Used most often for radioactive dating
To explain those rules, I'll need to talk about some basic atomic physics. Hydrogen-1's nucleus consists of only a single proton.
Protons and neutrons together are called nucleons, meaning particles that can appear in the atomic nucleus.
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For example, hydrogen-1 and hydrogen-2 are both nuclides of the element hydrogen, but hydrogen-1's nucleus contains only a proton, while hydrogen-2's nucleus contains a proton and a neutron.
Uranium-238 contains 92 protons and 146 neutrons, while uranium-235 contains 92 protons and 143 neutrons.
Because of the short length of the carbon-14 half-life, carbon dating is only accurate for items that are thousands to tens of thousands of years old. Geologists must therefore use elements with longer half-lives.
Since all atoms of the same element have the same number of protons, different nuclides of an element differ in the number of neutrons they contain.Geologists do not use carbon-based radiometric dating to determine the age of rocks.Carbon dating only works for objects that are younger than about 50,000 years, and most rocks of interest are older than that.Carbon-14 has a half life of 5730 years, meaning that 5730 years after an organism dies, half of its carbon-14 atoms have decayed to nitrogen atoms.Similarly, 11460 years after an organism dies, only one quarter of its original carbon-14 atoms are still around.
Once the organism dies, it stops replenishing its carbon supply, and the total carbon-14 content in the organism slowly disappears.