The amount of strontium-86 in a given mineral sample will not change.
Therefore the relative amounts of rubidium-87 and strontium-87 can be determined by expressing their ratios to strontium-86: Rb-87/Sr-86 and Sr87/Sr-86 We measure the amounts of rubidium-87 and strontium-87 as ratios to an unchanging content of strontium-86.
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.
However, any escaping argon gas would lead to a determined age younger, not older, than actual.
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.
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.
An atom with the same number of protons in the nucleus but a different number of neutrons is called an isotope.