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,
Proton number of protons, each element has its own unique atomic number. The word "proton" is Greek for "first", and this name was given to the hydrogen nucleus by Ernest Rutherford in 1920. In previous years, Rutherford had discovered that the hydrogen nucleus (known to be the lightest nucleus) could be extracted from the nuclei of nitrogen by atomic collisions. Protons were therefore a candidate to be a fundamental particle, and hence a building block of nitrogen and all other heavier atomic nuclei. In the modern Standard Model of particle physics, protons are hadrons, and like neutrons, the other nucleon (particles