When I first got involved in the creationism/evolution controversy, back in early 1995, I looked around for an article or book that explained radiometric dating in a way that nonscientists could understand. Young-Earth creationists -- that is, creationists who believe that Earth is no more than 10,000 years old -- are fond of attacking radiometric dating methods as being full of inaccuracies and riddled with sources of error.   Contents: The half-life of a radioactive isotope is defined as the time it takes half of a sample of the element to decay.

A mathematical formula can be used to calculate the half-life from the number of breakdowns per second in a sample of the isotope.

So, if we know how much of the isotope was originally present, and how much there is now, we can easily calculate how long it would take for the missing amount to decay, and therefore how long it's been since that particular sample was formed.

That's the essence of radiometric dating: measure the amount that's present, calculate how much is missing, and figure out how long it would take for that quantity of the isotope to break down.

Because it's a statistical measurement, there's always a margin of error in the age figure, but if the procedure is done properly, the margin is very small. We must know the original quantity of the parent isotope in order to date our sample radiometrically. In order to do so, we need an isotope that's part of a mineral compound. Because there's a basic law of chemistry that says "Chemical processes like those that form minerals can't distinguish between different isotopes of the same element." This is because an element's chemical behavior depends only on the number of electrons it has, which is the same as its number of protons.

Obviously, the major question here is "how much of the isotope was originally present in our sample? So to a chemical process, U235 and U238 are identical.

Some isotopes can break down in more than one way -- in these cases, each different breakdown type has its own half-life.

The decay rate and therefore the half-life are fixed characteristics of an isotope. That's the first axiom of radiometric dating techniques: the half-life of a given isotope is a constant.

Some isotopes have very long half-lives, measured in billions or even trillions of years.

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