### UNSTABLE NUCLEI

When the stable nuclides are plotted on a graph Z against N, a jagged, slightly curving line is seen, the line of stability. The further away the nuclide is from this line of stability, the more unstable it is and the faster it will decay into the more stable nuclides/elements. Nuclides above the line have an excess of protons and may decay by emitting a proton (not usual), or by inverse beta decay or electron capture. Nuclides below the line have an excess of neutrons and may decay by emitting a neutron (again unlikely), or by beta decay. Nuclides with atomic weight greater than 140 are more likely to decay by emitting a helium nucleus (alpha decay), whilst those heavier than uranium by spontaneous fission (SF).
Some nuclides have such enormously long half-lives (5.5×10^{24} years) compared to the present age of the Universe (approx. 13.81×10^{9} years) that they may be classed as stable but slightly radioactive. When a nuclide decays, it may transmute into a different element of lower or identical Atomic Weight (A).

Elements with Atomic Number, Z, greater than 92 (uranium) are only produced in high energy sub-atomic particle accelerators, nuclear reactors, super novae, or nuclear explosions; as they are all unstable. There are no stable isotopes for Z=43 (technetium), or any element above and including Z=84 (polonium). Similarly, there are no stable isotones for N=19, 21, 35, 39, 45, 61, 71, 89, 115, and 123, all odd numbers. Also, A=5 and A=8 are the only isobars below A=211 for which no stable isobars exist.

Shown: arsenic-73 (electron capture), zinc-65 (inverse beta decay), aluminium-26 (isomer & inverse beta decay), chromium-50 (Dbl Inverse Beta), krypton-89 (otherwise stable isomer [Internal Transition gamma emitter]), copper-64 (dual-mode beta/inverse beta), tellurium-128 (Dbl beta decay), strontium-90 (beta decay), thorium-228 (alpha decay), fermium-258 (Spontaneous Fission) and uranium-236 (alpha decay & fertile isomer [SF]).