The Atomic Clock: Radiometric Dating Explained

If radioactive decay happens at a predictable rate, then we can measure how long it takes for half of a sample to change. If exactly half of the parent atoms become daughter atoms, then that time period is one half-life.

If relative dating only tells us the order of events, then we need a different method for specific dates. If a technique provides a numerical value in years, then it is providing the absolute age.

If one half-life passes (1,000 years), then 100g becomes 50g. If a second half-life passes (another 1,000 years), then half of that 50g decays. If half of 50 is 25, then 25 grams remain.

If sedimentary rock is made of recycled bits of older rocks, then dating a grain would give the age of the original source rock. If the goal is to date the new layer, then dating the individual older grains provides the wrong information.

If we cannot guess when one atom will pop, but we know exactly how long a billion atoms take to lose half their number, then the group behavior is reliable. If it is reliable, then it functions like a clock.

If a sample is ancient and has very little parent isotope left, then adding fresh parent material from the modern world will skew the ratio. If the ratio shows more parent, then the clock appears to have just started.

If the parent turns into the daughter over time, then the balance between them changes every second. If we measure that balance (the ratio), then we can use the known half-life to calculate exactly how much time has passed.

If 1 half-life passes, 1/2 remains. If 2 pass, 1/4 remains. If 3 pass, 1/8 remains. If the math shows 1/2^3 = 1/8, then exactly 3 half-lives have passed.

If Carbon-14 is created in the atmosphere and taken in through breathing or eating, then only biological things will have it. If a rock does not eat or breathe, then it does not contain the Carbon-14 needed for this specific test.

If the parent atom does not disappear but simply changes its form, then every "lost" parent atom becomes a "new" daughter atom. If this conversion happens, then the daughter amount must go up as the parent goes down.

If an atom has the same number of protons but a different mass, then it must have a different number of neutrons. If these variations exist, then they are called isotopes.

If chemical reactions are affected by temperature, then we might expect isotopes to be too; however, if radioactive decay is a nuclear process, then it remains constant regardless of the environment.

If a radioactive atom is about to decay into a more stable form, then it is the starting point of the process. If it is the starting point, then it is called the parent.

If a rock is as old as the Earth, then it needs an isotope that decays very slowly. If Uranium-238 has a multi-billion year half-life, then it is the tool used for ancient geological dating.

If an atom's nucleus is balanced and will never change into a different element, then it is not radioactive. If it does not change, then it is classified as a stable isotope.

If the calculation relies on a ratio of parent to daughter atoms, then adding or losing atoms from the outside would ruin the math. If the decay rate is predictable and isotopes are present, then the date can be determined.

If we assume the decay rate is a constant law of physics and that no atoms leaked out, then our math will be correct. If we also assume the daughter start-amount was zero (or a known level), then we can calculate the age.

If Carbon-14 has a half-life of only 5,730 years, then it disappears after about 50,000 years. If a dinosaur bone is 65 million years old, then there is no Carbon-14 left to measure.

If geologists need to date different time spans, then they use elements with different half-lives. Carbon, Uranium, Potassium, and Argon are standard; however, Gold is stable and does not decay, so it cannot be used for dating.

If scientists need to weigh atoms to tell the difference between parent and daughter isotopes, then they use a machine that sorts them by mass. If it sorts them by mass and records the data, then it is a mass spectrometer.