An isotope is one of the different possible forms of an atom of the same simple substance (the same element). The atoms still count as that element because the number of plus-charges in the middle is the same, so they sit in the same place on the chart of elements; they differ in how many extra uncharged bits they carry, so some are a little heavier than others. Carbon, for example, has a very common isotope and a rarer one that scientists use to date old bones and wood. Some isotopes stay as they are; some slowly break down and give off energy, and those are the radioactive ones. Though actually, news English often uses isotope as if it meant the dangerous glowing kind. Most isotopes around you are quiet and ordinary — they are just the slightly heavier or lighter brothers of the atom you already know, not a different substance and not, by themselves, a poison or a weapon.