Pb204

Pb-204, or lead-204, is a stable, non-radioactive isotope of the element lead (atomic number 82). It has 82 protons and 122 neutrons, giving a nominal atomic mass of about 203.97 u. Among natural lead isotopes, 204Pb is the least abundant.

Natural abundance and relation to other isotopes. In ordinary lead, the isotopic composition is about 204Pb (roughly 1.4%), 206Pb (~24.1%), 207Pb (~22.1%), and 208Pb (~52.4%). Unlike 206Pb, 207Pb, and 208Pb, which accumulate from the radioactive decay of uranium and thorium over geological time, 204Pb is considered primordial and not produced by such decay chains.

Geochemical and geochronological significance. 204Pb serves as a non-radiogenic reference isotope in lead–lead dating and related isotopic systems. By comparing ratios such as 206Pb/204Pb and 207Pb/204Pb (and 208Pb/204Pb) in rocks and meteorites, scientists can estimate the initial lead composition and determine the ages of geological samples, often spanning billions of years. This makes 204Pb essential for reconstructing early solar system history and planetary differentiation.

Occurrence and production. 204Pb occurs naturally in lead-bearing minerals, such as galena, and remains part of the natural lead inventory due to its primordial origin. It can be produced in trace amounts by nucleosynthetic processes in stars, but on Earth its presence is primarily due to ancient solar-system material rather than in situ radioactive decay.