197Au

197Au is the only stable, naturally occurring isotope of the element gold. It has a proton number Z of 79 and a neutron number N of 118, giving a mass number A of 197. The atomic mass is 196.966569 u, and gold’s standard atomic weight is essentially this value, reflecting its monoisotopic composition.

The nucleus of 197Au has a ground-state spin I of 3/2− and is highly stable, with a large binding energy per nucleon of about 7.9 MeV. As a stable isotope, it does not undergo radioactive decay and has no measurable half-life.

In astrophysical processes, gold is produced predominantly through neutron-capture processes in stellar environments, such as the r-process occurring in extreme events like neutron-star mergers or certain supernovae. The solar system’s gold is overwhelmingly 197Au, reflecting the stability and predominant formation pathway of this isotope.

In practical applications, 197Au finds use in nuclear science and materials research. The neutron-capture reaction 197Au(n,γ)198Au is widely used to monitor neutron flux in reactors and for neutron activation analysis. Gold foils serve as targets in experiments and as calibration references in spectrometry. As the sole stable isotope, 197Au also provides a stable isotopic standard for calibrating measurements of gold in various analytical techniques.

See also: Isotopes of gold, Nuclear binding energy, Nucleosynthesis.