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The valley of beta stability a region of the chart of possible isotopes (arrange by atomic numbers versus neutron count) in which beta decay occurs at low frequencies (i.e., the isotopes are stable against beta decay) with a "valley bottom" of the most stable isotopes, and the instability of isotopes nearby tending to slope upward away from these. The stability is due to higher binding energy keeping the nucleus intact. This energy is generally high when the difference between proton and neutron counts is small, this factor in the calculated binding energy termed symmetry energy or nuclear symmetry energy. Isotopes at or near the bottom of the valley tend to be the ones we find, because after a less-stable isotope is synthesized, it is likely to beta decay until it reaches such a state. The valley helps explain the observed abundances of elements. The timescale of beta decay varies widely by isotope, from under a second to orders-of-magnitude longer than the age of the universe.