Absolute and relative dating quiz
Under ordinary conditions, the disintegration of each radioactive isotope proceeds at a well-defined and characteristic rate.
The last term, the so-called pairing energy, takes on any one of three values depending on whether He) by emitting an electron.
Some isotopes, however, decay so slowly that they persist on Earth today even after the passage of more than 4.5 billion years since the last significant injection of freshly synthesized atoms from some nearby star.
Examples of such long-lived radioisotopes include -14, may at first seem puzzling.
The great importance of the atomic number derives from the observation that all atoms with the same atomic number have nearly, if not precisely, identical chemical properties. Similarly, mesothorium was shown to be chemically indistinguishable from .
A large collection of atoms with the same atomic number constitutes a sample of an element. As chemists used the criterion of chemical indistinguishability as part of the definition of an element, they were forced to conclude that ionium and mesothorium were not new elements after all, but rather new forms of old ones.