# S4 dating 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.Radioactive dating gives the Find out how many times you need to multiply (1/2) by itself to get the observed fraction of remaining parent material. If some material has been decaying long enough so that only 1/4 of the radioactive material is left, the sample is 2 half-lives old: 1/4 = (1/2) × (1/2), n =2.Having been there herself, Jill shares 5 questions to consider before entering into the online dating scene.There are several ways to figure out relative ages, that is, if one thing is older than another.() is the ``natural logarithm'' (it is the ``ln'' key on a scientific calculator).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?Here are the steps: a result of radioactive decay (call that isotope ``B'' for below).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.

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.

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 solidified.

After yet another half-life, there is 1/2 of that 1/4 left = 1/2 × 1/2 × 1/2 = 1/8 of the original amount of the parent left (which is the fraction asked for).

So the rock is 1 half-life 1 half-life 1 half-life = 3 half-lives old (to get the age in years, simply multiply 3 by the half-life in years).   