Lead-207 has 82 neutrons. It is a stable isotope of lead. Because it is non-radioactive, it does not have a half life. It is used in medical and scientific applications.
According to the Georgia State University hyperphysics website, one of the scientific applications of lead-207 is uranium-lead dating. Lead-207 is the stable end-product of the uranium-235 decay series, a process that takes millions of years. Scientists use this information along with the ratio of other lead isotopes in the material to estimate the age of an object. GSU's hyperphysics website states that uranium-lead dating is the most precise method available to determine the age of the Earth.