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.
Because argon is an inert gas, it is not possible that it might have been in the mineral when it was first formed from molten magma.
Potassium-Argon dating: The element potassium (symbol K) has three nuclides, K39, K40, and K41. K40 can decay in two different ways: it can break down into either calcium or argon.
The ratio of calcium formed to argon formed is fixed and known.
The sum of protons plus neutrons is the mass number.
We designate a specific group of atoms by using the term "nuclide." A nuclide refers to a group of atoms with specified atomic number and mass 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.
Because of radioactivity, the fraction of rubidium-87 decreases from an initial value of 100% at the time of formation of the mineral, and approaches zero with increasing number of half lives.
At the same time, the fraction of strontium-87 increases from zero and approaches 100% with increasing number of half-lives.
The decrease in the amount of potassium required to form the original mineral has consistently confirmed the age as determined by the amount of argon formed.
Carbon-14 dating: See Carbon 14 Dating in this web site.
If we knew the fraction of a radioactive element still remaining in a mineral, it would be a simple matter to calculate its age by the formula To determine the fraction still remaining, we must know both the amount now present and also the amount present when the mineral was formed.