Isotope
All chemical elements (building stones of the more complex molecules - any particle of 2 or more atoms) may exist as several isotopes. Typically most of these isotopes are unstable (radioactive) and only one (or a few) are stable. For a given element all isotopes have the same chemical properties, and can not be separated from other isotopes of the same element by chemical means. All isotopes of a given element have the same number of protons (small particles with a positive electrical charge present in the nucleus of the atom) and electrons (even smaller particles with a negative electrical charge rotating around the nucleus of the atom). But for a given element the number of neutrons (particles of the same size/mass as protons, but without an electrical charge, also present in the atom nucleus) may vary considerably. Put differently, the protons and electrons determine the chemical properties of an element, whereas the number of protons and neutrons together determine its radioactive properties.
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