The teacher should tell the students that there are two basic principles used by geologists to determine the sequence of ages of rocks.
This is a stable condition, and there are no more changes in the atomic nucleus.
A nucleus with that number of protons is called lead (chemical symbol Pb). This particular form (isotope) of lead is called Pb-207.
For example, U-235 is an unstable isotope of uranium that has 92 protons and 143 neutrons in the nucl eus of each atom.
Through a series of changes within the nucleus, it emits several particles, ending up with 82 protons and 125 neutrons.
This happens at any time when addition of the fleeting "weak nuclear force" to the ever-present electrostatic repulsion exceeds the binding energy required to hold the nucleus together.
Very careful measurements in laboratories, made on VERY LARGE numbers of U-235 atoms, have shown that each of the atoms has a chance of decaying during about 704,000,000 years.Some half lives are several billion years long, and others are as short as a ten-thousandth of a second.Return to top A tasty way for students to understand about half life is to give each team 100 pieces of "regular" M & M candy.If the nucleus has not yet decayed, there is always that same, slight chance that it will change in the near future.Atomic nuclei are held together by an attraction between the large nuclear particles (protons and neutrons) that is known as the "strong nuclear force", which must exceed the electrostatic repulsion between the protons within the nucleus.Where the amounts of parent and daughter isotopes can be accurately measured, the ratio can be used to determine how old the rock is, as shown in the following activities.