Radionuclide

Okay so let's imagine tiny little particles called atoms that make up everything around us. Now, some atoms can have extra bits called protons and neutrons that make them heavier than other atoms. These heavy atoms are called isotopes.

Now, some isotopes are unstable and can break down into smaller pieces over time. When they break down, they release energy in the form of radiation, which can be harmful to living things.

A radionuclide is just another name for an unstable isotope that emits radiation as it breaks down. Some types of radionuclides are used in medicine, like radioactive iodine used to treat thyroid cancer. But if too many radionuclides are released into the environment, like in a nuclear disaster, it can be dangerous for people and the environment.

So basically, a radionuclide is a type of atom that isn't very stable and can give off harmful energy.