U-235 is the parent isotope of Pb-207, which is the daughter isotope.Many rocks contain small amounts of unstable isotopes and the daughter isotopes into which they decay.Principle of cross-cutting relations: Any geologic feature is younger than anything else that it cuts across.Some elements have forms (called isotopes) with unstable atomic nuclei that have a tendency to change, or decay.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.Part 2a Activity At any moment there is a small chance that each of the nuclei of U-235 will suddenly decay.
A nucleus with that number of protons is called lead (chemical symbol Pb). This particular form (isotope) of lead is called Pb-207.
Students should be able to understand the principles and have that as a background so that age determinations by paleontologists and geologists don't seem like black magic. Geologists in the late 18th and early 19th century studied rock layers and the fossils in them to determine relative age.
William Smith was one of the most important scientists from this time who helped to develop knowledge of the succession of different fossils by studying their distribution through the sequence of sedimentary rocks in southern England.
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.
2) To familiarize students with the concept of half-life in radioactive decay.