Ask A Scientist Chemistry Archive

(Created prior to 1993)

Question:
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?
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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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