History of Earth - Radiometric Dating

The remains of an exploding star or supernova.
The remains of an exploding star or supernova.

History of Earth - Radiometric Dating

Chemical elements are defined by the number of protons in their nucleus. For example an atom with 6 protons in its nucleus, is by definition, carbon. The number of neutrons can however vary. Most carbon atoms have 6 neutrons, but some have 8, which makes them unstable and radioactive.

Atoms of the same chemical element with different amounts of neutrons are called isotopes. In general an isotope is identified by its atomic mass, which is the total amount of protons and neutrons in its nucleus. For example, Carbon-12 refers to the stable isotope of carbon which have 6 protons and 6 neutrons. Carbon-14 on the other hand is the radioactive isotope with 6 protons and 8 neutrons.

As atoms increase in size, it becomes increasingly difficult for for the neutrons to hold the protons together. Calcium, which contains 20 protons and 20 neutrons, is the last chemical element whose most stable isotope have a 1:1 ratio concerning protons and neutrons. Zirconium, which is higher up the periodic table, have 40 protons and needs 51 neutrons to keep its nucleus together, while lead, the last of the stable elements, needs 125 neutrons to keep its 82 protons together.

Supernovas, the remains of an exploding star, can create chemical elements with more than 82 protons, but no force in nature can keep such a large nucleus together forever. At some point they will split into smaller, more stable pieces through a process known as radioactive decay. Even though the rates of decay vary from isotope to isotope, there is a constant known as half-life which is assigned to a given isotope. Therefor the half-life of a radioactive isotope is the time it takes for one half of an isotope to decay into a more stable form. This property has led to the development of a geological method know as radiometric dating.

Some pretty stable isotopes have half lives of billions of years, while highly unstable isotopes can exist for less than a second. Uranium for instance, has atoms with 92 protons, which puts them well above the stable limit of 82. All isotopes of uranium are therefor radioactive, but some are less stable than others. Uranium-234 is relatively unstable and has a half life of 245 000 years, while U-238 is more stable because it has more neurons, and has a half-life of 4.5 billion years. In the end, both U-234 and U-238 decay into lead-206(Pb-206). Therefore, a rock containing equal parts of U-234 and Pb-206 would be 245 000 years old. A different rock containing equal amounts of U-238 and Pb-206 would be 4.5 million years old.

Radiometric Dating - Scientists can calculate the age of certain rocks by comparing the relative amounts of radioactive and stable material within them, and therefore can tell us much about the history of the earth.

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    Source: The Bedside Baccalaureate

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