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" Another way of explaining it is that when geologists talk about isotopes, they are talking about one element of differing masses.Isotopes of an element are atoms that all have the same atomic number (or number of protons in the nucleus) but have different atomic masses (hence different numbers of neutrons in the nucleus).Radioactive decay involves the spontaneous transformation of one element into another.
However, oxygen atoms can have between 8 and 10 neutrons in the nucleus and therefore the isotopes of oxygen have atomic masses of 16, 17, and 18 a.m.u.(and none are radioactive! Samarium (Sm) has 7 naturally occurring isotopes (3 are radioactive).
Even if they have been given the definition, they interpret the term to mean one-half the life of the system.