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The fourth one is that we know what the concentration of atmospheric C14 was when the organism lived and died.The story of radiocarbon dating shows science at its finest.The new atom doesn't form the same kinds of chemical bonds that the old one did. It may not even be able to hold the parent atom's place in the compound it finds itself in, which results in an immediate breaking of the chemical bonds that hold the atom to the others in the mineral. (The exact details of this are rather complicated, so I won't go into them here.) When the number of electrons change, the shell structure changes too.So when an atom decays and changes into an atom of a different element, its shell structure changes and it behaves in a different way chemically. That's the sum total of the chemical and physical basis of radiometric dating.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. All these methods point to Earth being very, very old -- several billions of years old.The vast majority of carbon atoms, about 98.89%, are C12. And since carbon is an essential element in living organisms, C14 appears in all terrestrial (landbound) living organisms in the same proportions it appears in the atmosphere. Animals and fungi get C14 from the plant or animal tissue they eat for food. The C14 already in the organism doesn't stop decaying, so as time goes on there is less and less C14 left in the organism's remains.If we measure how much C14 there currently is, we can tell how much there was when the organism died, and therefore how much has decayed.
Different isotopes of the same element can have substantially different half-lives.) It's important to understand that the half-life is a purely statistical measurement. A sample of U238 ten thousand years old will have precisely the same half-life as one ten billion years old.If an element has more than one isotope present, and a mineral forms in a magma melt that includes that element, the element's different isotopes will appear in the mineral in precisely the same ratio that they occurred in the environment where and when the mineral was formed. The third and final axiom is that when an atom undergoes radioactive decay, its internal structure and also its chemical behavior change.Losing or gaining atomic number puts the atom in a different row of the periodic table, and elements in different rows behave in different ways. Well, an atom's chemical activity pattern is a result of its electron shell structure.When we know how much has decayed, we know how old the sample is.Many archaeological sites have been dated by applying radiocarbon dating to samples of bone, wood, or cloth found there. One is that the thing being dated is organic in origin.