Radiometric dating 210pb
If the amount originally present cannot be determined, than the substance is not eligible for radiometric dating.
In order to have cases where the original amount can be known, an isotope that is part of a larger mineral compound is needed.
An isotope is one of two or more atoms which have the same number of protons in their nuclei, but a different number of neutrons.
Radioisotopes are unstable isotopes: they spontaneously decay (emitting radiation in the process -- thus making them radioactive).
Protons and neutrons together are called nucleons, meaning particles that can appear in the atomic nucleus.
A nuclide of an element, also called an isotope of an element, is an atom of that element that has a specific number of nucleons.
Certain chemical isotopes are stable, in that they will not change in structure until they are affected by an external force.
In this instance, Uranium-238 is called the "parent" and Lead-206 is called the "daughter".
These long time periods are computed by measuring the ratio of daughter to parent substance in a rock and inferring an age based on this ratio.