Isotopes do not just differ in reactivity, however.

Which isotope of an element you are dealing with in nuclear physics is very important. For instance, deuterons in heavy water are used in particle accelerators. They are an isotope of hydrogen, and regular hydrogen nuclei simply will not do the same job.

Another example is in preparing fuel for nuclear reactors. Before purified uranium is made into fuel pellets, it must be enriched - a process that calls for increasing the percentage of the isotope U-235 in the fuel from less than one percent, up to about five percent.

The reason for this is the major constituent of uranium, the isotope U-238. Neutrons are a scarce resource in a nuclear reactor, and the feasibility of the fission process depends on getting a chain reaction to occur. When a U-235 nucleus fissions, it releases one or more neutrons. When one of these neutrons hits another U-235 nucleus, it causes that nucleus to fission also, releasing more neutrons, and so on.

U-238 nuclei, on the other hand, will simply 'gobble up' any neutron that hits them without fissioning. (They later decay to plutonium (?), but not fast enough for our purposes, and they do not emit neutrons when they do.) As a result, if there is too much U-238 present, the chain reaction simply will not proceed.

If the percentage of U-235 is increased, the chain reaction becomes sustainable, and generating power from fission becomes possible.

(Incidentally, breeder reactors make more fuel than they burn, by altering the process so that a lot of plutonium gets formed during the fission of enriched uranium fuel. After reprocessing, they can then use the plutonium as fuel.)