Or on a slightly smaller scale, where can paleontologists find a clock to tell the age of fossils, or how can archeologists determine how old ancient pottery and buried artifacts are? They are mostly empty space with a denser tiny area called the nucleus and a cloud of electrons surrounding the nucleus.
The particles emitted in the process are what we call radiation. Now, try to link the clock characteristics you just listed to the characteristics of radioactive decay that appeal to geologists: Could you link these to your list of characteristics of a good clock?
It is now time to explore why geologists are so interested in these radioactive decay processes as a means of dating objects. This example might help clarify the processes and terms just introduced: Looking at the parent isotope potassium-40 (abbreviated as K-40) that decays into the daughter isotope argon-40 (abbreviated as Ar-40), scientists measured the half-life time to be 1.25 billion years.
While an element always has the same atomic number, meaning it has the same number of protons in its nucleus, it can have a different number of total nucleons in its nucleus.
Scientists call these different variations of the same element isotopes of each other. Radioactive refers to the characteristic that these isotopes are unstable and tend to fall apart.
That display is scheduled to remain open through late winter 2018; you can check out details at the Rock Hall's official site.