Neutrino: A weightless, uncharged particle which carries off energy in the process of beta decay.
Neutron: A neutral particle, one of the basic constituents of the nucleus. A neutron weighs slightly more than a proton, and when free, decays into a proton plus an electron and a neutrino.
Noble gases: Helium, neon, argon, krypton, and xenon. They do not combine chemically with any elements including themselves.
Nuclear bomb: A bomb which derives its energy from nuclear fission or fusion.
Nuclear reactor: A machine for maintaining a controlled chain reaction.
Nucleus: The core of an atom, consisting of neutrons and protons. Its charge is equal to the number of protons. Its weight is equal to the number of protons plus the number of neutrons.
Periodic system: The chemical elements arranged in order of increasing atomic charge. Elements with similar chemical properties occur periodically.
Plutonium: Element with charge 94, produced by capturing a neutron in uranium²³⁸ followed by two beta emissions. Like uranium²³⁵, plutonium is valuable as an atomic fuel.
Positron: The positive counterpart of the electron.
Potassium⁴⁰: A natural radioactive isotope. It has a half-life of one billion years and emits beta and gamma rays.