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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.

After 1 half-life, there is 1/2 of the original amount of the parent left.

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).

() is the ``natural logarithm'' (it is the ``ln'' key on a scientific calculator).

There are several ways to figure out relative ages, that is, if one thing is older than another.

For example, looking at a series of layers in the side of a cliff, the younger layers will be on top of the older layers.

That number is also the amount of parent that has decayed (remember the rule #parent #daughter = constant). in the age measurements of less than 100 million years.

The narrow range of ages is taken to be how long it took the parent bodies of the meteorites to form.

All atoms of an element have the same number of protons in their nucleus and behave the same way in reactions.

The atoms of an isotope of a given element have same number of protons AND neutrons in their nucleus.

When plants absorb carbon-dioxide in the photosynthesis process, some of the carbon dioxide has the carbon-14 atom in the molecule.

Assuming that our atmosphere's composition and the cosmic ray flux has not changed significantly in the last few thousand years, you can find the age of the organic material by comparing its carbon-14/carbon-12 ratios to those of now-living plants.

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.