'' The age can still be determined but you have to be more clever in determining it.
One common sense rule to remember is that the number of parent isotope atoms the number of daughter isotope atoms = an unchanging number throughout time.
Or you can tell that certain parts of the Moon's surface are older than other parts by counting the number of craters per unit area.
The old surface will have many craters per area because it has been exposed to space for a long time. If you assume that the impact rate has been constant for the past several billion years, then the number of craters will be proportional to how long the surface is exposed.
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?
Different isotopes of a given element will have the same chemistry but behave differently in Radioactive isotopes will decay in a regular exponential way such that one-half of a given amount of parent material will decay to form daughter material in a time period called a half-life. When the material is liquid or gaseous, the parent and daughter isotopes can escape, but when the material solidifies, they cannot so the ratio of parent to daughter isotopes is frozen in.
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.
After another half-life, there is 1/2 of that 1/2 left = 1/2 × 1/2 = 1/4 of original amount of the parent left.
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).
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.
When plants absorb carbon-dioxide in the photosynthesis process, some of the carbon dioxide has the carbon-14 atom in the molecule.