Final answer:
The measure used in radioactive dating to denote the average time it takes for a particular radioactive nucleus to decay is known as its half-life, which is essential for determining the age of rocks and geologic materials.
Step-by-step explanation:
In radioactive dating, scientists measure how long a particular radioactive nucleus will take to decay on average using a term called its half-life. The half-life is a constant that indicates the time required for half of the nuclei in any sample of a radioactive isotope to decay. This concept is critical in radioactive dating techniques, such as those using the decay of uranium-238 (238 U) to lead-206 (206 Pb), to estimate the age of rocks and geologic materials. Knowing the half-life allows researchers to calculate the ratio between parent (original radioactive isotope) and daughter (resulting isotope) atoms and thus determine how many half-lives have passed since the rock solidified.
For instance, 238 U has a half-life of 4.5 × 109 years, which makes it useful for dating materials that are millions to billions of years old. This enables scientists to conclude that the oldest rocks on Earth solidified about 3.5 × 109 years ago.