Radiometric Dating . These radioactive elements spontaneously decay, meaning that they release electrons or neutrons and protons and subsequently form a different element. The average rate of decay is constant. When an igneous rock is created, it has a set amount of a radioactive material and that material begins to decay. If the decay rate is known, the scientists can calculate the time when only parent elements and no daughter elements existed.
Scientists speak in terms of half lives.
A half life is when one half of the original parent element has decayed into the daughter element. So, in the first half life, half of the parent element has decayed and half remains. The instruments that measure the amount of parent material and daughter material are sensitive. However, there are limits to how small of a quantity they can measure. Thus, depending on the rate of decay, some radioactive elements offer a long time span over which they can be measured, and others offer only a short time span because they decay so quickly.