom molecule will. There is one chance over infinity that it does not. A single collision is enough to change how it rotates.

    Again, a single atom doesn't have anything to rotate around.

    Molecules store energy this way, by vibration, rotation and by translation. How much depends on temperature, or it total energy. Or you can turn this around and say that the temperature of a substance depends on molecular energy.

    Van der Walls force might play a role. How much of a role would depend on temperature, what molecules are involved, pressure (how many collisions per time interval), etc.
  6. Sep 8, 2004 #5


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    You can turn this question on its head.

    How did that single atom get introduced to the room? Was it injected? Then it has some initial velocity. Even if it were injected gently, it still had a temperature, and thus had its own momentum, and was moving quite rapidly.

    Was it already in the room, and you left it there, while all others were removed? Then it has whatever momentum it was left with from the last collision.

    Regardless, it will keep its initial momentum with respect to the room. Its momentum will be proportional to its temperature when you stopped manipulating it.

    And yes, it will move in a straight line until it collides with a wall, another molecule or a magnetic field.

    If you decide that doesn't count, and to try to "steady" it first - stop it from moving after everything else has been removed, then you're going to have to slow it down until it stops careening around - i.e. you're going to have to supercool it with lasers or somesuch method.

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