Radiometric dating tests
Some nuclides have very long half-lives, measured in billions or even trillions of years.
So, if we know how much of the nuclide was originally present, and how much there is now, we can easily calculate how long it would take for the missing amount to decay, and therefore how long its been since that particular sample was formed. We must know the original quantity of the parent nuclide in order to date our sample In order to do so, we need a nuclide thats part of a mineral compound. Because theres a basic law of chemistry that says "Chemical processes like those that form minerals cannot distinguish between different nuclides of the same element." They simply cant do it.
In electron absorption, a proton absorbs an electron to become a neutron.
In other words, electron absorption is the exact reverse of beta decay.
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.