How do you know how many isotopes an element has without being told? I know that some elements have 5 isotopes while others could only have 3 but how do I know how many?  

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Isotopes of an element have the same number of protons, but different numbers of neutrons.  (Remember it is the number of protons that give an element its identity. ) You would not know how many isotopes an element has without being told or looking up the information.  Light elements usually have about the same number of neutrons as protons.  Heavier elements have more neutrons than protons to hold all those protons together.   Each element does not have an infinite number of isotopes.  There seem to be some combinations of protons and neutrons that make the an element more stable than other combinations.  If an isotope is unstable it is said to be radioactive.  Unstable elements will kick out particles and turn themselves into new stable elements.

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