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An elementary reaction is a chemical reaction that occurs in a single step, without any intermediates. It represents the simplest and most basic unit of a chemical reaction.
A substance formed temporarily during a multi-step reaction but does not appear in the overall balanced equation.
Rate-determining step: The slowest step in a multi-step reaction that determines the overall rate of the reaction.
Transition state: A high-energy, unstable arrangement of atoms during a chemical reaction, representing the peak energy point before forming products.