An atom with the same number of protons in the nucleus but a different number of neutrons is called an isotope.
For example, uranium-238 is an isotope of uranium-235, because it has 3 more neutrons in the nucleus.
So, when a mineral forms – whether from molten rock, or from substances dissolved in water – it will be initially argon-free, even if there is some argon in the liquid.
However, if the mineral contains any potassium, then decay of the K isotope present will create fresh argon-40 that will remain locked up in the mineral.
About 89.28% of the time, it decays to calcium-40 ( Potassium-40 is especially important in potassium–argon (K–Ar) dating.
Argon is a gas that does not ordinarily combine with other elements.
The core also likely contains radiogenic sources, although how much is uncertain.It has the same number of protons, otherwise it wouldn't be uranium.The number of protons in the nucleus of an atom is called its atomic number.The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock. The reason for this is that Rb has become distributed unequally through the Earth over time.We can see how do deal with this if we take a particular case. For example the amount of Rb in mantle rocks is generally low, i.e. The mantle thus has a low If these two independent dates are the same, we say they are concordant.To see how we actually use this information to date rocks, consider the following: Usually, we know the amount, N, of an isotope present today, and the amount of a daughter element produced by decay, D*.By definition, D* = N-1) (2) Now we can calculate the age if we know the number of daughter atoms produced by decay, D* and the number of parent atoms now present, N.Potassium occurs in two stable isotopes (Ar atoms trapped inside minerals.What simplifies things is that potassium is a reactive metal and argon is an inert gas: Potassium is always tightly locked up in minerals whereas argon is not part of any minerals. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content.The potassium-argon (K-Ar) isotopic dating method is especially useful for determining the age of lavas.Developed in the 1950s, it was important in developing the theory of plate tectonics and in calibrating the geologic time scale.