a neutral (non-charged) subatomic particle (atom part). It hangs out with positively-charged protons in an atom’s dense central nucleus, surrounded by a sea of negatively-charged electrons. Neutrons contribute to an atom’s mass, but not an atom’s charge. Elements are defined by the number of protons they have (atomic number) but the number of electrons can change as can the number of neutrons. Different versions of the same element with different numbers of neutrons are called nuclear isotopes. Some isotopes are unstable and thus radioactive (they let off radiation when they rearrange themselves into a more stable form) but other isotopes are just “heavy” (they have more neutrons than the main isotope, but they are stable). ³²P is an example of a radioactive isotope (radioisotope) whereas ¹⁵N (aka “heavy nitrogen”) is an example of a stable heavy isotope.
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