Nuclear Stability

Nuclear Forces Nucleons- protons and neutrons Nuclide- specific nucleus Strong force holds the nucleus together acts between nucleons that are very close together If nucleons are too far apart the repulsion forces are larger than the strong force and the nucleus will fall apart

Arrows describe magnitudes of the strong force acting on protons

Quarks Smaller parts of protons and neutrons Six known flavors of quarks- up, down, top, bottom, strange, and charm Only up and down make protons and neutrons

Binding Energy Energy released when nucleons make a nuclide Energy comes from the conversion of mass to energy. E=mc2 The mass of the whole is less than its parts Difference in mass is the mass defect

Nuclear Stability Find the N/Z ratio N = # neutrons Z = # protons Small atoms- atomic number 20 and under Ratio =1 Larger atoms- atomic number over 20 Ratio =1.5 More neutrons are needed between the protons Magic numbers of protons and neutrons are stable No atoms with atomic numbers larger than 83 and mass number bigger than 209 are stable

Magic Numbers Nuclear shell model Nucleons exist in energy levels (shells) in the nucleus Numbers of nucleons that represent completed nuclear energy levels- 2, 8, 20, 28, 50, 82, and 126- are magic numbers

Example: 10B 11B 194Hg 202Hg

Rules for Predicting Stability Except for hydrogen-1and helium-3, in stable nuclei, #neutrons ≥ #protons. A nucleus that has an N/Z number that is too large or too small is unstable Nuclei with even numbers of neutrons and protons is more stable Nuclei that have so-called magic numbers of protons and neutrons tend to be more stable No atoms that have atomic numbers larger than 83 and mass numbers larger than 209 are stable