Nuclear Stability

A nucleus contains most of an atom's mass and is tiny compared with the entire atom. The average nuclear density is nine trillion times greater than the density of osmium: the densest element!

The earth would be 30,000 times smaller if it had the density of the nucleus. Why does the nucleus have such a high density?

The nucleons are held together by the close-range strong nuclear force. The balance between the proton–proton repulsions and the nucleon–nucleon attractions determines the stability of the nucleus. When the proton–proton repulsions outweigh the attractive nuclear forces, the nucleus disintegrates.

Plotting nuclides by the numbers of protons and neutrons illustrates that the stable nuclei occupy a central region, indicated in blue, which is called the belt or valley of stability.

Lighter nuclides with neutron-to-proton ratios of one, like carbon-12, enjoy great stability. As the atomic number increases beyond 20, more neutrons are required to counterbalance the proton–proton repulsions.

Neutrons are attracted to each other by nuclear forces, whereas there are no repulsive interactions amongst them. Thus, an increase in the number of neutrons fortifies the nuclear force significantly. All stable heavier nuclides have neutron-to-proton ratios of greater than one.

Radionuclides with higher neutron-to-proton ratios typically undergo beta-minus decay, which converts a neutron into a proton. Thus, the neutron-to-proton ratio decreases to yield a daughter nuclide that is closer to the stability belt on the chart.

Radionuclides with lower neutron-to-proton ratios emit positrons or undergo electron capture to convert protons into neutrons and thereby move closer to the stability belt.

Interestingly, just like electron pairs residing in the orbitals, proton–proton and neutron–neutron pairing is observed in the nucleus. When the numbers of protons and neutrons are both even, nuclei are remarkably stable, as pairing is possible for all nucleons. Only five nuclides with odd numbers of neutrons and protons are stable.

Nuclei with certain numbers of protons or neutrons are more stable than expected, leading these numbers to be called magic numbers. Nuclei with magic numbers of both protons and neutrons are called doubly magic.

All nuclei with atomic numbers higher than 82 are radioactive. However, bismuth-209, atomic number 83, has an exceptionally long half-life among radionuclides.

For a radionuclide far from the stability belt, its decay chain is the series of decay processes by which it ultimately reaches a stable nuclide.