Kinetic isotope effects (KIE) refer to the impact on reaction rate when one of the atoms in a molecule is replaced by its isotope. For example, replacing hydrogen (H) in a molecule with its isotope deuterium (D), which is heavier by one neutron, can slow down a reaction by up to 20 times. From rate information scientists learn about what happens to molecules during a reaction. They gain information about the changes as the reactants form new products during the course of the reaction,.

Atomic isotopes are closely related. Consider, for example, the isotopes of radium. They are atoms of the same element, so they have the same number of protons and electrons. The only way they differ is in the number of neutrons. One isotope has 202 neutrons, and the other isotope has 203 neutrons. The isotope with the extra neutron is a little heavier than...