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It is know that entropy is zero when a pure crystalline substance is at absolute zero. Theoretically, does this mean that atomic motion (electrons, etc.) stops also?

Zero entropy means that the system is in a single quantum mechanical state, in particular, the lowest, the ground state. This means that the particles of the system have adjusted their motions to lower their energy as much as possible. However, because of quantum mechanical effects there is still residual motion of the particles. This is measurable and detectable and is often called zero point motion. It is particularly observable in the motion of the atoms when they are studied by x-rays. Even in the lowest energy state of a metal, the electrons are still moving around keeping the energy as low as possible.

Sam Bowen

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