U
uncertainty principle
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The idea that a theoretical limit exists on the accuracy with which the product of the position and momentum of a particle can be determined.
unimolecular process
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An elementary step in a reaction mechanism; formally no species collide in such a step, but in actuality such processes usually involve a collision with a species that does not react.
unit cell
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The smallest fragment of the structure of a solid that by repetition can generate the entire structure.
unsaturated
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Describing 1) a solution that contains less than the maximum possible equilibrium concentration of a solute, or 2) an organic compound containing double or triple bonds.
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