Example of radioisotope dating
As an example, the potassium-40 isotope (which contains 19 protons, 40 nucleons, and is represented by the atomic symbol K) will change into the argon-40 isotope (which contains 18 protons, 40 nucleons, and is represented by the symbol Ar).
When this happens, potassium-40, which is emitting particles in its conversion to a more stable form, is called the parent isotope.
Geologists (along with paleontologists, archeologists, and anthropologists) actually turn to the elements for answers to their geological time questions. The nucleus itself is made of protons and neutrons, collectively called nucleons.
Yes — as long as they use a big enough sample so statistical fluctuations average out.
To take it a step further, once only 1/4 of the original amount of K-40 isotopes are left (half of the half left over after 1.25 billion years), geologists can say that 2.5 billion years (double the half-life time) have gone by.
Now, can you predict how much time has gone by if only 1/8 is left?
Geologists use those radioactive isotopes to date volcanic ash or granite formations like the giant Half Dome in Yosemite National Park.
Yes, radioactive isotopes present in rocks and other ancient material decay atom by atom at a steady rate, much as clocks tick time away.