How do energy levels change when atoms collide?

Depends.
If you're in normal chem, I think your answer will be that energy levels don't change.
If you're in AP chem, see below:
     If the collisions are completely elastic collisions, like in ideal gases, energy        level does not change.
     However, if the atoms have IMF's(intermolecular forces) then kinetic energy      is lost.

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