A chemical reaction fundamentally involves the cleavage and formation of bonds between atoms. In general, the electrons and nuclei don't move at precisely the same time; more often than not, a rapid electronic rearrangement precedes a slower nuclear rearrangement, after which the electron distribution may adapt once again. In this context, the question arises of when exactly a bond can be considered broken. Should the criterion be electronic arrangement? Nuclear separation? Some combination of the two?
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