The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock. The reason for this is that Rb has become distributed unequally through the Earth over time.
We can see how do deal with this if we take a particular case. For example the amount of Rb in mantle rocks is generally low, i.e. The mantle thus has a low If these two independent dates are the same, we say they are concordant.
This gives us the impression that all but a small percentage of the dates computed by radiometric methods agree with the assumed ages of the rocks in which they are found, and that all of these various methods almost always give ages that agree with each other to within a few percentage points.
This technique relies on the property of half-life.
Half-life is defined as the time it takes for one half of a radioactive element to decay into a daughter isotope.
One way that helps scientists place fossils into the correct era on the Geologic Time Scale is by using radiometric dating.
Also called absolute dating, scientists use the decay of radioactive elements within the fossils or the rocks around the fossils to determine the age of the organism that was preserved.
As radioactive isotopes of elements decay, they lose their radio activity and become a brand new element known as a daughter isotope.