By using radiometric dating to determine the age of igneous brackets, researchers can accurately determine the age of the sedimentary layers between them. These layers are like bookends - they give a beginning and an end to the period of time when the sedimentary rock formed. So to determine the age of sedimentary rock layers, researchers first have to find neighboring layers of Earth that include igneous rock, such as volcanic ash. The extreme temperatures of the magma would just destroy the bones. Fossils can't form in the igneous rock that usually does contain the isotopes. But this sediment doesn't typically include the necessary isotopes in measurable amounts. Fossils, however, form in sedimentary rock - sediment quickly covers a dinosaur's body, and the sediment and the bones gradually turn into rock. Each of them typically exists in igneous rock, or rock made from cooled magma. Unfortunately, these elements don't exist in dinosaur fossils themselves. Some of the isotopes used for this purpose are uranium-238, uranium-235 and potassium-40, each of which has a half-life of more than a million years. To determine the ages of these specimens, scientists need an isotope with a very long half-life. Dinosaur bones, on the other hand, are millions of years old - some fossils are billions of years old. The half-life of carbon-14 is only 5,730 years, so carbon-14 dating is only effective on samples that are less than 50,000 years old. But carbon-14 dating won't work on dinosaur bones. This is what archaeologists use to determine the age of human-made artifacts. The most widely known form of radiometric dating is carbon-14 dating. Read on to see what it takes to date a fossil and what volcanic ash has to do with it. The short half-life is only part of the problem when dating dinosaur bones - researchers also have to find enough of the parent and daughter atoms to measure. This means that isotopes with a short half-life won't work to date dinosaur bones. Scientists can't tell whether the clock ran down a few days or millions of years ago. Once all the parents have become daughters, there's no more basis for comparison between the two isotopes. The half-life of the isotope being measured determines how useful it is at dating very old samples. The more parent isotopes there are - and the fewer daughter isotopes - the younger the sample. The ratio of parents to daughters can tell the researcher how old the specimen is. To read the time on this radioactive clock, scientists use a device called a mass spectrometer to measure the number of parent and daughter atoms. The element's half-life is the amount of time it takes for half the parent atoms in a sample to become daughters. But you can predict how long it will take a large group of atoms to decay. You can't predict when a specific unstable atom, or parent, will decay into a stable atom, or daughter. The result is like a radioactive clock that ticks away as unstable isotopes decay into stable ones. Eventually, some of the blocks can fall away, leaving a smaller, more stable structure.

The same is true if you take a block away from one of the pyramid's sides, making the rest unstable. If you try to add extra blocks to the sides pyramid, they may stay put for a while, but they'll eventually fall away. Think of the nucleus as a pyramid of building blocks. If, however, there are too many or too few neutrons, the atom is unstable, and it sheds particles until its nucleus reaches a stable state. Atoms may have an equal number of protons and neutrons.

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