Principles of radiometric dating
A nucleus with that number of protons is called lead (chemical symbol Pb). This particular form (isotope) of lead is called Pb-207.U-235 is the parent isotope of Pb-207, which is the daughter isotope.
Developed in the beginning of the last century mainly to determine the absolute ages of rocks and minerals, radiometric chronology now plays a central role in a broad range of Earth and planetary sciences - from extra-solar-system processes to environmental geoscience.They are: Principle of superposition: Younger sedimentary rocks are deposited on top of older sedimentary rocks.Principle of cross-cutting relations: Any geologic feature is younger than anything else that it cuts across.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.It wasn't until well into the 20th century that enough information had accumulated about the rate of radioactive decay that the age of rocks and fossils in number of years could be determined through radiometric age dating.
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Learn about different types of radiometric dating, such as carbon dating.