Radiometric dating is possible because the radioactive decay of large numbers of radioactive atoms follows a predictable pattern.
Uranium 235 decays to lead 207, and Thorium 232 decays to lead 208.
In addition there is another stable isotope, lead 204, that is entirely primordial and does not form via radioactive decay at all.
Thus any of the radioactive isotopes and its lead daughter product can be used for dating, or a combination may be used.
In addition, some of the longer-lived intermediate daughter products have uses in dating.
The original radioactive atom is known as a parent isotope, while the atom produced by the decay process is known as a daughter isotope. For example Uranium-235 and Uranium-238 are both Uranium atoms with the same number of protons, but they have a different number of neutrons.
The number used to identify the isotope refers to the total number of particles in the nucleus of each atom.Zircon is especially useful because it frequently contains uranium in substitution for zirconium, but does not incorporate lead (as shown by the absence of Lead-204).Thus all the lead in the zircon can be assumed to be radiogenic.It has become increasingly clear that these radiometric dating techniques agree with each other and as a whole, present a coherent picture in which the Earth was created a very long time ago.Further evidence comes from the complete agreement between radiometric dates and other dating methods such as counting tree rings or glacier ice core layers.Uranium minerals themselves are too uncommon to be very useful in dating.