Radiometric isochron dating
The half-life of a radioactive nuclide is defined as the time it takes half of a sample of the element to decay.
A third, very rare type of radioactive decay is called electron absorption.The mass number doesnt change, while the atomic number goes down by 1.So an atom of potassium-40 (K40), atomic number 19 can absorb an electron to become an atom of argon-40 (Ar40), atomic number 18.The decay rate and therefore the half-life are fixed characteristics of a nuclide. Thats the first axiom of radiometric dating techniques: the half-life of a given nuclide is a constant.(Note that this doesnt mean the half-life of an element is a constant.