Prout's hypothesis being correct. Nevertheless, the rule was not found to predict isotope masses better than this for all isotopes, due mostly to mass-defects resulting from release of binding energy in atomic nuclei, when they are formed. Although all elements are the product of nuclear fusion of hydrogen into higher elements, it is now understood that atoms consist of both protons (hydrogen nuclei) and neutrons. The modern version of Prout's rule is that the atomic mass of an isotope of proton number (atomic number) "Z" and neutron number "N" is equal to sum of the masses of its constituent protons and neutrons,
Nucleon upon equally by the strong interaction, which is invariant under rotation in isospin space. According to the Noether theorem, isospin is conserved with respect to the strong interaction. Protons and neutrons are best known in their role as nucleons, i.e., as the components of atomic nuclei, but they also exist as free particles. Free neutrons are unstable, with a half-life of around 13 minutes, but they are common in nature and have important applications (see neutron radiation and neutron scattering). Singular protons, not bound to other nucleons, are usually regarded as the nuclei of hydrogen atoms or ions, but in