# Absolute dating rock layers

Most of the radioactive isotopes used for radioactive dating of rock samples have too many neutrons in the nucleus to be stable.

Recall that an isotope is a particular form of an element.

The number of parent isotopes decreases while the number of daughter isotopes increases but the total of the two added together is a constant.

You need to find how much of the daughter isotopes in the rock (call that isotope ``A'' for below) are the result of a radioactive decay of parent atoms.

There are always a few astronomy students who ask me the good question (and many others who are too shy to ask), ``what if you don't know the original amount of parent material?

'' or ``what if the rock had some daughter material at the very beginning?

The parent isotope can only decay, increasing the amount of daughter isotopes. The number n is the number of half-lives the sample has been decaying.Isotopes of a given element have the same chemical properties, so a radioactive rock will incorporate the NONradioactively derived proportions of the two isotopes in the Multiply the amount of the non-daughter isotope (isotope B) in the radioactive rock by the ratio of the previous step: (isotope B) × R = initial amount of daughter isotope A that was not the result of decay.Subtract the initial amount of daughter isotope A from the rock sample to get the amount of daughter isotope A that IS due to radioactive decay.You then subtract this amount from the total amount of daughter atoms in the rock to get the number of decays that have occurred since the rock solified.Here are the steps: a result of radioactive decay (call that isotope ``B'' for below).