the emission of particulate or electromagnetic radiation as a result of decay of the nuclei of unstable elements, a property of all chemical elements of atomic number above 83, and possibly inducible in all other known elements. The chemical elements are made up of atoms, each consisting of a nucleus around which orbits a cloud of negatively charged electrons. The nucleus itself is made up of two kinds of particles: neutrons (which have no electrical charge) and protons (each of which has a single positive charge). A neutral atom has an equal number of protons and electrons and no electric charge. The atomic number of an element is the number of protons in the nucleus of each of its atoms. The mass number of an element is the sum of the number of protons and neutrons in the nucleus.