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For isobars (i.e. nuclides with a constant mass number $A$), the binding energy $E$ as a function of the atomic number $Z$ describes a parabola, the so-called valley of β-stability (see also Weizsäcker’s formula or semi-empirical mass formula).
$$E = a \cdot Z^2 + b \cdot Z + c \pm d/A^{3/4}$$
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