208Pb

208Pb is the heaviest stable isotope of the element lead (Pb). It has 82 protons and 126 neutrons, giving a mass number of 208. The nucleus has spin and parity of 0+, and, as a stable nuclide, it has no radioactive half-life.

In nature, lead exists as a mixture of isotopes; 208Pb accounts for roughly half of natural lead, about 52%. It is formed primarily as the end product of the thorium-232 decay series, making 208Pb a radiogenic isotope produced by the decay of long‑lived naturally occurring radionuclides. By contrast, the uranium decay chains terminate at 206Pb and 207Pb, while 208Pb is the stable endpoint of the 232Th series.

Because 208Pb is stable, it is used in radiometric dating, especially uranium–lead dating, where ratios involving 206Pb/238U, 207Pb/235U, and 208Pb/232Th are used to determine ages and to construct concordia diagrams. Minerals such as zircon, which retain lead over geological timescales, provide reliable records for crustal evolution and planetary dating. The relatively high abundance of 208Pb in the Earth's crust also makes it a convenient reference in isotopic studies and mass spectrometric calibration.

In nuclear structure terms, 208Pb is notable for its doubly magic configuration (Z = 82, N = 126), which contributes to its exceptional stability and makes it an important anchor point in understanding nuclear shell structure.