Pb208

Pb-208, or lead-208, is the isotope of lead with mass number 208. It has 82 protons and 126 neutrons. It is the most abundant stable isotope of natural lead, accounting for about half of the lead found in nature. Natural lead consists primarily of four stable isotopes: 204Pb, 206Pb, 207Pb, and 208Pb, with 208Pb being the most abundant.

Pb-208 is the end product of the thorium-232 decay chain and is produced in trace amounts by other long-lived decay pathways. It is a stable nucleus, meaning it does not undergo radioactive decay under ordinary conditions. The nucleus has Z = 82 and N = 126, a configuration described as doubly magic, which contributes to its high binding energy and notable stability. The ground state of Pb-208 has spin and parity 0+.

Because of its stability and well-measured mass, Pb-208 serves as a benchmark in nuclear physics and is used in geochemical and cosmochemical applications. Isotopic ratios involving Pb, including 208Pb, are used in lead-lead (Pb-Pb) dating methods to study the ages of rocks, meteorites, and the early solar system. Pb-208 also plays a role in calibrating mass spectrometry measurements and in understanding nucleosynthetic processes that produce heavy elements.

In comparison with the heaviest common isotope, 209Bi, which is technically radioactive with an extremely long half-life, Pb-208 is widely regarded as the heaviest stable nucleus.