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We said that a regular atom needs to have the same number of protons as electrons, but we didn’t say anything about neutrons. Neutrons are special because their number can vary in a regular atom. In some oxygen atoms, there may be eight neutrons in the nucleus, while there may nine or ten in others. These different versions of the same element are called isotopes.
While we wish protons and neutrons would always live together in a peaceful coexistence in the nucleus, things don’t always work that well. That's because some isotopes are radioactive.
Radioactive isotopes are unstable and their nuclei can break apart, emitting energy in the process. Radioactive isotopes have really neat practical applications, like their use in positron emission tomography (PET) scans or the dating of fossils and sediments. We don't mean that they're dating each other, obviously. That would be awkward. Rather, radioactive isotopes help us figure out how old fossils and sediments are.
Ready for more atoms? Here are three isotopes of oxygen:
These days, it's well-known that radiation is bad for you, but not too long ago, a kind of ceramic plate was sold that had significant quantities of radioactive uranium oxide in the glaze. It was called Fiesta dinnerware and was very popular in the early 20th century. Oopsie.