“You know how everything is made of atoms, right? Well, you got different kinds of atoms, Helium, Nitrogen, and so on. This are called elements. Every atom of the same element has the same numbers of protons. Every Carbon atom in the world has 6 protons. But, sometimes, the elements has different numbers of neutrons.So, you could find Carbon with 5 neutrons, or 6, or 7. We called every “flavor” of Carbon an Isotope. And this is what isotopes are. “Flavors” of the elements, with different numbers of neutrons.
Sometimes a particular isotope goes by a transformation, and gives another isotope, of the same,or another, element. When the isotope does that, radiates energy, that can be harmful for humans. We call this isotopes radioactive, so, we places signs to alert other ones when they are present."