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mfb- t1_jcdb4p7 wrote

A single lead atom (or ion) still moves so fast that it's going to collide with some random side of the room with almost equal probability in vacuum. If the wall has the same temperature and the atom doesn't get stuck there then it has no reason to lose kinetic energy over time, although its energy will vary randomly from each collision. In practice lead atoms tend to stick to something pretty quickly at room temperature.