An isotope is an atom which is related to an element by the number of protons in the nucleus, and differs by the number of neutrons. Thus, they vary in physical properties, but all have very similar chemical properties, and electronic structure. The relative atomic mass of an element is configured using isotopes. It is the average atomic mass of all the element's isotopes. Therefore, if one isotope among all the others has an atomic mass which is not similar to the other isotopes, it could create a great change in the atomic mass of an element.
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