Uranium-Lead dating is a radiometric dating method that uses the decay chain of uranium and lead to find the age of a rock.
The four isotopes are uranium-235, uranium-238, lead-207, and lead-206.
The process of dating finds the two ratios between uranium-235 and lead-207; and uranium-238 and lead-206.
Uranium comes in two common isotopes with atomic weights of 235 and 238 (we'll call them 235U and 238U).
Both are unstable and radioactive, shedding nuclear particles in a cascade that doesn't stop until they become lead (Pb).
The age of a rock can be calculated if its ratio of uranium to lead is known.