Radioactive decay and radiometric dating
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.
Therefore the amount of argon formed provides a direct measurement of the amount of potassium-40 present in the specimen when it was originally formed.
At the same time, the fraction of strontium-87 increases from zero and approaches 100% with increasing number of half-lives.
An atom with the same number of protons in the nucleus but a different number of neutrons is called an isotope.