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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 the number of neutrons can vary in a regular atom. In some oxygen atoms, there might be 8 neutrons in the nucleus, and in others, 9 or 10. These different versions of the same element are called isotopes.

Remember all that stuff about those BFF protons and neutrons living together in peace and harmony in the nucleus? We lied…it doesn’t always work like that. Scandal alert. 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 PET, or positron emission tomography, scans and 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:

Brain Snack

It's pretty intuitive 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. Oopsies.

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