Above bismuth, the repulsive forces are greater than the strong force which binds the nucleus together, thus the nucleus is thermodynamically unstable. The rate of decay is sometimes very slow, however. The nucleus is now known to exhibit a shell structure analogous to electron orbitals. This shell structure, originally comprehended by Maria Meyer, significantly impacts the stability of the nucleus. Although the ratio of protons to neutrons can vary, it generally increases as atomic number increases.
Nucleons like to be paired.
Only 4 out of 273 stable nuclei have odd numbers of both protons and neutrons. Elemental distributions are portrayed Fig 2.6 in your text . During beta decay, a neutron in the nucleus splits to form a proton and an electron. The electron is ejected from the nucleus as a b -particle. The radioactive decay of carbon-14 involves b emission to form nitrogen.