Small particles of dust collected together into larger and larger objects – pebbles, rocks, boulders, etc – until there were many planetoids in the Solar System.These planetoids collided together and eventually enough came together to become Earth-sized.These are uranium-234, uranium-235, and uranium-238. Since each atom of uranium has 92 protons, the isotopes must have 142, 143 and 146 neutrons respectively. Atomic mass has very little bearing on chemical reactions; therefore the reactivity and chemical reactions of an element's different isotopes are almost identical.The physical properties of atoms, however, do depend on mass.Isotopes Example 1 Carbon atoms exist naturally with 6, 7 or 8 neutrons.Since each atom of carbon has 6 protons, the isotopes must have atomic masses of 12, 13 and 14. Carbon-14 is unstable, decaying with a half-life of about 5,700 years.How do scientists know Earth is 4.54 billion years old?

At any particular time all living organisms have approximately the same ratio of carbon 12 to carbon 14 in their tissues.For each dating or chronological method there is a link in the box at right to take you to that section of this page.There, you will find a brief description of the method, plus links to take you to other webpages with more extensive information.Overview of Methods Superposition Stratigraphy Dendrochronology Radiocarbon C14 Radiometric Dating Methods Obsidian Hydration Dating Paleomagnetic/Archaeomagnetic Luminescence Dating Methods Amino Acid Racemization Fission-track Dating Ice Cores Varves Pollens Corals Cation Ratio Fluorine Dating Patination Oxidizable Carbon Ratio Electron Spin Resonance Cosmic-ray Exposure Dating This is an excellent overview of dating methodologies, and is a chapter in a textbook on Archaeology.You may find it useful for the clear definitions, and for excellent links on a variety of topic.(Since atomic mass = mass protons mass neutrons) These isotopes are called carbon-12, carbon-13 and carbon-14. It is produced in earth's atmosphere by cosmic ray bombardment of nitrogen-14.

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