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Atoms are made up of a positively charged nucleus and negatively charged electrons orbiting around it. Atomic nucleus consists of two of the heavier particles, the protons and the neutrons and contains all the positive charge. The number of protons (equal to the number of electrons) is characteristic of a particular atom, and is called the atomic number (Z), wheras the total number of protons and neutrons is called its mass number (A). Atoms of the same element can have different numbers of neutrons; the different possible versions of each element having different mass numbers are called isotopes. For example, considering atomic number 1, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons. These isotopes may have great variation in their stability. One (or more) of these isotopes are stable while others fall apart out of their own, giving rise to spontaneous decay associated with emission of electromagnetic energy through a process known as radioactivity.
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