For example Carbon 14 is radioactive, but Carbon 12 isn't. Uranium 235 is very radioactive, but Uranium 238 is only mildly radioactive. If two isotopes of one element are mixed together, it is very difficult to separate them. No chemical method works, because both isotopes have identical chemistry. Usually we use centrifuges, or else distillation.
Distillation works because lighter molecules are a little quicker to evaporate.
Light Year One light year is the distance that light travels in one year. This means evolution at the level of a species, or higher. For example, the evolution of land animals into whales would be macroevolution. Many Creationists say that their argument is with macroevolution, and not microevolution. Most scientists consider macroevolution to just be microevolution which has gone on for a long time.