Whole number rule called the neutron. The fact that the atomic nucleus is composed of protons and neutrons was rapidly accepted and Chadwick was awarded the Nobel Prize in Physics in 1935 for his discovery. The modern form of the whole number rule is that the atomic mass of a given elemental isotope is approximately the mass number (number of protons plus neutrons) times an atomic mass unit (approximate mass of a proton, neutron, or hydrogen-1 atom). This rule predicts the atomic mass of nuclides and isotopes with an error of at most 1%. Whole number rule The whole number rule states that
Valley of stability atomic and neutron numbers, forming what has been called an island of stability outside of the valley of stability. All atomic nuclei are composed of protons and neutrons bound together by the nuclear force. There are 287 different nuclei that occur naturally on earth, each corresponding to a unique number of protons, called the atomic number, Z, and a unique number of neutrons, called the neutron number, N. The mass number, A, of a nuclide is the sum of atomic and neutron numbers, A=Z+N. Not all nuclides are stable, however. According to Byrne, stable nuclides are defined as those having