Radiometric dating exam questions
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.
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.
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.
Presented with a new method that gave answers different than existing methods, the scientists involved did not simply assume that either the old method or the new one was wrong.
They viewed the problem as a challenge, dug into it with all their energy, and didn't stop until they understood exactly why their C14 dates disagreed with traditional dates, what was wrong with their C14 procedures, and how to compensate for the problems in the future. When Professor William Libby developed the C14 dating system in 1949, he assumed that the amount of C14 in the atmosphere was a constant.
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Radiometric dating methods are the strongest direct evidence that geologists have for the age of the Earth.