Working with several collaboraters, Libby established the natural occurrence of radiocarbon by detecting its radioactivity in methane from the Baltimore sewer.
In contrast, methane made from petroleum products had no measurable radioactivity.
Isotopes participate in the same chemical reactions but often at differing rates.
When isotopes are to be designated specifically, the chemical symbol is expanded to identify the mass (for example, C is not stable.
Ninety-nine percent of these also contain six neutrons.
The 6 proton 6 neutron atoms are said to have a mass of 12 and are referred to as "carbon-12." The nuclei of the remaining one percent of carbon atoms contain not six but either seven or eight neutrons in addition to the standard six protons.
That means Argon atoms which are normally just gaseous become trapped in solid igneous rock.
So we know that 100% of all the Argon atoms in an igneous rock got there from decaying Potassium, after it solidified.
Carbon-14 is most abundant in atmospheric carbon dioxide because it is constantly being produced by collisions between nitrogen atoms and cosmic rays at the upper limits of the atmosphere.
During the lifetime of an organism, the amount of c14 in the tissues remains at an equilibrium since the loss (through radioactive decay) is balanced by the gain (through uptake via photosynthesis or consumption of organically fixed carbon).
However, when the organism dies, the amount of c14 declines such that the longer the time since death the lower the levels of c14 in organic tissue.
You should instead stick with the more general concept of RADIOMETRIC dating. Argon is a gas, but Potassium is a mineral that is mixed in with lava.
So when a volcano explodes, it deposits fresh liquid lava with 0% Argon, because any gaseous content bubbles out of the much denser lava.