By dividing the above equation by a common factor, a more useful equation can be created: This ratio of radiogenic strontium to a non-radiogenic isotope, Strontium-86, over time is more useful than the absolute amount of Strontium present in a sample.
Putting all this information together can tell us the history of a rock sample.
It has the same number of protons, otherwise it wouldn't be uranium.
The number of protons in the nucleus of an atom is called its atomic number.
Radiometric dating is a means of determining the "age" of a mineral specimen by determining the relative amounts present of certain radioactive elements.
By "age" we mean the elapsed time from when the mineral specimen was formed.
Some background on Rubidium and Strontium: Rb is a highly incompatible element.
Sr is fairly incompatible in mafic systems and relatively incompatible in silica-rich systems.
From this partitioning, the mantle will become depleted in Rb relative to Sr.
Certain natural phenomena or processes, such as Earth's year-long solar orbit, and the resulting annual climatic variations that govern the growth of tree rings, can be used as "natural clocks." If we can find and date a rock that we know has been around since the Earth formed, we can measure the age of the Earth.
Can we find in rocks a natural clock that has been operating since they formed?
The above equation tells us the absolute amount of Strontium-87 in the rock sample.
However, because this is a small number and counting atoms isn’t easy, it is more useful to use the ratio of one isotope to another.