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Chemist: Werner Heisenberg
Contribution: The Principle of Indeterminacy
The German Werner Heisenberg proposed his Principle of Indeterminacy in 1927, also known as Heisenberg's Uncertainty Principle. Basically, this principle states that you can't know the position and velocity of a particle at the same time. You can tell where something was and how fast it is going now, but not where it is or how fast it was going. This happens because particles are so small you can't help but interfere with them when making calculations.
Information Credits: Atomic Structure Timeline, HowStuffWorks